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20 April 2022, Volume 44 Issue 2

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Research paper

GEOLOGICAL CHARACTERISTICS AND ERUPTION HAZARDS TYPES OF BINGMAJIAO: A COASTAL VOLCANO IN EMAN, HAINAN

ZHAO Yong-wei, LI Ni, CHEN Zheng-quan, WANG Li-zhu, FENG Jing-jing, ZHAO Bo

2022, 44(2):  281-296.  DOI: 10.3969/j.issn.0253-4967.2022.02.001

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Bingmajiao volcano is a coastal volcano, located in Eman Town, Leiqiong volcanic field, China. In this paper, based on satellite image and unmanned aerial vehicle(UAV)image data interpretation, as well as field investigation, typical cross sections at different locations of the coastal volcanic cone were analyzed to identify the volcanic eruption sequence and determine the physical mechanism of eruption. The origin of pyroclasts was analyzed under microscope and scanning electron microscope. There are three types of pyroclasts in Bingmajiao volcano. The first type is in the shape similar to ropes or tree root and experienced obvious plastic deformation. The micro-plastic lava droplets with different sizes and irregular shapes are agglomerated on the surface of clasts. The vesicular structure in the clasts is extremely developed. All lines of evidence support this type of pyroclasts derived from magmatic explosive eruption without significant water-involving. The second type of pyroclasts is featured by crusted and moss-like surface with superficial cracks. The rigid shell surface fragmented, forming a large number of sheet-pieces that were re-disordered cemented. Under the surface, fine-honeycomb-like vesicular structure appears. The surface cracking supports the quenching by water under high temperature, and the interior vesicular structure shows that the core part may not be affected. These features indicate moderate water-magma interaction in the pyroclasts. The third type of pyroclasts shows no distinction between the surface and the interior. Irregular vesicles account for the major volume in the pyroclasts. Thin film-like lava separates these vesicles. Some lava broke into a large number of sheet-like pieces and agglomerated, forming strongly brittle-ductile deformed pyroclasts. Abundant cracks appear on the surface of lava. These features support this type of pyroclasts formed in relatively strong water-magma interaction. The study shows that the Bingmajiao volcano erupted in littoral environment, with the characteristic of transition from submarine volcano to terrestrial volcano. In the early stage of volcanism, submarine “fire fountain” type eruption prevailed, and pyroclastic deposits dominated by the third type of pyrolcasts formed underwater. Most were composed of sharp-hornlike volcanic lapilli. The pyroclastic deposit is loose and has no bedding, and the particle size sorting is not obvious. There is a large number of black fluidal juvenile lava with highly vesicular structure. As the eruption continued, when the pyroclastic deposits rose above the water surface, the volcanism transformed into the phreatomagmatic eruption, resulting in surge current and tuff deposit, which has obvious parallel bedding and cross-bedding. The second type of pyroclasts formed in this stage. In the late period of volcanic activity, Strombolian and Hawaiian type eruption were the main types, which formed black and red welding aggregates. Finally, the eruption turned into an overflow of lava, forming a lava platform. According to the eruption physics of Bingmajiao volcano, it is speculated that the potential eruption hazards of littoral volcano in the future include underwater “fire fountains”, surging currents, ballistic falling volcanic bombs, lava fountains and lava flows. Among them, the surge current may move at a high speed close to the sea level, affecting a range of 10km around the crater, which is the most dangerous type of volcanic eruption hazard.

LATE QUATERNARY ACTIVE TECTONICS OF THE NORTH ALTYN FAULT

YE Yu-hui, WU Lei, WANG Yi-ping, LOU Qian-qian, CHEN Li-qi, GAO Shi-bao, LIN Xiu-bin, CHENG Xiao-gan, CHEN Han-lin

2022, 44(2):  297-312.  DOI: 10.3969/j.issn.0253-4967.2022.02.002

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The~1600km long, left-reverse strike-slip active Altyn Tagh fault system defines the northern edge of the Tibetan plateau, and serves as an important tectonic boundary in models describing the northward expansion of the plateau. The Altyn Tagh fault system has complex geometries, and consists mainly of the left-lateral South Altyn Fault to the south, the left-reverse or reverse-dominated North Altyn Fault to the north, and the intervening Altyn Shan. Most of the existing studies focus on the more active South Altyn Tagh Fault, but few has paid attention to the North Altyn Fault, which separates the Tarim Basin to the north from the Altyn Shan to the south, and figures importantly in understanding the tectonic evolution of the entire fault system. The kinematics of the North Altyn Fault in the Cenozoic remains disputed in whether it is a left-reverse or reverse-dominated fault. Herein, we used tectonic geomorphology analysis to systematically study the characteristics of active tectonics on the North Altyn Fault in the Quaternary. There are dozens of rivers in the Altyn Shan between the South Altyn Tagh Fault and North Altyn Fault, the majority of which originate near the South Altyn Tagh Fault and flow northward across the North Altyn Fault into the Tarim Basin. These rivers contain abundant information about the Quaternary tectonic activity of the North Altyn Fault. We used SRTM DEM data to extract the geomorphic features of 18 rivers and related catchment basins flowing across the North Altyn Fault. Geomorphic index, such as river longitudinal profiles, standardized river length-gradient index(SLK), normalized river steepness index(K_sn), area-elevation curves and their integrals(HI)of catchment basins, are analyzed. The conclusions are drawn as follows.
The geomorphological indexes show that the eastern part of the North Altyn Fault is geomorphologically more active than the western part. Along the western part of the North Altyn Fault, the river longitudinal profile and the area-elevation curves of the corresponding catchment basins are both concave upward, with many small knickpoints on the river profile and relatively low SLK, K_sn, and HI values. On the contrary, most of the river profiles in the eastern part of the fault are convex or linear, with much larger knickpoints on the hanging wall of the North Altyn Fault, coinciding with high SLK and K_sn values. The associated area-elevation curves are mainly S-shaped and convex, and the HI values are relatively large. Tectonic geomorphic index is generally affected by lithology, climate and tectonics. The lithology of the hanging wall of the North Altyn Fault is relatively simple, consisting mainly of Precambrian metamorphic rocks intruded by some granite. There is no obvious difference in rock strength between the entire eastern and western sections. In addition, since the rivers are all located in the Altyn Shan and the area involved is not large, there is also no significant climatic variation along the strike of the North Altyn Fault in the Quaternary. Therefore, the difference of geomorphological activities between the parts should not be caused by difference in lithology and climate. Instead, we found that the eastern part of the North Altyn Fault is located to the north of the Akato restraining double bend, which features intense crustal shortening due to change of the fault strike, on the active South Altyn Tagh Fault. As such, we infer that the strong geomorphic activity of the eastern part of the North Altyn Fault likely results from intense lateral contraction from the Akato restraining double bend to the south, suggesting intimate interplay between the South Altyn Tagh Fault and the North Altyn Fault.
Our findings also imply that the North Altyn Fault likely changed from a strike-slip-dominated fault to a reverse-dominated fault in the late Cenozoic. It can be seen from the extracted river morphology that all rivers are relatively straight when passing through the North Altyn Fault, without systematic left-lateral deflection. The geomorphic indexes, such as the locations of river knickpoint, high SLK and K_sn value, which reflect where the relatively rapid tectonic uplift has occurred, all appear in the hanging wall of the North Altyn Fault. Moreover, a south-dipping frontal fault is discovered in the north of the North Altyn Fault. This fault cut and uplifted the Quaternary alluvial fan in the hanging wall, and the amount of uplift decreases gradually from middle to both sides until it vanishes, forming a bilaterally symmetric anticline approximately parallel to the fault. The rivers across through the fault are straight and undeflected systematically. All these show typical characteristics associated with a thrust fault. We thus infer that the North Altyn Fault is dominated by reverse dip-slip in the late Quaternary. Together with the Cenozoic strike-slip motion on the North Altyn Fault by the measurement of kinematic indicators, a transition from strike-slip-dominated to reverse-dominated in the late Cenozoic is thus expected.

PRESENT DEFORMATION OF~90° INTERSECTING CONJUGATE FAULTS AND MECHANICAL IMPLICATION TO REGIONAL TECTONICS: A CASE STUDY OF 2019 M_W≥6.4 PHILIPPINES EARTHQUAKE SEQUENCE

WANG Yu-qing, FENG Wan-peng, ZHANG Pei-zhen

2022, 44(2):  313-332.  DOI: 10.3969/j.issn.0253-4967.2022.02.003

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Conjugate faults are a pair of faults developed under the identical regional tectonic stress fields with cross-cutting structures and opposite shear senses. They have been applied to restore the ancient regional tectonic stress fields, and the mechanics of local crust during its formation can be reflected by their dihedral angle. The ~60° intersecting conjugate fault occurs under brittle environment as proposed by the Anderson theory, while the 110° intersecting conjugate fault could be formed under the conditions of ductile environment as explained by maximum effective moment(MEM)criterion. In addition, there is another kind of conjugate faults with ~90° intersecting angle, which have been observed globally, but the mechanism of their formation still remains unsolved.
Conjugate faults have been intensively studied using traditional geological methods and laboratory rock experiments. Interferometric synthetic aperture radar(InSAR), as an important geodetic mapping tool with an unprecedented precision and spatial resolution, provides a potential for investigating conjugate faults by exploring three-dimensional geometric structures. In this study, we investigated the 2019 M_w≥6.4 Philippines earthquake sequence as an example to link the present deformation characteristics of the ruptured conjugate faults to the regional tectonic stress.
From October to December 2019, four M_W≥6.4 earthquakes occurred in Mindanao, Philippines. The epicenters were located in the Philippine Sea plate, at the junction of the Eurasian plate, the Pacific plate and the Indian Ocean plate. Affected by three-sided subduction, the plate boundaries are almost convergent boundaries with active tectonic movement and frequent seismic activities. The target earthquake sequence occurred in Mindanao where the Philippine Sea plate collided with the Sunda plate. According to the GCMT earthquake catalog, this earthquake sequence shows similar focal mechanisms to the eight M_W≥5.0 earthquakes in the study area before this earthquake sequence from 1992, which will have certain implications for the research on local mechanical background.
This study collected both C-band Sentinel-1 TOPS and L-band ALOS-2 SAR images in ascending and descending tracks to retrieve surface deformation of the earthquake sequence. Four Sentinel-1 interferograms and three ALOS-2 interferograms were obtained using an InSAR open source package: GMTSAR. Based on the latest global atmospheric model, ERA5, the atmospheric phase delay correction was conducted, and the standard deviations(SDs)of the used Sentinel-1 and ALOS-2 interferograms before and after correction were reduced from 1.94cm and 3.55cm to 1.93cm and 3.46cm, respectively. The improved InSAR deformation products were used for earthquake fault modelling with a geodetic inversion package PSOKINV, which is based on the elastic half-space dislocation model, also called “Okada Model”. The obtained faults were further divided into several sub-faults with small patch-sizes to determine the accumulated distributed slip. The predicted interferograms from the obtained slip models can fit the original interferograms well, and the SDs of the residuals of Sentinel-1 and ALOS-2 interferograms were 1.55cm and 3.36cm, respectively, which were lower than the noise levels of the original InSAR data.
The inversion results show that the four earthquakes mainly resulted from the ruptures of one dextral strike-slip fault(F₁)of strike 48.8°, dip 74.5° and slip angle -174.1°, and the other sinistral strike-slip fault(F₂)of strike 318.2°, dip 68.9° and slip angle 9.6°. The surface intersection of the two faults is nearly orthogonal, while the minimum spatial rotation angle between the two slip vectors is 29.28°. The latter indicates that two slip vectors are not completely conjugate in the seismological sense. The angle bisector of F₁ and F₂ is basically consistent with the azimuth of the regional principle compressive stress derived from seismic data, which also agrees with the horizontal components of the GPS velocities observed in the island. Given that the oblique direction of converging between the Philippine Sea and Sunda plates, a clear rotation of the regional stress conditions could have happened across the Philippine strike-slip fault.
Furthermore, 4790 aftershocks in the study area from October to December 2019 recorded by the local seismic network show that the aftershocks are evenly distributed above a depth of 31km, which is the depth of the Moho based on previous studies. Therefore, the seismogenic faults of the earthquake sequence could have extended to the Moho boundary, indicating that it is likely that they may have formed in the ductile mechanical environment originally. The Coulomb stress change(CSC)analysis indicates that the rupture of one branch of the conjugate faults can release stress on the both fault planes in the vicinity of their interaction, and pose positive CSC in the far fields simultaneously, in which CSC on itself is larger. Meanwhile, combined with 14 sets of conjugate faults collected globally in this study, L-shaped characteristics of the conjugate faults turn to be common. The phenomenon having different rupture lengths and slip magnitudes for each fault branch in a set of conjugate faults is likely related to the significant variations of the fault physical properties.

A COMPARATIVE STUDY OF SOURCE EFFECT BASED ON MINI-MULTICHANNEL SEISMIC PROFILE IN MARINE ACTIVE FAULT DETECTION

YI Hu, ZHAN Wen-huan, MIN Wei, WU Xiao-chuan, LI Jian, FENG Ying-ci, REN Zhi-kun

2022, 44(2):  333-348.  DOI: 10.3969/j.issn.0253-4967.2022.02.004

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The neotectonic activity is intense in the Taiwan Straits and the coastal area of South China. This region is one of the earthquake-prone areas of the world. In history, earthquakes of magnitude 6-7 occurred repeatedly in this region with a high recurrence rate. Therefore, this area has always been the focus of seismicity research and coastal earthquake prevention and disaster reduction. The exploration of active faults is the basis for seismic zoning, but the detection and identification of active faults in sea area are more difficult because of the coverage of sea water, which leads to a large number of “blind areas” in marine fault exploration for a long time. Seismic exploration methods are economical, suitable and efficient in detecting active faults in the sea area. This study compares the detection effect of different seismic sources.
In this study, geophysical exploration of active faults was carried out in the southeast Fujian uplift zone in the Taiwan Straits. A mini-multichannel seismic profile of GI gun source and sparker source at the same location was selected for comparative analysis and illustration. Five reflection interfaces(T₁—T₄, Tg)were interpreted on the GI gun profile, and five sets of seismic sequences(A—E)were classified. Six reflection interfaces(T'₁, T₁—T₄, Tg)were interpreted on the sparker source profile, and six sets of seismic sequences(A—D and E₁—E₂)were classified. Three basement faults and two shallow faults with small vertical extension were found, which are active since the late Pleistocene. Among them, the scale of fault F₁ is large, the displacement of the basement fault F₁ is 51ms, and the overall displacement of (T₁—T₄) in the sediments is 35ms. Faults F₂—F₅ are located on the continental side of fault F₁ and can be combined into grabens and horsts in forms, which are inferred to be the associated faults of Fault F₁. It’s found that basement faults can be identified by both GI gun profile and sparker source profile, while the small faults can only be identified by the sparker profile. At the same time, the depth of upper breakpoint on the sparker profile is shallower, and the latest fault activity can be traced back to the Holocene. The locations and geometrical shapes of the three basement faults are similar on the two profiles, but there are imaging differences in the formation shapes around the faults and the distribution patterns of the secondary faults due to the influence of resolution. The similarity of fault detection results shows the effectiveness of the two methods, while the difference of profile imaging shows the necessity of combined detection in practical work.
According to the comparison of the two kinds of data, the sparker profile reveals a finer shallow structure than the GI gun profile does, and the GI gun profile can obtain a clearer basement structure. Based on the fusion results of the two kinds of data, the structural attributes of fault F₁ are further analyzed and explained in detail in this paper, and the Fault F₁ is the result of the reactivation of a basement pre-existing fault in the late Pleistocene and is a depression-boundary fault with an activity pattern of extensional normal faulting, and it is considered in this paper to be part of the South China Binhai fault zone. Therefore, it is necessary to attach importance to the combination of multiple detection methods in marine seismic zoning and marine seismic hazard assessment in order to obtain more detailed fault information.

RESEARCH ON SHALLOW STRUCTURAL CHARACTERISTICS IN THE BANQUAN SEGMENT OF ANQIU-JUXIAN FAULT ZONE BASED ON SHALLOW SEISMIC REFLECTION PROFILING

QIN Jing-jing, LIU Bao-jin, WANG Zhi-cai, FENG Shao-ying, DENG Xiao-juan, HUA Xin-sheng, LI Qian

2022, 44(2):  349-362.  DOI: 10.3969/j.issn.0253-4967.2022.02.005

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The Anqiu-Juxian Fault is the latest active fault in Tanlu fault zone, which is also the seismogenic fault of Tancheng M8.5 earthquake in 1668. In order to probe the shallow structure and the characteristics of faults in the eastern graben of Tanlu fault zone, we applied the high-resolution shallow seismic reflection method with multifold overlaying and stacking. In addition, we laid out two shallow seismic reflection lines across the Anqiu-Juxian Fault and the eastern graben of Tanlu fault zone. The shallow seismic profiles clearly reveal the stratigraphic interface morphology and shallow fault characteristics. The results show that the eastern graben of Tanlu fault zone is a graben basin consisting of multiple faults, and the thickness of Quaternary strata and graben structure characteristics are obviously affected and controlled by Changyi-Dadian Fault F₁ and Baifenzi-Fulaishan Fault F₂. Also, the eastern and western sides of the graben are the basement uplift areas, and the sediment thickness of the Quaternary strata in uplift areas is less than 30m. There are thick Cenozoic strata deposited in the barben, the stratigraphic morphology changes greatly laterally, showing an inclined form which is shallow in the west and deep in the east, and the Cenozoic strata are in angular unconformity contact with the overlying strata. The deepest part of Quaternary strata in the graben is located near the horizontal distance of 7400m, and its depth is about 190m. The Anqiu-Juxian Fault revealed by the shallow seismic reflection profile is composed of two branch faults dipping in opposite direction, which merge into one fault in the deep section. According to the discernible buried depth of the upper breakpoints of these faults and the characteristics of the Quaternary activity, the activity of Baifenzi-Fulaishan Fault on the western boundary of the eastern graben of Tanlu fault zone is relatively weak and the discernible depth of the upper breakpoint is 53m, we infer that the Baifenzi-Fulaishan Fault is a pre-Quaternary fault. The Changyi-Dadian Fault on the eastern boundary of the eastern graben of Tanlu fault zone not only cut the bedrock’s top interface, but also revealed signs of dislocation since Quaternary. The discernible depth of the upper breakpoint of Changyi-Dadian Fault is about 26~33m. The Anqiu-Juxian Fault is the latest active fault in the study area, which possess the characteristics of large scale and large penetration depth. The fault controls the deposition of the Cenozoic strata in the graben and plays an important role in the formation of the the eastern graben of Tanlu fault zone. The discernible depth of the upper breakpoint of Anqiu-Juxian Fault is about 17~22m. Therefore, we infer that the active ages of Changyi-Dadian Fault and Anqiu-Juxian Fault are the late Pleistocene and Holocene, respectively. The research results can provide seismological evidence for further understanding of activity mode and activity age of the seismogenic fault of the 1668 Tancheng M$8\frac{1}{2}$ earthquake, as well as the near-surface characteristics and activity of the Banquan segment of the Tanlu fault zone.

CHARACTERISTICS OF THE CRUSTAL STRESS FIELD AND ITS DIRECTION CONVERGENCE BEFORE THE WENCHUAN EARTHQUAKE

WANG Xiao-shan, WAN Yong-ge

2022, 44(2):  363-377.  DOI: 10.3969/j.issn.0253-4967.2022.02.006

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The occurrence of earthquake is closely related to the crustal stress field. Earthquakes are caused by the failure of faults, driven by tectonic stress build-up in the Earth’s crust. The change of the stress field before a large earthquake is directly related to the earthquake preparation process. In order to understand the relationship between the tectonic stress field and the low-level seismicity of the Longmenshan Fault and adjacent region before the 2008 Wenchuan earthquake, the composite focal mechanism method based on P wave first motions of small and medium earthquakes is used to determine the tectonic stress field before the Wenchuan earthquake and analyze the temporal and spatial characteristics of the composite focal mechanisms.
Accurate earthquake location is a necessary factor to determine the focal mechanism and the stress field, especially to invert the focal mechanism and the stress field using P wave first motion of the near-field and local earthquake. Firstly, we estimated the hypocentral location and its uncertainty of a large number of small and medium earthquakes in Sichuan, China with a relatively accurate earthquake location method by considering the arrival time uncertainty. Secondly, the azimuth and take-off angle of the P wave first motion of a large number of small and medium earthquakes were calculated, whose focal mechanisms usually cannot be determined from small amount of P wave first motions, and the different weight values were given to the P wave first motion according to the hypocentral distance. Then we determine the composite focal mechanisms on the 0.5°×0.5° grid point in Sichuan area before the Wenchuan earthquake by using the composite focal mechanism method. The results show that the principal compressive stress(P)axes and principal tensile stress(T)axes of the composite focal mechanisms have obvious zoning characteristics, divided roughly by the Longmenshan Fault, the Xianshuihe Fault, and the Huayingshan Fault. The direction of the compressive axis of the northern Sichuan block from the west of the Longmenshan fault zone to the Longriba Fault is near EES-WWN, and that of the extension axis is nearly vertical, which results in the movement pattern of thrusting with right-lateral strike-slip in the Longmenshan fault zone and promoted the accumulation of stress field before the Wenchuan earthquake. The composite focal mechanisms in the south of the Xianshuihe Fault show a strike-slip pattern, which perfectly explains the sliding behavior of a series of major strike-slip earthquakes on the Xianshuihe Fault. The southeast segment of Huayingshan Fault presents a thrust pattern, which is consistent with the paleostress model proposed by predecessors. Thirdly, in order to understand the temporal variation of the crustal stress field before the Wenchuan earthquake, we calculate the focal mechanism rotation angles(FMOAs)of the annual composite focal mechanisms taking the Wenchuan earthquake as the time end to the focal mechanism of the Wenchuan earthquake obtained by different authors and institutions before the Wenchuan earthquake. It is found that the FMOAs of all the focal mechanisms of different authors and institutions reached its minimum value and were lower than its standard deviation 1 year before the Wenchuan earthquake. In view of the large rupture scale of the Wenchuan earthquake, we calculate the FMOAs of the annual composite focal mechanisms to the focal mechanisms of the Yingxiu-Hongkou initial rupture segment and Beichuan rupture segment before the Wenchuan earthquake. The results show that the FMOA of the Yingxiu Hongkou section decreased obviously, which indicates that this method can predict the location of future earthquake to some extent. Finally, in order to verify the uniqueness of convergence of stress field before the Wenchuan earthquake, we calculated the FMOAs of the annual composite focal mechanisms to the focal mechanisms of the other four reference points except the location of the Wenchuan earthquake in Sichuan area, and the results did not show the phenomenon that the stress direction of the four points tends to be consistent.
Above all, the temporal and spatial variation characteristics of the FMOAs of the stress field show that the focal mechanism and location of the Wenchuan earthquake are closely related to the convergence of the composite focal mechanism around the epicenter before the Wenchuan earthquake, which illustrates that the convergence tendency of the stress field to the Wenchuan earthquake rupture may provide a new idea to explore large earthquake precursor from tectonic stress field.

CRUSTAL VELOCITY STRUCTURE BENEATH THE SOUTHERN LIAONING PROVINCE DERIVED FROM DOUBLE DIFFERENCE TOMOGRAPHY

WANG Liang, JIAO Ming-ruo, QIAN Rui, ZHANG Bo, YANG Shi-chao, SHAO Yuan-yuan

2022, 44(2):  378-394.  DOI: 10.3969/j.issn.0253-4967.2022.02.007

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In recent years, the southern Liaoning Province is the main area of seismic activity in Liaoning Province, and the main geological structure units in this area include the Liaohe rift and Liaodong uplift in the east. As an important manifestation of modern tectonic activity, earthquakes are less distributed in Liaohe rift. Most of the seismic activities are concentrated in eastern Liaoning uplift area on the east side of Liaohe rift. The structure in this area is relatively complex. The revival of old faults during Quaternary is obvious, and there are more than 10 Quaternary faults. Among them, Haichenghe Fault and Jinzhou Fault are the faults with most earthquakes. The 1975 Haicheng M_S7.3 earthquake occurred in the Haichenghe Fault and the 1999 Xiuyan M_S5.4 earthquake occurred in the east of the fault.
In this paper, the seismic phase bulletins are used for earthquakes from August 1975 to December 2017 recorded by 67 regional seismic stations of Liaoning Province. These stations were transformed during the Tenth Five-year Plan period. Using the double-difference tomography and tomoDD program, we relocated the earthquakes and inversed the velocity structures of the southern Liaoning area.
In the study, grid method is used for model parameterization of seismic tomography, ART-PB is used for forward calculation, damped least square method is used in inversion, and checkerboard test is used for the solution evaluation. The theoretical travel time is forward calculated by taking the checkerboard velocity model of imaging meshing and plus or minus 5% of anomaly as the theoretical model. The checkerboard test results show that the checkerboard P-wave velocity model at the depths of 4km, 13km, 24km and 35km in the study area can be restored completely, and most areas at the depth of 33km can also be restored completely.
We calculated and got the relocations of almost all of the earthquakes in southern Liaoning area and obtained a better distribution of P wave velocities at the depth of 4km, 13km, 24km and 33km. The results show that earthquakes mainly concentrated in two areas: the Haicheng aftershock area and the Gaizhou earthquake swarm activity area. The distribution of seismicity in this area is obvious in NW direction.
The result of P-wave tomography in 4km depth indicates the consistent characteristics of shallow velocity structure with the surface geological structure in southern Liaoning Province area. The two sides of the Tanlu fault zone are characterized by different velocity structures. The high and low velocity discontinuities are located in the Tan Lu fault zone, which is in good agreement with the geological structure of the region. In Haichenghe Fault in the Haicheng aftershock area, there are high-velocity zone in the shallow layer and low-velocity zone in the depth of 4~12km, and the low-velocity zone intrudes and deepens eastward. The Xiuyan earthquake with M_S5.4 in 1999 occurred on the boundary section of high and low velocity zones. At the same time, there is a gap between Xiuyan and Haicheng sequences, which is located at the junction of high and low velocities, and there is a significant low-velocity zone underground in the region. From the perspective of mechanism of the seismogenic model, this velocity structure model may generate large earthquakes.

There are high-velocity zones at the ends of different segments of Jinzhou Fault, and the Gaizhou earthquake swarm occurred in the high-velocity area at the end of the fault. It is speculated that the activity of the Gaizhou earthquake swarm may be caused by the rise of water saturation in rocks due to the intrusion of liquid under the condition of stress accumulation.

THE CHARACTERISTICS OF MAJOR FAULTS AND STRESS FIELD IN WEIHE-YUNCHENG BASIN CONSTRAINED BY SEISMIC ACTIVITY AND FOCAL MECHANISM SOLUTIONS

YU Zhan-yang, SHEN Xu-zhang, LIANG Hao, ZHENG Wen-jun, LIU Xu-zhou

2022, 44(2):  395-413.  DOI: 10.3969/j.issn.0253-4967.2022.02.008

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In this paper, the seismic phase bulletin of 14381 earthquakes from January 1, 2009 to June 30, 2018 in the Weihe-Yuncheng Basin and its adjacent region were selected and analyzed. After removing the records with incomplete event information and insufficient station information, 11856 seismic events remained. A basic requirement for the double difference location method is that the distance between the pairs of seismic events is much smaller than the distance between the events and the stations and the linear scale of the velocity inhomogeneous body on the wave propagation path, so that the travel time difference between two earthquakes and the same station is only determined by the relative position between the two seismic events and the velocity of the seismic wave. In this case, the error caused by insufficient understanding of crustal structure can be effectively reduced and the result of relocation can be more accurate. Due to the large area, the whole study region was divided into three smaller parts for relocation of the events in order to reduce the influences of local structures. 8106 seismic events recorded by 52 stations were relocated using the double-difference location algorithm. It is found that the results constrained by the grid searching method are basically consistent with those obtained by other methods. The reliability of focal mechanism is affected by the number of initial motion and the azimuth distribution of the station. Therefore, when inversion of focal mechanism solution is carried out, earthquakes with more than 10 clear initial motion phases are selected, and the maximum azimuth gap between two stations with clear initial motion is required to be less than 90°. The azimuth coverage of the initial motion on the source sphere was measured according to azimuth and take-off angle distributions, and the focal mechanism solutions with poor coverage were eliminated. The contradiction ratio of focal mechanism solutions is less than 0.2. The average difference of b-axis of the best fitting solutions is less than 20°. Finally, the focal mechanism solutions of 346 seismic events with M_L≥2 were determined with initial motion of P and S waves. Normal type and strike-slip type earthquakes are widely distributed, accounting for more than 60% of all seismic events, and most of them are concentrated near fault zones. Before the formal inversion, the study area was divided into 1°×1° grids, and a series of damping coefficients were set to obtain the trade-off curve between the residual error of data fitting and the length of the stress field inversion model. The crustal stress field of 1°×1° grid in Weihe-Yuncheng Basin was obtained based on focal mechanism solution and stress tensor damping inversion method, and a certain number of depth profiles vertical to the faults were constructed for the analysis. The results show that compared with the original locations of seismic phase bulletin, the distribution of seismic events after relocation is more concentrated along the fault strike in plane. Vertically, they are densely distributed along the fault plane. There are more earthquakes in and around Shanxi graben, but the magnitude is generally small. The seismic activity in Weihe rift is relatively weak. Before the relocation, the focal depth distribution was concentrated in 5~10km, but after the relocation, the focal depth distribution changed significantly. The earthquakes were concentrated in the range of 10~25km, the overall focal depth was concentrated in the range of 20km, and a small number of earthquakes occurred in the range of 25~35km. The focal depth in the basin is relatively shallow with depth range of 5~15km. The focal depth at both ends of the basin tends to deepen, and the deepest depth can reach about 30km, which is consistent with the results of previous studies. The results of the depth profiles show that most of the fault planes in the study area have a large dip angle, similar to the occurrence of the surface, and some fault planes are even nearly vertical. The motion properties of fault structure and focal mechanism indicate that the faults in the study area are mainly normal and strike-slip ones. The results of stress field inversion indicate that the R values, which indicate the stress state, of the other regions are all less than 0.5 except for some areas in the southeastern margin of the research area. The stress state of Weihe-Yuncheng Basin tends to be tensile, and the maximum horizontal principal stress direction is nearly EW in Weihe rift and NNE and NEE in southern Shanxi rift, which is basically consistent with previous studies.

STUDY ON THE DENSITY VARIATION CHARACTERISTICS OF UNDERGROUND MEDIUM IN HUTUBI GAS STORAGE BASED ON HIGH-PRECISION TIME-VARYING MICROGRAVITY METHOD

LIU Dai-qin, XUAN Song-bai, CHEN Shi, LI Jie, WANG Xiao-qiang, LI Rui

2022, 44(2):  414-427.  DOI: 10.3969/j.issn.0253-4967.2022.02.009

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In this paper, based on microgravity time-varying signals, the gravity field and underground medium density change of Hutubi gas storage were simulated and calculated, and the response relationship between gravity change and injection-production pressure was analyzed. By using the 7 phases of mobile gravity data of Hutubi underground gas storage, adopting the classical adjustment method and selecting the absolute gravity points of HKPN, HKPS and Urumqi(BJ00) and Shihezi gravity point(BJ06) of CMONOC around the gas storage area as the calculation basis, the relative gravity variation of each monitoring point in the study area was obtained with the precision ranging (3~5)×10^-8m/s² for each point in each phase. Combined with the relevant data of gas storage injection-production pressure, the response relationship image between the spatial-temporal variation characteristics of gravity field and injection-production pressure in this area was acquired. The research shows that the gravity change in the entire survey area exhibits zoning characteristics. The gravity change in the outer area of the gas storage south of Hutubi Fault is relatively small, and the gravity change in the gas storage area increases and decreases alternately. Especially in the east side of the reservoir area, the gravity change shows obvious characteristics of decreasing in spring and increasing in autumn, which causes the natural gas in the gas storage to basically drop to the lowest in March, thus resulting in the minimum internal stress in the gas storage. According to the theory of crustal stress equilibrium, when the pressure inside the gas storage tends to increase or decrease, the stress outside the gas storage will be adjusted correspondingly. When the gas injected into the gas storage spreads between the rocks and their gaps in the gas storage, it will exert a certain pressure on the rocks, causing the medium density in the underground gas storage cavity to vary in different degrees, thus resulting in the changes in the gravity values of the surface measuring points in the gas storage area. Finally, based on the dynamic change data of gravity field observed on the surface of Hutubi underground gas storage, the constraint of depth weighting function was added in the calculation process to eliminate and weaken the multi-solution and skin effect, and the compact gravity inversion algorithm of spatial distribution of underground density variation anomaly body was adopted to simulate and calculate the underground material density change image of Hutubi gas storage and the morphological structure distribution characteristics inside the gas storage. In this paper, according to the structural framework of about 1km/layer in Hutubi gas storage, all slices are constructed in the vertical direction of 1km to the crust, and a total of 9 layers are cut into them. That is, they are divided from the surface to the interior of the gas storage from 0 to 9km. Based on the change amount of gas injection and production in Hutubi gas storage, combining with the density images of underground media in different periods, it can be clearly seen that the internal cavity shape distribution inside the gas storage is irregular, so the stress on each point in the gas storage will be uneven, resulting in different density changes of the medium in different depths. The density distribution of underground medium in this gas storage varies with time, and the density variation is relatively different, but it has a certain change rule. Most density variation images show four quadrant distribution characteristics, especially at the depth of about 3000~4000m of the gas storage, where the migration degree of underground medium substances is the largest, resulting in the largest density variation in this area, with the maximum density variation of about 0.7kg·m^-3. At this stage, the gas storage is just at the peak points of gas injection and production, that is, the maximum and minimum peak points of stress. In addition, the density change image has showed that the internal structure of the gas storage is in NW-SE direction, which is basically consistent with the geological structure distribution characteristics of Hutubi gas storage. Therefore, using gravity data, the structural form of Hutubi underground gas storage and the whole process of medium density changing with injection-production pressure can be clearly explained.

GEOCHEMICAL CHARACTERISTICS OF SOIL GAS IN ACTIVE FAULT ZONE IN NORTHWEST YUNNAN AND ITS ENLIGHTENMENT TO FAULT ACTIVITY

WANG Bo, ZHOU Yong-sheng, ZHONG Jun, HU Xiao-jing, ZHANG Xiang, ZHOU Qing-yun, LI Xu-mao

2022, 44(2):  428-447.  DOI: 10.3969/j.issn.0253-4967.2022.02.010

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14 survey lines, with a total of 167 measuring points, were laid out in the northern section of the Red River Fault, the Longpan-Qiaohou Fault, the Heqing-Eryuan Fault, the eastern piedmont fault of Yulong Mountains, and the Lijiang-Jianchuan Fault in the northwest of Yunnan Province, China. Cross-fault soil gas radon, hydrogen, and carbon dioxide have been measured on the above-mentioned faults. The concentration intensity and distribution characteristics of soil gas in the study area were calculated and analyzed. The results show that:
(1)The concentrations and distribution patterns of soil gas radon and hydrogen vary greatly in different faults. The concentrations of radon vary from 6.18Bq/L to 168.32Bq/L, while that of hydrogen are between 7.72ppm to 429ppm, and carbon dioxide are from 0.73% to 4.04%.
(2)The average results of soil gas measurement show that the concentrations of radon are higher than 40kBq/m³ in the sampling sites of Yinjie, Niujie, Gantangzi, while the concentrations of radon in En’nu and Tiger Leaping Gorge measuring lines are smaller; The concentrations of hydrogen are higher than 60ppm in the sampling sites of Yangwang village, Houqing, Dawa, Yangcaoqing and Tiger Leaping Gorge, while the concentrations in Gantangzi and Niujie measuring lines are smaller.
(3)The spatial distribution characteristics of soil gas concentration in faults in northwest Yunnan are obvious, and the intensity of radon and hydrogen concentrations in different active fault zones vary greatly. The intensities of radon and hydrogen concentration are higher and have good consistency in Yinjie and Yangwang village measuring lines located in the northern section of the Red River Fault, the Houqing survey line located in Longpan-Qiaohou Fault, Dawa survey line in the Lijiang-Jianchuan Fault and Yangcaoqing in the south of Chenghai Fault. The soil gas concentration in such sample sites is high and the degassing ability is strong, indicating the different activity characteristics of different segments of the above faults to some extent.
Under the action of tectonic stress, the fault will slip and the rock properties and material structure of the fault will change, thus causing changes of underground material, gas transport channel and transport mode, which is characterized by the change of the concentration and distribution characteristics of escaped soil gas. Combined with the active characteristics of faults, slip rate and geomorphological features, the characteristics of concentration and spatial distribution of two soil gases(radon and hydrogen)are discussed, and the following conclusions are obtained.
(1)There is a large difference in the concentration of escaped soil gas from different faults in the study area, indicating that the content of soil gas is controlled by regional geochemical background values, and there are certain differences in the gas concentration of different sections of the same fault, indicating that the local concentration/flux change is greatly affected by the transport.
(2)The concentration of fault soil gas is related to fault activity, and for different faults, the higher the degree of fault activity is, the higher the concentration of fault gas will be. From the point of fault gas concentration characteristics, the concentrations in the survey lines in the northern section of the Red River Fault and Heqing-Eryuan Fault in the study area vary greatly, suggesting that the fault segmentation is obvious. Compared with other faults, the northern section of the Red River Fault has a higher concentration of soil gas, indicating that the fault is more active. However, there is no simple linear relationship between the soil gas concentration and the fault slip rate, and it may also depend on its material source, transport channel structure, etc.
(3)The concentration of soil hydrogen at the outcrop of faults(especially normal faults and strike-slip faults)is generally higher, which shows that hydrogen has better indicative significance in revealing the location of fault rupture, and the distribution pattern of soil gas radon concentration is a good indicator for analyzing the characteristics of fault movement.

THE CHARACTERISTIC AND DEEP STRUCTURE OF THE GEOMAGNETICS IN AND AROUND THE DUNHUA-MISHAN FAULT ZONE

WANG Yu-dan, ZHANG Jing-fa, TIAN Tian

2022, 44(2):  448-460.  DOI: 10.3969/j.issn.0253-4967.2022.02.011

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This paper focuses on the in-depth analysis of the aeromagnetic characteristics of the Dunhua-Mishan fault zone and its surrounding areas using wavelet multi-scale analysis. In order to analyze the anomalies of the crustal structure at different depths, wavelet multi-scale decomposition is used to separate the deep field from the shallow field sources, superimpose the aeromagnetic anomalies on different anomalies of different geological bodies, extract the required information, analyze the local field anomalies caused by the field sources, and invert and interpret the geological bodies. In this paper, wavelet multi-scale analysis is used to decompose the aeromagnetic data, separate the deep and shallow field sources of aeromagnetism in the study area, and obtain wavelet detail maps of order 1 to 4. The wavelet transform detail maps are a response to high frequency anomaly information, and also a reflection of local field aeromagnetic anomaly information, which can be used to infer information such as fault depth and basement depth of basin. The experimental results are used to analyze the anomaly characteristics at different depths, invert and analyze the characteristics of the aeromagnetic anomalies and crustal structure at different depths, explore the deep basement and fault tectonic features and the intersection relationship between the Dunhua-Mishan Fault and the surrounding faults, calculate the approximate field source depth by wavelet detail map and power spectrum method, and infer the fault cut-through depth. The results of the analysis can provide geophysical research information for the study of geotectonics and the evaluation and exploration of hydrocarbon resources.
Based on the original aeromagnetic anomaly map, aeromagnetic anomalies ranging from -494~2022nT can be obtained, with the highest anomaly located at about 50km from Baoqing County. The anomalies in the central part of the study area are high, while those in the eastern and western parts are low. The deposition of basal and ultramafic magmatic rocks in the Dunhua-Mishan area has caused massive high anomalies, while deep and large faults caused basement uplift or decline, shown as high and low anomaly zones. In the aeromagnetic shallow source field, the shallow surface and upper crustal media are more complex, and the Dunhua-Mishan fault zone shows multi-pearl-like small-scale anomalies, resulting mainly from the intrusion of basal or ultramafic magmatic rocks in the shallow part of the fault. In the deep source field, the magnetic anomalies in middle and lower crust are mainly caused by different magnetic properties of basin bedrock. The large fault zone presents as the dividing line of different trajectory feature zones, and the deep large fault cuts deeper and presents as the dividing line of different trajectory feature zones. The cut-through depth of the deep major faults is larger and affects the aeromagnetic characteristics of the deep tectonic zone. The paper further discusses the cut-through depth of the major faults of this region by analyzing the characteristics of the aeromagnetic anomalies at different depths and finds that there are the three deep major faults in the region, namely, the Dunhua-Mishan Fault, the Dahezhen Fault and the Yilan-Yitong Fault, while the Hulin River Fault, the Muling River Fault, the Fujin-Xiaojia River Fault and the Nanbeihe-Boli Fault only cut through the shallow crust; the Muling River Fault, the Dunhua-Mishan Fault, the Dahezhen Fault and the Fujin-Xiaojia River Fault only intersect in the shallow crust. The Parker method was used to invert the depth of the Curie points in the area, and the results show that the depth of the Curie points in the area ranges from 22.3~29.9km, with the deepest area in the south of Hulin County, which is a depressional basin formed by plate subduction and extrusion, and the Dunhua-Mishan fault zone has a controlling effect on the morphology of the Curie points. Seismic activity is low in the region as a whole, and earthquakes are densely distributed in the northwest of the study area along the Yilan-Yitong fault zone, and less distributed along the Dunhua-Mishan fault zone and the Dahezhen fault zone. In the vicinity of the Dunhua-Mishan fault zone, small earthquakes are mainly concentrated in the area south of the Mishan sub-uplift, and the northern section of the Dunhua-Mishan fault zone is generally more stable. The gravity field in this area has been studied in depth by previous authors. The area belongs to the Moho surface uplift zone in Heilongjiang Province, with the Moho depth of about 30~32.5km. The Yilan-Yitong rift zone is deep to the Moho surface, and the Moho surface often shows uplift in the seismically active area. The local deformation and uplift of the crust-mantle provides the possibility of stress concentration, while the existence of deep major faults provides a channel for material transport. The overall level of seismic activity in the region is low, and the areas with intense activity are mainly concentrated in the Yilan-Yitong fault zone, with small earthquakes also gathering near the Jixi area. Seismicity of Qitaihe-Jixi area is mainly influenced by the Mudanjiang Fault and the Nanbei River Fault. The Dunhua-Mishan Fault has a strong influence on the distribution of Curie points and also influences the formation of several major tectonic units. So, more attention should be paid to the crustal activity of areas around the faults and at the intersections of faults in the future.

Focus: Mechanical understanding of the surface ruptures of the 2021 Madoi earthquake

DISTRIBUTED CHARACTERISTICS OF THE SURFACE DEFORMATIONS ASSOCIATED WITH THE 2021 M_W7.4 MADOI EARTHQUAKE, QINGHAI, CHINA

LIU Xiao-li, XIA Tao, LIU-ZENG Jing, YAO Wen-qian, XU Jing, DENG De-bei-er, HAN Long-fei, JIA Zhi-ge, SHAO Yan-xiu, WANG Yan, YUE Zi-yang, GAO Tian-qi

2022, 44(2):  461-483.  DOI: 10.3969/j.issn.0253-4967.2022.02.012

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Earthquake surface ruptures are the key to understand deformation pattern of continental crust and rupture behavior of tectonic earthquake, and the criteria to directly define the active fault avoidance zone. Traditionally, surface fissures away from the main rupture fault are usually regarded as the result triggered by strong ground motion. In recent years, the earth observation technology of remote sensing with centimeter accuracy provides rich necessary data for fine features of co-seismic surface fractures and fissures. More and more earthquake researches, such as the 2019 M_W7.3 Ridgecrest earthquake, the 2016 M_W7 Kumamoto earthquake, the 2020 M_W6.5 Monte Cristo Range earthquake, suggest that we might miss off-fault fissures associated with tectonic interactions during the seismic rupture process, if they are simply attributed to effect of strong ground motion. Such distribution pattern of co-seismic surface displacement may not be isolated, it encourages us to examine the possible contribution of other similar events. The 22 May 2021 M_W7.4 Madoi earthquake in Qinghai Province, China ruptured the Jiangcuo Fault which is the extension line of the southeastern branch of the Kunlun Fault, and caused the collapse of the Yematan bridge and the Cangmahe bridge in Madoi County. The surface rupture in the 2021Madoi earthquake includes dominantly ~158km of left-lateral rupture, which provides an important chance for understanding the complex rupture system.
The high-resolution UAV images and field mapping provide valuable support to identify more detailed and tiny co-seismic surface deformation. New 3 to 7cm per pixel resolution images covering the major surface rupture zone were collected by two unmanned aerial vehicles (UAV) in the first months after the earthquake. We produced digital orthophoto maps (DOM), and digital elevation models (DEM) with the highest accuracy based on the Agisoft PhotoScan^TM and ArcGIS software. Thus, the appearance of post-earthquake surface displacement was hardly damaged by rain or animals, and well preserved in our UAV images, such as fractures with small displacement or faint fissures. These DOM and DEM data with centimeter resolution fastidiously detailed rich details of surface ruptures, which have been often easily overlooked or difficult to detect in the past or on low-resolution images. In addition, two large-scale dense field investigation data were gathered respectively the first and fifth months after the earthquake. Based on a lot of firsthand materials, a comprehensive dataset of surface features associated with co-seismic displacement was built, which includes four levels: main and secondary tectonic ruptures, delphic fissures, and beaded liquefaction belts or swath subsidence due to strong ground motion. Using our novel dataset, a complex distributed pattern presents along the fault guiding the 158km co-seismic surface ruptures along its strike-direction. The cumulative length of all surface ruptures reaches 310km. Surface ruptures of the M_W7.4 Madoi earthquake fully show the diversity of geometric discontinuities and geometric complexity of the Jiangcuo Fault. This is reflected in the four most conspicuous aspects: direction rotation, tail divarication, fault step, and sharp change of rupture widths.
We noticed that the rupture zone width changed sharply along with its strike or geometric complexity. Near the east of Yematan, on-fault ruptures are arranged in ten to several hundred meters. Besides clearly defined surface ruptures on the main fault, many fractures near the Dongo section and two rupture endpoints are mainly along secondary faulting crossing the main fault or its subparallel branches. Lengths of fracture zones along two Y-shaped branches at two endpoints are about 20km. At the rupture endpoints, the fractures away from the main rupture zone are about 5km. Some authors suggested the segment between the Dongcao along lake and Zadegongma was a “rupture gap”. In our field investigation, some faint fractures and fissures were locally observed in this segment, and these co-seismic displacement traces were also faintly visible on the UAV images.
It is also worth noting that near the epicenter, Dongo, and Huanghexiang, a certain amount of off-fault surface fissures appear locally with steady strike, good stretch, and en echelon pattern. Some fissures near meanders of the Yellow River, often appear with beaded liquefaction belts or swath subsidences. In cases like that, fissure strikes are, in the main, orthogonal to the river. Distribution pattern of these fissures is different from usual gravity fissures or collapses. But they can’t be identified as tectonic ruptures because clear displacement marks are always absent with off-fault fissures. Therefore, it is difficult to determine the mechanism of off-fault co-seismic surface fissures. Some research results suggested, that during the process of a strong earthquake, a sudden slip of the rupturing fault can trigger strain response of surrounding rocks or previous compliant faults, and result in triggering surface fractures or fissures.
Because of regional tectonic backgrounds, deep-seated physical environments, and site conditions(such as lithology and overburden thickness), the pattern and physicalcause of co-seismic surface ruptures vary based on different events. Focal mechanisms of the mainshock and most aftershocks indicate a near east-west striking fault with a slight dip-slip, but focal mechanisms of two M_S≥4.0 aftershocks show a thrust slip occurring near the east of the rupture zone. On the 1︰250000 regional geological map, the Jiangcuo Fault is oblique with the Madoi-Gande Fault and the Xizangdagou-Cangmahe Fault at wide angles, and with several branches near the epicenter and the west endpoint at small angles. Put together the surface fissure distribution pattern, source parameters of aftershocks and the regional geological map, we would like to suggest that besides triggered slip of several subparallel or oblique branches with the Jiangcuo Fault, inheritance faulting of pre-existing faults may promote the development of off-fault surface fissures of the 2021Madoi earthquake. Why there are many off-fault distributed surface fissures with patterns different from the gravity fissures still needs further investigation. The fine expression of the distributed surface fractures can contribute to fully understanding the mechanism of the seismic rupture process, and effectively address seismic resistance requirements of major construction projects in similar tectonic contexts in the world.

DETAILED MAPPING OF THE SURFACE RUPTURE NEAR THE EPICENTER SEGMENT OF THE 2021 MADOI M_W7.4 EARTHQUAKE AND DISCUSSION ON DISTRIBUTED RUPTURE IN THE STEP-OVER

HAN Long-fei, LIU-ZENG Jing, YAO Wen-qian, WANG Wen-xin, LIU Xiao-li, GAO Yun-peng, SHAO Yan-xiu, LI Jin-yang

2022, 44(2):  484-505.  DOI: 10.3969/j.issn.0253-4967.2022.02.013

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Detailed mapping of coseismic surface rupture can provide valuable information for understanding the geometrical complexities, dynamic rupture processes and fault mechanisms. Fault geometrical complexities, such as bends, branches, and stepovers are common in strike-slip fault systems and can control the coseismic surface rupture characteristics to a certain extent. Observational studies of surface ruptures in past earthquakes suggested that special rupture characteristics would form distributed ruptures and rupture gaps. The detailed mapping has become an important way to study the surface rupture. According to the China Earthquake Networks Center(CENC), the M_W7.4 earthquake occurred at 2:04 PM on May 22, 2021, in Madoi County, Qinghai Province. The epicenter is about 70km south of the eastern Kunlun Fault on the northern boundary of the Bayan Kera block. It is the largest earthquake that hit the Chinese mainland since the Wenchuan M_S8.0 earthquake in 2008. After field investigation and rupture mapping on the computer, Yao et al.(2022)estimated that the length of surface rupture of this earthquake is 158km. Surface ruptures of the M_W7.4 Madoi earthquake broke through the geometric discontinuities such as step-overs and bends, and formed various coseismic surface fractures, especially in the middle segment. In the survey of the Madoi earthquake, we rapidly acquired aerial image data using UAV aerial photogrammetry and obtained high-resolution digital orthograph models(DOMs)and digital elevation models(DEMs)using PhotoScan software based on the SfM algorithm processing. Those data provide an opportunity for detailed mapping of seismic rupture and also provide an important reference for fieldwork. Based on high-resolution topographic data, we carried out detailed surface rupture mapping, classification, geometric structure and strike analysis for the ~30km section of the epicenter segment. At the same time, we conducted field work to supplement and proofread the maps.
According to the characteristics of surface ruptures in the epicenter area, we divided the ruptures into six segments. The surface ruptures along segment S₁ and segment S₆ are concentrated near the main fault, while the surface ruptures in the stepover(segment S₃, S₄, and S₅)are distributed dispersively, and the secondary ruptures along the segment S₂ are also distributed scatteredly, with the main rupture missing. To reveal the distribution characteristics of surface fractures more clearly, the surface ruptures are divided into the main rupture, secondary rupture, surface rupture and collapse rupture, among which the genesis of the surface rupture is uncertain. There are a lot of typical tensile ruptures with left-lateral component in segment S₁, the strike of the ruptures is consistent with the strike of the main fault or intersects the main fault with a small angle. The maximum width of the main rupture in segment S₁ is ~50m. The main ruptures in segment S₆ are developed along with the preexisting tectonic topography and the offset of the left-lateral displaced gully is up to tens of meters in magnitude. The surface ruptures are distributed in an echelon pattern, and all intersected with the strike of the main fault at a large angle. The location and size of the step-over are determined according to the topography and rupture morphology of faults in segment S₁ and segment S₆. The surface ruptures on the floodplain and banks of the Yellow River are in various forms and difficult to classify accurately. Therefore, only the typical seismic ruptures developed along the accumulated tectonic topography are labeled as main ruptures, and other typical seismic ruptures inconsistent with the location of the main fault are labeled as secondary ruptures. The typically collapse ruptures distributed along the river bank or lake bank are labeled as collapse ruptures, while the rest are labeled as surface ruptures. Surface ruptures in segment S₃ are distributed on the planar graph, but they have a dominant strike in the NE direction that can be seen from the diagram map. In the floodplain of the Yellow River, there are typical “grid” cracks, “explosive” cracks, and tensile cracks, and many cracks are accompanied by sand liquefaction which is beadlike, single, and distributed along the cracks. After the earthquake, the geodesic and geophysical data obtained quickly from the InSAR co-seismic deformation map and precise positioning of aftershocks revealed the basic morphological characteristics of earthquake rupture and provided valuable information such as earthquake rupture length, which provided an important reference for the design of UAV aerial photography and fieldwork. Compared with the rupture trace in field investigation by Pan et al.(2021), the surface rupture coverage obtained by mapping based on UAV aerial photogrammetry technology in this study is more extensive and accurate.
In general, surface ruptures of the Madoi earthquake are widely distributed, and we have classified those ruptures into the main seismic ruptures, secondary seismic ruptures, collapse cracks, and other surface ruptures. In addition to the seismic rupture with the same strike, there are also a variety of distributed surface ruptures with different strikes from the main fault. In these distributed surface ruptures, there are also many surface ruptures whose cause is not clear and they may be affected by tectonics or strong quake. For example, the “grid” and “explosive” surface ruptures on the Yellow River floodplain may be related to the strong quake near the epicenter or may also be related to the three-dimensional dynamic ruptures process in the initial stage. In this study, the characteristics of earthquake surface rupture in the step-over and adjacent sections near the epicenter has been described in detail, which provides a deeper understanding of the distributed coseismic surface rupture in the strike-slip fault.

COSEISMIC DISPLACEMENT MEASUREMENT AND DISTRIBUTED DEFORMATION CHARACTERIZATION: A CASE OF 2021 M_W7.4 MADOI EARTHQUAKE

SHAO Yan-xiu, LIU-ZENG Jing, GAO Yun-peng, WANG Wen-xin, YAO Wen-qian, HAN Long-fei, LIU Zhi-jun, ZOU Xiao-bo, WANG Yan, LI Yun-shuai, LIU Lu

2022, 44(2):  506-523.  DOI: 10.3969/j.issn.0253-4967.2022.02.014

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The coseismic displacements are required to characterize the earthquake rupture and provide basic data for exploring the faulting mechanism and assessing seismic risk in the future. Detailed field investigation is still an important way to acquire reliable coseismic displacements comparing to geodetic measurements. Combining with previous research on other earthquakes, this study tries to discuss distributed deformation along the strike rupture and its implications. The M_W7.4 Madoi earthquake ruptured the southeast section of the Kunlun Shankou-Jiangcuo Fault on May 22, 2021, in Qinghai Province. It is a typical strike slip event, and its epicenter locates at~70km south of the East Kunlun Fault, which is the north boundary of the Bayan Har block. Field investigation results show that the surface rupture extends along the piedmont. The deformation features mainly include compression humps, extensional and shear fissures, and scarps. After the earthquake, we used the unmanned aerial system to survey the rupture zone by capturing a swath of images along the strike. The swath is larger than 1km in width. Then we processed the aerial images by commercial software to build the orthoimage and the digital elevation model(DEM)with high resolutions of 3~5cm. We mapped the surface rupture in detail based on drone images and DEM along the western section. Meanwhile, we also got the commercial satellite images captured before the earthquake, on 2nd January 2021. The images were processed with geometrical rectification before comparison. The spatial resolution of satellite images before earthquake is about 0.5m.
At the south of the Eling Hu(Lake), the clear offset tire tracks provide an excellent marker for displacement measurement. We located the positions of tracks precisely based on remote sensing images, and compared between the tracks lines after earthquake and the corresponding positions before earthquake, then extracted distance difference, which is defined as coseismic displacements. The results show that the total displacement is about 3.6m, which contains the distributed deformation of about 0.9m. The off-fault deformation is about 33% of the on-fault and about 25% of the total deformation. The ratios are similar to previous studies on earthquake worldwide. The fault zone width is probable about 200m. The total horizontal displacement measured by this study is similar to the slip in depth by InSAR inversion, which implies that there is no slip deficit at the west rupture section of the earthquake.
The results also present the asymmetry of distributed deformation that most distributed deformation occurs at the south of the surface rupture zone. Comparing with other earthquakes in the world, it is likely that the asymmetrically distributed deformation is common in strike-slip earthquakes and the asymmetric feature is not related to the property of the material. The characteristics of distributed deformation might be related to fault geometry at depth or local stress state. More work is needed to resolve this question in the future. This study implies that we probably underestimated the slip rates resulting from ignoring distributed deformation in the past. In order to avoid underestimation of slip rates, we can correct the previous results by the ratio of distributed deformation to total slip. It is also suggested that the study sites should be on the segment with narrow deformation and simple geometry.

RAPID EXTRACTION OF FEATURES AND INDOOR RECON-STRUCTION OF 3D STRUCTURES OF MADOI M_W7.4 EARTHQUAKE SURFACE RUPTURES BASED ON PHOTOGRAMMETRY METHOD

WANG Wen-xin, SHAO Yan-xiu, YAO Wen-qian, LIU-ZENG Jing, HAN Long-fei, LIU Xiao-li, GAO Yun-peng, WANG Zi-jun, QIN Ke-xin, TU Hong-wei

2022, 44(2):  524-540.  DOI: 10.3969/j.issn.0253-4967.2022.02.015

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Exact characteristics of surface rupture zone are essential for exploring the mechanism of large earthquakes. Although the traditional field surface rupture investigation methods can obtain high-precision geomorphic data in a local area, it is difficult to rapidly get an extensive range of high-precision topographic and geomorphic data of the entire fault due to its limited measurement range and low efficiency. In addition, manual measurement is of tremendous workload, high cost, time-consuming and laborious, and the subjective differences in the judgment standards during the manual operation process may also cause the measurement results to be inconsistent with the actual terrain characteristics. In recent years, the development of photogrammetry technology has provided another more effective technical means for the rapid acquisition of high-precision topographic and geomorphic data, which has dramatically changed the way of geological investigation, improved the efficiency of fieldwork. At the same time, it also makes it a reality to reproduce the 3D tectonic features of field tectonic deformation indoors.
Structure from Motion(SfM)multi-view mobile photogrammetry technology is widely concerned for its convenience, fast and low-cost acquisition of high-resolution 3D topographic data in a working area of tens-kilometers scale. The emergence of this method has greatly improved the automation degree of photogrammetry. The technology obtains image sets by motion cameras, uses a feature matching algorithm to extract homonym features from multiple images(at least three images), determines the relative positional relationship of cameras during photography, and continuously optimizes by the nonlinear least square algorithm. Finally, the pose of cameras is automatically solved, and 3D scene structure is reconstructed. The technology can restore the original 3D appearance of the object in the computer by a set of digital images with a certain degree of overlap. In the applications of terrain mapping, this technology only needs to combine a small number of ground control points(GCPs)to quickly establish digital orthophoto maps(DOMs)and digital elevation models(DEMs)with high-precision. In this way, low altitude remote sensing platforms such as small and medium-sized UAVs have provided a foundation for SfM photogrammetry technology.
After the Madoi M_W7.4 earthquake occurred on May 22, 2021, our research team rushed to the site as soon as possible and conducted the rapid photogrammetry of the entire coseismic surface rupture zone in a short period with the use of the CW-15 VTOL fixed-wing UAV. We completed the collection of topographic data in an area with ~180km length and ~256km² area and collected 34302 aerial photographs. We used Agisoft PhotoScan TM software to process the images and generate DOMs quickly. The DOM resolution of the entire surface rupture was 2~7cm/pix, most of which were 3~5cm/pix. Then we used GIS software to vectorize the surface rupture. The centimeter-scale high-resolution DOMs could clearly display the coseismic surface rupture’s spatial distribution and the relative width. On this basis, the surface rupture could be accurately interpreted, and related parameters such as coseismic offsets could be extracted. In this study, the horizontal offsets measured by orthophoto images were basically consistent with the field measurement results, which proved the authenticity and reliability of the data obtained by the UAV photogrammetry method.
In order to obtain more detailed surface rupture vertical offset data, we used DJI Phantom 4 Pro V2.0 UAV to collect terrain information of several areas with the most significant rupture deformation. The DEM resolution obtained could reach centimeter-scale, and the accuracy was greatly improved. The high-resolution topographic and geomorphic data obtained by this method could accurately identify tiny fault features, clearly display sub-meter-level vertical offset features, significantly improve the accuracy of offset measurement, and achieve high-resolution 3D reconstruction of fault geomorphic.
In addition, we selected typical surface ruptures in the field, such as compressional stepovers, tensional cracks, and pressure ridges, and collected their 3D structural features using the iPhone 12 Pro LiDAR scanner. The 3D Scanner application was used to optimize the image, completely restore the “real object” in 3D to realize the indoor reconstruction of the 3D structure of surface ruptures and pressure ridges. The augmented reality(AR)imaging models could truly reflect the characteristics and details of surface ruptures, forming the same effect as field observations. This technology, which creates 3D models of close-range environments without any prior preparation, provides a novel, economical, and time-saving method to rapidly scan morphological features of small and medium-sized landforms(from centimeters to hundreds of meters)at high spatial resolution. This is the fastest and most convenient way to collect 3D models in field geological investigation without using external equipment, which provides a new idea for future geological teaching and scientific research.
Although photogrammetry technology still has some limitations, such as the short flight time of the flight platform, being easily affected by factors such as weather and altitude, and unsatisfactory aerial photography in densely vegetated areas, it is believed that these problems will be solved with the advancement of technology. Once solved, photogrammetry will become an essential technical means in quantitative and refined research on active tectonics.

DISCUSSION ON COSEISMIC SURFACE RUPTURE LENGTH OF THE 2021 M_W7.4 MADOI EARTHQUAKE, QINGHAI, CHINA

YAO Wen-qian, WANG Zi-jun, LIU-ZENG Jing, LIU Xiao-li, HAN Long-fei, SHAO Yan-xiu, WANG Wen-xin, XU Jing, QIN Ke-xin, GAO Yun-peng, WANG Yan, LI Jin-yang, ZENG Xian-yang

2022, 44(2):  541-559.  DOI: 10.3969/j.issn.0253-4967.2022.02.016

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Coseismic surface rupture length is one of the critical parameters for estimating the moment magnitude based on the empirical relationships and later used in assessing the potential seismic risk of a region. On 22 May 2021, the M_W7.4 Madoi earthquake occurred in the northeastern part of the Tibetan plateau(Madoi County in Qinghai Province, China)and ruptured the poorly known Jiangcuo Fault along the extension line of the southeastern branch of the Kunlun Fault. We began our data acquisition using aerial photogrammetry by UAV three days after the earthquake. Between 24 May and 15 June 2021, more than 40000 high-resolution low-altitude aerial photos were acquired covering a total length of 180km along the surface rupture. Based on detailed field investigations, combined with a fine interpretation of sUAV-derived orthophotos and high-resolution DEMs, we determined a total length of~158km of the coseismic surface rupture extending to the eastern end at 99.270°E, which is basically consistent with the position given by previous geophysical methods. Although the extending segment is located beyond the end of the continuous surface rupture trace near Xuema Township, it should be included in the calculation of the length of the surface rupture as part of the tectonic surface rupture. The surface rupture is segmented into four sections, named from west to east: the Eling Lake, Yematan, Yellow River, Jiangcuo branch sections. Additionally, to the east of Dongcaoa’long Lake, we mapped semi-circular arc-shaped continuous tension-shear fractures in the dune area with a short gap(~3km)connecting to the east of the Jiangcuo branch. The surface ruptures along the southeastern Youyunxiang segment also sporadically appear in several sites, locally relatively continuous, covered by the sand dune with vertical displacements of up to 30cm. After passing through the dunes, the surface rupture of the Youyunxiang segment began to spread widely, extending continuously with a strike of nearly east-west. However, it should be noted that the rupture lengths of the Youyunxiang segment and other branches are not counted in the total earthquake rupture length. By comparing the current research results, we believe that the critical factors causing the significant differences of the measured length of coseismic surface ruptures would depend on: 1)more extensive and detailed field investigations combined with a fine interpretation of high-resolution images; 2)avoidance of repeated calculation of superimposed sections on both sides of the complex geometrical area. In this study, combined with the fine interpretation of high-precision image data, many surface rupture traces in the dunes of the Youyunxiang segment were identified(verified and confirmed by field inspection)and more continuous surface rupture segments on the F1 fault, which is difficult to reach by human beings, were discovered, also highlights the important role of digital photogrammetry in the study of active tectonics. The studies of the strong historical earthquakes around the Bayan Har block show that the coseismic surface rupture length is larger than that estimated by the empirical relationships. Further research thus is highly necessary to uncover its mechanism and indicative significance.