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20 April 2023, Volume 45 Issue 2

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GRAIN SIZE AND MICROSTRUCTURE CHARACTERISTICS OF HOLOCENE MEGAFLOOD SLACK WATER DEPOSITS IN THE MIDDLE REACHES OF THE YARLUNG TSANGPO RIVER

XU Bo, WANG Ping, WANG Hui-ying, GUO Qiao-qiao, SHI Ling-fan, SHI Yu-xiang

2023, 45(2):  305-320.  DOI: 10.3969/j.issn.0253-4967.2023.02.001

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The terrain in southeastern Tibet is steep and the valleys are crisscrossed. Since the Quaternary, glacial ice and debris have blocked the course of the Yarlung Tsangpo River and its tributary river valleys to form giant dammed lakes, and the huge flood deposits formed by the dammed lake outburst floods are often associated with moraines, ice water deposits, lacustrine deposits, aeolian sand or other running water sediments to form complex river valley accumulation landforms. Different types of sediments in alpine and canyon areas are similar in morphology, structure and fabric, and are difficult to distinguish. Grain size and morphological characteristics are the most important structural characteristics of sediment, and the distribution rules are controlled by many factors such as sedimentary environment, physical properties of detrital material, transporting medium and transporting mode, etc., which is an important proxy index for restoring paleoclimate and inverting paleoenvironment. However, the relevant research on identifying sediment types in alpine valley area of southeast Tibet by grain size and morphology index is still in the exploratory stage. In order to understand the particle size characteristics and spatial differentiation laws of outburst flood sediments and the micromorphological characteristics of particle surfaces, we collected 33 samples of Holocene flood retention sediments preserved along the river within about 350km from the outlet of the Jiacha Gorge in the middle reaches of the Yarlung Tsangpo River to Pai Town, and measured them with Malvern 3000 laser diffraction particle size meter and Zeiss Signma scanning electron microscope, combined with digital geomorphology(DEM)data extracted river channel width and steepness coefficient. The features of spatial distribution law of particle size are analyzed, and the following understanding is obtained. The particle size of outburst flood retention deposits is characterized on the whole by fine-silty sand(2.57~5.18Φ)with poor sorting, positive skew and narrow peak state. Two end element models are obtained: The main peak of EM1 terminal element is 3.16Φ, with an average percentage content of 42.7%, which may represent the alluvial characteristics of higher energy of outburst floods in alpine valley areas, and the main peak of EM2 terminal elements is 2.06Φ with an average percentage content of 55.6%, which can be used to indicate the accumulation process of the outburst flood lag deposits. Affected by the width of the river, the EM1 content has a tendency to increase downstream, while EM2 has the opposite trend. The surface microstructure of quartz particles in the outburst flood lag deposits is mainly characterized by mechanical scratches, shell-like fractures, upturn cleavage and cleavage steps, with low structural maturity, mostly angular shape, and rare denudation pores of chemical origin. As a typical representative of climbing sand dunes in the valley area of the semi-humid monsoon area, the genesis of the dunes is of great guiding significance for revealing the source of sand dunes in the valley area of the alpine valley area, identifying paleoflood deposit and aeolian deposit, distinguishing aeolian deposit and paleoflood slackwater deposits on both sides of the riverbank, and windbreak and sand fixation engineering in the Yarlung Tsangpo River. By comparing the particle size and surface micromorphology characteristics of the known outburst flood deposits of the Yarlung Tsangpo River, we believe that the sand source of the Fozhang dunes is mainly from the outburst flood deposits and was transformed later by wind forces.

PALEOSEISMOLOGIC STUDY ON THE YUEXI FAULT IN THE MIDSECTION OF THE DALIANGSHAN FAULT ZONE SINCE THE LATE QUATERNARY

LIU Qing, LIU Shao, ZHANG Shi-min

2023, 45(2):  321-337.  DOI: 10.3969/j.issn.0253-4967.2023.02.002

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The Xianshuihe-Xiaojiang fault system(XXFS)is a strongly active left-lateral strike-slip fault zone on the eastern edge of the Qinghai-Tibetan plateau. It controls the eastern boundary of the Sichuan-Yunnan block, Which is one of the most active tectonic zones in the north-south seismic belts. There have been 36 destructive earthquakes since 1327AD. The historical strong earthquakes in the middle section of the XXFS fault system are mainly distributed along Anning River faults and Zemu River faults, such as M7.0 in 814AD, M7^1/2 in 1536AD, M6^3/4 in 1732AD, M7^1/2 in 1850AD and M6^3/4 earthquakes in 1952AD. However, as an important part of the middle of XXFS, the Daliangshan fault zone only recorded a magnitude of M5^1/2 in 1480AD, and there was a lack of earthquake records above a magnitude of 6 which may be due to the quiet period of earthquakes, or the location of remote mountainous areas where historical records are missing. The paleoseismic study revealed that there were surface rupture events along the Butuo and Jiaojihe faults in the southern section of the Daliangshan fault zone in 970-1510AD and 1310-1660AD respectively, with a magnitude of not less than 6.5; Along the Puxiong fault in the middle section of the Daliangshan fault zone, there was a surface rupture event in 927-1360AD, with a magnitude of not less than 7.0. However, there are no corresponding historical records of the earthquakes in these three historical periods, indicating that strong historic earthquakes in the Daliangshan fault zone may be missing.

The Yuexi fault is the only branch fault in the Daliangshan fault zone dominated by thrust slip. The fault spreads in an arc shape, with a total length of about 50km, and controls the quaternary basins such as Zhenxi, Xinmin, and Yuexi. The topographic height difference between the fault’s two sides is about 2 000m. The middle section of the fault is the eastern boundary fault of the Yuexi Basin, which cuts through the piedmont alluvial fan, forming fault scarps several meters to tens of meters high. Together with the Puxiong fault on the east side, which is dominated by left laterally slipping, a positive flower-type structure is formed in the middle section of the Daliangshan fault zone. There are previous discoveries about fault scarps of the Yuexi fault on the piedmont alluvial fans, but no paleoseismic research has been reported up to now.

On the basis of remote sensing interpretation and field geological and geomorphological survey of the Yuexi fault, a big trench was excavated across the 12m-high fault scarp on the late quaternary alluvial fan in the Yuexi Basin, which revealed four paleoseismic events since the late quaternary and the coseismic vertical slip of the last one is ~1.2m. Based on trench analysis, 14 stratigraphic units are defined from which carbon samples are acquired for geochronological analysis. Through radioactive carbon dating and correction of the dating data by the OxCal software, and OxCal model building to limit the age of paleoearthquake events, the ages of the four events were 25260-23880BC, 23930-23500BC, 20980-1400BC, and 270-1500AD. According to historical records, a destructive earthquake occurred in Yuexi County on September 13, 1480AD, which triggered landslides, 7 earthquakes on that day, and more than 20 aftershocks as of the 27th, with a tremor range of 150km. We consider that the latest event should be the Yuexi earthquake in 1480AD according to the historical records of earthquake damages. Based on the paleoearthquake research, this event very likely led to a coseismic rupture of the Yuexi and the Puxiong faults. According to the empirical scaling laws between magnitude and rupture length, the magnitude of the surface ruptured paleoearthquake is estimated to be more than M7.0. The results provide basic data for evaluating seismic activity and analyzing seismic risk in this area.

NEW ACTIVITY PHENOMENA REVEALED BY TRENCH ON THE NORTH SIDE OF NÜSHAN LAKE IN THE TANLU FAULT ZONE AND DISCUSSION

ZHAO Peng, LI Jun-hui, TAO Yue-chao, SHU Peng, FANG Zhen

2023, 45(2):  338-354.  DOI: 10.3969/j.issn.0253-4967.2023.02.003

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The Tan-Lu fault zone is a huge seismic-tectonic belt in the eastern China. It can be generally divided into three segments: the north, the middle, and the south segment. Among them, recent activity of the middle segment has been most thoroughly studied. The junction section between Jiangsu and Anhui Province is located in the transition zone between the middle and the south segment of the fault zone. Due to the complex tectonic structure, unevenly distributed Quaternary deposits and severely transformed surface landscape, it is difficult to study the recent Quaternary activity of the fault. Research in recent years have shown that the faults in the Fushan and Ziyang areas to the south of the Huaihe River were active during late Pleistocene-early Holocene, and their activities were characterized by thrusting, normal faulting, tension and twisting. How is the fault activity extending southwards to Nüshan Lake and whether the late Quaternary activity occurred at Nüshan Lake are issues worthy of attention.

Geomorphology of the study area is characterized by slope plains and uplands. The uplands mostly extend in near north-south direction and are obviously controlled by the faults. In the remote sensing satellite images, linearity features of the fault from Huaihe River to Nüshan Lake are distinct. Field investigations confirmed that in the farmland to the east of Liugudui Village, north of Nüshan Lake, there are scarps extending in NNE direction and distributing intermittently due to faulting. In this study, we chose relatively clear scarps and excavated trenches across the fault. The trench revealed abundant faulting phenomena. The trench wall revealed a fault deformation zone as wide as 2~4 meters, consisting of 3 fault branches. Among them, faults f₁ and f₃ are the boundary faults while fault f₂ is developed within the deformation zone. The latest activity of fault f₃ on the west side has ruptured the overlying horizon of late Pleistocene strata, and the rupture extended upwards to the surface. OSL dating samples were collected in the uppermost layer of the faulted horizons. Dating results show that the fault has been active at least in late Pleistocene. The scratches and steps developed on the fault plane indicate that the fault has experienced thrusting and dextral faulting. The deformation zone appears dark brown, which is conspicuously different from the horizons on both sides. Materials in the fault zone are compacted, crumpled and deformed, and the alignment direction is consistent with the fault. The deformation zone contains gravels and calcium tuberculosis of different sizes. Two brownish-yellow clay masses in irregular shape are deposited near the upper part of the fault plane. Among them, the clay mass tk1 on the south wall of the trench is quite clear, with the upper part connected with f₁ and the middle part obliquely cut by f₂. OSL dating samples were collected from clay masses from two trench walls. The dating results are consistent with the late Pleistocene horizons, indicating that the brownish-yellow clay masses were involved in the fault zone when faulting occurred in the middle-late Pleistocene, and the faulting event occurred roughly between(50.92±4.65)kaBP and(27.12±2.26)kaBP. Our research shows that late Quaternary activity of the most active fault of the eastern branch of the Tanlu fault zone extended southwards to Nüshan Lake in Mingguang, but intensity of the fault activity has weakened.

The segment from Sihong in Jiangsu Province to Mingguang in Anhui Province is the structural node between the middle segment and the southern segment of the Tanlu fault zone. Trench exposures in Wangqian, Sunpaifang, Dahongshan in Sihong and Santang, Ziyang, Zhuliu in Mingguang and other places revealed a variety of faulting phenomena such as wedges, wedge-shaped mass, normal faulting, negative flower-shaped structure, clay mass, etc. These show that faults that were dominantly thrusting led to the local and abundant phenomena near surface in this region. The reasons for these different phenomena may be related to the influence of regional complex stresses and their changes on large-scale fault systems at different time and spaces scales.

EVIDENCE OF LATE QUATERNARY TECTONIC ACTIVITY OF THE BEIDA SHAN FAULT, SOUTHERN MARGIN OF THE ALASHAN BLOCK

ZUO Yu-qi, YANG Hai-bo, YANG Xiao-ping, ZHAN Yan, LI An, SUN Xiang-yu, HU Zong-kai

2023, 45(2):  355-376.  DOI: 10.3969/j.issn.0253-4967.2023.02.004

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The southern Alashan block is located at the crustal front of the northern Tibetan plateau. It was initially considered as a relatively stable area with weak tectonic activity. In recent years, an increasing number of studies have shown that the Alashan block has undergone significant tectonic deformation since the Cenozoic. Multiple active faults with a horse-tail distribution are developed in the southern margin of the Alashan block. However, there is still controversy over the tectonic deformation patterns of these active faults. One view is that the fault system in the southern margin of Alashan is the result of the eastward extension of the Altyn Tagh Fault and belongs to the tail structure of the strike-slip fault. Another view is that the fault system in the southern Alashan block is the result of the revival of the pre-existing fault caused by the northward compression and thrust of the Tibetan plateau. Therefore, deciphering fault’s kinematics and slip rates since the late Quaternary in the southern Alashan block is crucial to understand the tectonic deformation pattern of the block and its response to Tibet’s northward growth. In this paper, combined with interpretations of remote sensing images and field investigations, we documented the Quaternary activity of the Beida Shan Fault, one of the major faults in the southern Alashan block, along the segment developed in Quaternary alluvium.

The Beida Shan Fault is a sinistral strike-slip fault with paralleled north and south branches that displaced the late Quaternary alluvial fans and terraces, forming offset gullies and fault scarps. According to the geometric distribution characteristics, activity and the landforms along the fault, we divided the fault into three segments: the Langwa Shan segment, the northern branch of the Jiapiquan Shan segment, and the southern branch of the Jiapiquan Shan segment. The fault is east-west trending, and the offset geomorphic features along the fault reveal that there are differences in the activity of different segments. The Langwa Shan segment is 10km long and developed at the junction of bedrock and alluvial fan. The fault trace is straight, and a series of gullies and ridges offset by the fault indicate that it is a sinistral strike-slip fault. The Jiapiquan Shan segment is 35km long and divided into two parallel north and south branches with a spacing of about 1.5km. The north branch fault strikes NE on the east side of Langwa Shan and has an angle of about 30° with the south branch fault. After extending about 2km to the northeast direction and entering the north side of Dahong Shan, the fault turns to the EW direction and is parallel to the south branch fault. It is distributed along the boundary between the bedrock and the alluvial fan with the south or north fault scarps and the secondary branch faults. To the east, the north branch fault is developed in bedrock, which is mainly characterized by offset gullies and ridges. The southern branch fault offset multi-stage alluvial fan, forming fault scarps of different heights and left-lateral offset gullies of different scales, and the exposed fault profiles show high angle reverse faults, which dip south or north, indicating that this segment is sinistral strike-slip.

Based on the 1.5m resolution DEM data obtained from UAV-SfM, we measured the horizontal displacement of fault landforms using the LaDiCaoZ software developed by Zielke et al.(2012) on the MATLAB platform. Combined with field survey data, we obtained the left-lateral horizontal displacements of 70 sites along the Beida Shan Fault. The sinistral offset of~1m is not included in slip distribution statistics due to limitations of the quantity and data accuracy. Statistical analysis of the displacements reveals that the left-lateral displacements along the fault are concentrated between 3m to 20m, with the majority in two pronounced peaks at 5.3m and 10.1m. The 5.3m peak contains the most data points, with 17 displacements data, accounting for 24% of the total, while the 10.1m peak contains 6 data points, accounting for 9% of the total. This indicates that the Beida Shan Fault has experienced multiple seismic events involving the displacement and rupture of stratigraphic layers on the surface.

An~8km-long surface rupture is discovered on the south fault branch, and it is represented by of fault scarps and of tens of centimeters 1~2m left-lateral displacement of small gullies. Fresh surface rupture and left-lateral offset gullies indicate the latest fault activity. Using the previously dated alluvial fan ages in Taohuala Shan, ~30km south of the Beida Shan, we calculated the late Pleistocene sinistral slip rate of 0.3~0.6mm/a along the Beida Shan Fault, which is consistent with the slip rate of the Taohuala Shan Fault estimated by Yu et al.(2017). Compared with the fault slip rate accommodated in the Hexi Corridor area and regional GPS rates, the southern Alashan block plays a significant role in absorbing deformation in response to the northern Tibetan growth.

Research paper

THE PRESENT CRUSTAL DEFORMATION CHARACTERISTICS OF THE HAIYUAN-LIUPANSHAN FAULT ZONE FROM INSAR AND GPS OBSERVATIONS

JIANG Feng-yun, JI Ling-yun, ZHU Liang-yu, LIU Chuan-jin

2023, 45(2):  377-400.  DOI: 10.3969/j.issn.0253-4967.2023.02.005

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The Haiyuan-Liupanshan tectonic belt is one of the most significant tectonic deformation areas in the northeastern Qinghai-Tibetan plateau with frequent strong earthquakes. It is an important opportunity to study the northeast extension of the Qinghai-Tibetan plateau and an ideal place to study the earthquake breeding process.

The published GPS observations show that the southwest side of the Haiyuan fault may still be undergoing deformation caused by the crustal viscoelastic relaxation effect of the 1920 Haiyuan M8.5 earthquake. And the publicly published leveling data results show local vertical deformation of the crust in the area west of the Liupanshan fault is significant. According to the seismic geological data, there exist historical earthquake rupture gaps in the middle and south sections of the Liupanshan fault and the southeast section of the Xiangshan-Tianjingshan fault in the Haiyuan-Liupanshan structural area, which have the background of strong earthquakes above M7.0. In view of the low spatial resolution of GPS and leveling observations, we need to use high-resolution crustal deformation fields to further study the crustal deformation characteristics of the above regions. Therefore, we further discuss the above issues in combination with InSAR observations.

The Sentinel-1A/B SAR data of two orbits covering the Haiyuan-Liupanshan fault from 2014 to 2020 were processed to obtain the current crustal deformation field in the line-of-sight direction. Furthermore, the high-density regional crustal deformation field was obtained by integrating InSAR and published GPS observations of the horizontal crustal movement velocity field on a time scale of 20 years. By comparing the observations of GPS, leveling and InSAR and high-resolution three-dimensional deformation integrated GPS-InSAR field, the characteristics of crustal deformation and strain field in the region are analyzed and discussed. The main conclusions are as follows:

(1)GPS and InSAR observations show that the post-seismic viscoelastic relaxation effect of the 1920 Haiyuan M8.5 earthquake may still be pronounced on the south side of the Haiyuan fault, but this conclusion is still speculative and needs to be confirmed by further observations;

(2)The high-resolution horizontal deformation field from GPS-InSAR shows that the decrease of the sinistral slip rate of the Haiyuan fault along the fault strike mainly occurs in the Middle East section. In contrast, the decrease of the middle and west sections is not significant, which may be related to the transformation of the left-lateral strike-slip to thrust nappe structure between the Haiyuan fault and the Liupanshan fault.

(3)GPS vertical and leveling observations both show that the vertical crustal deformation characteristics in the middle and south sections of the Liupanshan fault are similar to the vertical deformation of the Longmenshan fault before the Wenchuan earthquake. Considering the similar structural characteristics of the Liupanshan fault and the Longmenshan fault, and combining with the seismic and geological data, we believe that the Liupanshan fault may be in the relatively late stage of the earthquake breeding process. It can also be recognized by the high-resolution horizontal deformation and strain field derived from GPS-InSAR data. According to the fault motion parameters obtained in our study and the existing seismic and geological data, it is estimated that the maximum moment magnitude of an earthquake in the middle-south section of Liupanshan Mountain is approximately 7.5.

(4)The areas with rapid maximum strain accumulation in the study region are mainly concentrated in the vicinity of the Haiyuan fault and the left lateral shear zone between the Haiyuan fault and the Xiangshan-Tianjingshan fault. The dilatation strain rate west of the Liupanshan fault shows prominent compressive deformation characteristics corresponding to the nappe deformation in the Liupanshan tectonic area. The strain rate field in the southeast section of the Xiangshan-Tianjingshan fault is smaller than that of the surrounding area. There is a strain mismatch phenomenon, which may be related to the preparation for strong earthquakes. From the perspective of rotational deformation, the study area presents multiple deformation units, among which counterclockwise rotation corresponds to left-lateral strike-slip deformation(the left-lateral shear belt from the Haiyuan fault to the Xiangshan-Tianjingshan fault). In contrast, clockwise rotation corresponds to right-lateral strike-slip deformation(the right-lateral shear belt in the western margin of Ordos and Longxi block).

THE SURFACE RUPTURE CHARACTERISTICS BASED ON THE GF-7 IMAGES INTERPRETATION AND THE FIELD INVESTIGA-TION OF THE 2022 MENYUAN M_S6.9 EARTHQUAKE

WANG Liao, XIE Hong, YUAN Dao-yang, LI Zhi-min, XUE Shan-yu, SU Rui-huan, WEN Ya-meng, SU Qi

2023, 45(2):  401-421.  DOI: 10.3969/j.issn.0253-4967.2023.02.006

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On January 8th, 2022, an M_S6.9 earthquake occurred around Menyuan County(37.77°N, 101.26°E), Qinghai Province. The epicenter is located in the northeastern part of the Tibetan plateau, where the western section of the Lenglongling Fault meets the eastern section of the Tolaishan Fault. In order to know the spatial distribution of coseismic surface rupture zone as soon as possible, and determine the seismogenic structure, the post-earthquake GF-7 remote sensing images of the Menyuan M_S6.9 earthquake were analyzed. Moreover, combining the interpretation of the GF-7 images and the field investigation, the distribution of the co-seismic surface rupture was determined and the typical coseismic landforms, and the image recognition features of various co-seismic landforms are interpreted and summarized. The results show that the earthquake produced two major surface rupture zones with a left-stepped oblique spatial arrangement. The main northern branch rupture distributes on the west side of the Lenglongling Fault, with a length of about 22km and a strike of 100°N~120°E, the secondary rupture of the southern branch distributes along the eastern section of the Tuolaishan Fault, with a length of about 4km and a strike of N90°E. The total length of the two rupture zones is about 26km.

Along the rupture zones, a series of typical left-lateral strike-slip coseismic landforms were formed, such as tensional fractures, tensional-shear fractures, pressure ridges, pressure bulges, left-lateral strike-slip gullies, as well as left-lateral strike-slip roadbeds, etc. We divided the surface rupture into six segments to conduct detailed observation and analysis, that is, the west of Daohe segment, Liuhuanggou segment, Honggou segment, Yongan River segment and Yikeshugou segment, from west to east among the main rupture zone of the north branch, as well as the secondary rupture zone of the south branch. In general, each co-seismic landform has its distinctive image characteristics, and we obtained them from the interpretation and summarization of the GF-7 images. The shear fractures located at the two ends of the main rupture and in the areas where the surface rupture is weak are zigzaggy on the remote sensing images, while the shear fractures located in the areas where the surface rupture is intense are shown as dark, wide and continuously smooth stripes; thrust scarps are represented on remote sensing images as shaded, narrow and slightly curved strips; the pressure ridges and pressure bulges exhibit black elliptical feature on the images that are parallel or at a smaller angle to the main rupture; tensional-shear fractures are displayed as black strips arranged in en echelon with a 30°~45° intersection angle with the main shear rupture, and their linear features are not as straight as those of shear ruptures yet are still distinct; the coseismic scarps formed on the ice are manifested in the images as traction bend and texture change. Based on the GF-7 images, the cumulative dislocations of typical sinistral landforms along the co-seismic surface rupture on Lenglongling Fault are interpreted and futher compared with the previous study. This is the first time of application of GF-7 to the strong earthquake geohazards monitoring since it was officially launched in August 2020. From this study, it can be seen that with its high resolution, GF-7 can be used to accurately identify faulted features. Not only it could provide information of the geometric roughness, complexity and segmentation of the fracture, but also can record clear dislocations of the landforms. The study of the GF-7 images in the 2022 Menyuan earthquake has showed that the GF-7 images can provide strong data support for the geology and geological hazard studies.

Research paper

THE ANALYSIS AND IMPLEMENTATION OF THE AUTOMATIC EXTRACTING METHOD FOR ACTIVE THRUST FAULTS IN THE NORTH TIANSHAN MOUNTAINS BASED ON ARCGIS SOFTWARE PLATFORM

ZHANG Ling, MIAO Shu-qing, YANG Xiao-ping

2023, 45(2):  422-434.  DOI: 10.3969/j.issn.0253-4967.2023.02.007

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Digital topographic analysis, an important means in the research of active tectonics and tectonic geomorphology, has increasingly become one of the principal tools in the identification of active tectonic features and understanding of the development of the earth’s surface process. Indoor interpretation of surface fault trace plays a key role in the digital topographic analysis as it can provide the foundation for setting priorities and defining strategies in the subsequent field investigation. At present, the extraction of fault traces is often realized by assisting the traditional visual interpretation through the image enhancement method. The relevant subjective assessments lead to the amount of work and usually cause different results due to the differences in the interpretation experience of actual operators. At the same time, the field of quantitative research on geomorphic parameters is evolving very rapidly with the advances in the popularity of high-resolution digital topographic data. Therefore, intelligent and automatic extraction of surface fault traces has gradually become a promising research direction. The methods based on machine learning often rely heavily on the good programming foundation of the operator, which is a visible technical barrier. We present a semi-automated method using an ArcGIS toolbox with a set of tools to extract surface fault traces based on geomorphic constraints. The Hutubi and Dushanzi faults are two typical thrust faults located on the northern piedmont of the Tianshan Mountains and are chosen as examples. Excellent exposure of the surface fault traces in these two regions permits detailed mapping of fault traces and deriving shape factors of faults with high-resolution DEMs(digital elevation models). Additionally, they are two of the most-studied thrust faults in this area. Large-scale geological and geomorphological mappings of them and numerous achievements have been published. This creates possibilities for us to conduct comparison analysis on different major methods. Based on typical morphology characteristics of fault scarps, appropriate geomorphic parameters are selected. In practice, reverse fault scarps are distinctly defined into forward and backward ones according to whether their dip is the same as that of the neighboring geomorphic surfaces. Based on two sets of geomorphic constraints,two approaches are then illustrated, including slope calculation, gully extraction, data density analysis and process modeling. Through a detailed comparison of the final extraction results and previous visual interpretations of remote sensing data and field geomorphic investigations, the validity of the method proposed in this study is proven. This method provides a set of tools with user-friendly interfaces to realize step-by-step interpretation and emphasizes the importance of field-based geomorphic constraints at the same time. Moreover, many subtle fault traces which have not been recognized before are simultaneously revealed in the Dushanzi research area. The high-resolution DEMs guarantee the realization of picking out finer bits of fault information. Compared to traditional ways of working, the method has the advantage of automatically delineating reverse fault traces on the earth’s surface. This advantage can significantly reduce the efforts to manually digitize geomorphic features and improve efficiency. But many basic manual adjustment options for recognizing target characteristics also need to be set in extraction, because the distinguishing criterion of fault scarp and surrounding geomorphic landforms vary among different areas. In different specific circumstances, users can manually adjust relevant parameters for the extraction during the modeling process. Generally speaking, the more detailed constraints, the more confidence in the final delineation of fault traces. Subjective judgments are therefore particularly critical for conducting extraction under complex backgrounds. But improving the degree of automation of the whole process is still an important study direction. Future work is thus recommended to employ machine learning and explore appropriate evaluation methods to search for the optimal solution of intermediate parameters.

PROBABILISTIC SEISMIC HAZARD ANALYSIS ALGORITHM INTEGRATING THREE-DIMENSIONAL FAULT SOURCES AND POTENTIAL SEISMIC SOURCE ZONE USING RANDOM SAMPLING

CHEN Kun, GAO Meng-tan, YU Yan-xiang, XU Wei-jin, DU Yi, LI Xue-jin, LU Dong-hua

2023, 45(2):  435-454.  DOI: 10.3969/j.issn.0253-4967.2023.02.008

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Using the Monte Carlo random sampling method, a set of probabilistic seismic hazard analysis calculation programs that integrates our country’s traditional planar potential seismic source zone and three-dimensional fault sources is developed. The program is not only suitable for our country’s traditional regional area sources, but also considers the rupture scale of earthquakes and is compatible with the probabilistic seismic hazard calculation of three-dimensional fault sources. The algorithm developed in this paper efficiently realizes the three-dimensional simulation of the seismic event set of the fault source and introduces the earthquake rupture scale into the probabilistic seismic hazard analysis calculation in China, which significantly improves the rationality of the seismic hazard calculation in the near-fault area. In order to improve the execution efficiency of the program, the algorithm adopts the method of filling grid points in the planar potential seismic source zone in advance and randomly simulating the uniform distribution of seismic events in the planar potential seismic source zone. For the seismic hazard calculation of elliptical attenuation relationship, the algorithm uses pre-constructed three-dimensional matrices of the distance of the ellipse minor axis under different magnitudes, distances, and different angles between sites and the ellipse long axis direction of potential seismic source zone, and directly obtains the corresponding distance of ellipse minor axis through table look-up and interpolation. The algorithm developed in this paper avoids the problem of low computational efficiency in the iterative approximation of the distance of the ellipse minor axis. The mathematical expression of the three-dimensional fault source is based on the Frankel fault plane form of the 2002 edition of the National Seismic Hazard Map of the United States. The surface track and average dip Angle of the fault are used to create the rectangular fault plane, in which the dip direction of each rectangle is always perpendicular to the strike of its local fault segment. To maintain the coordination between the rupture area and the magnitude, the rupture of the earthquake occurring on the fault plane should not exceed the fault plane or the combination of fault planes. If the boundary of the rupture plane is outside the fault boundary, the entire rupture plane will move so that the boundary of the entire rupture plane matches the boundary of the fault plane. Using the probabilistic seismic hazard program of the Seismic ground motion parameters zonation map of China(2015)and the algorithm developed in this paper, the regional seismic hazard of the study area including Changsha-Zhuzhou-Xiangtan of the urban agglomeration in Hunan Province with moderate to strong seismic activity are calculated. Seismic hazard at different probability levels(return periods of 50.8, 475 and 2 475 years, respectively)for the Changde near-fault sources and Zhuzhou sites are also computed. The comparative study shows that the procedure of the Seismic ground motion parameters zonation map of China(2015)underestimates the seismic hazard near the three-dimensional fault source, and the degree of underestimation becomes more significant as the probability level decreases. Considering the influence of the earthquake rupture scale at the low exceedance probability level, the decomposition results of the seismic hazard for sites near fault show that the contribution of the seismic hazard is different from that of the traditional method of the Seismic ground motion parameters zonation map of China(2015), which mainly focuses on the earthquake of high magnitude. However, earthquakes of all magnitudes on the fault source can contribute to the seismic hazard, but the proportion of high magnitudes is the largest. Finally, an example verifying the probabilistic seismic hazard program(data set 1 case 10)from the Pacific Earthquake Engineering Research Center(PEER)is used to verify the reliability of the algorithm developed in this paper.

RESEARCH ON COMPREHENSIVE STANDARDIZATION FOR SURVEYING AND PROSPECTING OF ACTIVE FAULT

LI Yi-shi

2023, 45(2):  455-463.  DOI: 10.3969/j.issn.0253-4967.2023.02.009

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Active fault surveying and prospecting is the fundamental work for earthquake prevention and disaster reduction. China began to conduct quantitative research on active faults in the 1980s, and then carried out surveying and prospecting of active faults and seismic hazards assessment in several cities. The results provide a scientific basis for urban land planning, urban disaster prevention planning, construction project site selection and fault setbacks, potential seismic hazards investigation, earthquake emergency preparedness, etc.

Standards research in surveying and prospecting of active faults began at the beginning of this century in pace with the development of professional work. Since 2013, the research on the technical system and standards system about surveying and prospecting of active faults was carried out, and a series of standards for technical methods and outcomes were compiled successively. Currently, 1 national standard and 9 sectors standards have been released, and 11 standards are in processing. The national standard GB/T 36072 “Surveying and Prospecting of Active Fault” stipulates the process, content, outcomes, and main technical methods. The 9 sectors standards cover techniques and methods consisting of remote sensing survey, fault geomorphological survey, paleo-seismic trenching, drilling, and fault strip mapping, and stipulate the requirements for the steps, technical indicators, and outcomes of the corresponding technical methods. These standards have become important technical support for active fault survey and prospecting and the main basis for operational supervision.

However, there are still many gaps in the standards, and there are obvious contradictions between the supply and demand of the standards. At the same time, the compiling of standards for surveying and prospecting of active faults scattered in different periods and institutions, leading to the problems of function matching and technical indicators coordination among standards. This paper applies comprehensive standardization to surveying and prospecting of active faults, with the objectives to improve the work quality and the application benefit, by regarding the standardization object as a complete system, decomposing comprehensively the relevant elements in three aspects: business process, outcomes and application, and constructing the standard-complex of surveying and prospecting active faults. This is the first attempt to apply comprehensive standardization to the earthquake industry.

The working process of surveying and prospecting of active faults can be decomposed into six steps: preparation and revision of implementation plan, determination of fault spatial distribution and parameters, identification of fault activity, analysis of the deep seismic-tectonic environment, assessment of seismic hazards of active faults, and determination of fault deformation zone width. The preparation and revision of the implementation plan comprise data collection, controlled detection, preliminary identification of fault activity, and revision of the implementation plan; the determination of fault spatial distribution and parameters include the implementation and on-site investigation of technical methods such as high-resolution remote sensing interpretation, geological and geomorphic investigation, fault geomorphological survey, geophysical exploration, drilling, paleo-seismic trenching, and dating. The relevant elements of the business process mainly include the work content, technical methods, and technical requirements for project implementation of these links, as well as the technical requirements for project implementation plan preparation and outcomes check and acceptance.

The outcomes of surveying and prospecting active faults are divided into survey data, professional outcomes maps, reports, databases, etc. The relevant elements of the outcomes mainly include the technical requirements of the original data and the phased outcomes obtained from the analysis, professional outcomes maps, reports, and databases.

The application of surveying and prospecting of active faults is oriented to meet the needs of disaster reduction, and its outcomes are applied to the practice of earthquake prevention and disaster reduction. Relevant elements of application mainly include technical requirements for fault classification and fault cataloging, three-dimensional modeling, hazard assessment, fault avoidance, data management, and information service system construction.

Based on the analysis of relevant elements of business process, outcomes, and application, combined with the current status of existing standards, the framework structure of five sequences on surveying and prospecting of active faults standard-complex is put forward, namely, business foundation, project implementation, technical method, outcomes, and application, together with a detailed list of 41 standards. Among them there are 8 items of business foundation, 3 items of project implementation, 15 items of technology and methods, 10 items of outcomes, and 5 items of application.

The standard-complex of surveying and prospecting of active faults covers the standards required by the entire business chain, and the standards are interconnected and coordinated. Taking the advantage of the complete set of standards will lay a good foundation for further improving the standardization level of surveying and prospecting of active faults and accelerating the progress of developing standards, and also provide a beneficial demonstration for the high-quality innovative and standardization development of other business areas of earthquake prevention and disaster reduction.

SHALLOW STRUCTURE AND QUATERNARY ACTIVITY OF THE TAOCHUAN-HUXIAN FAULT, THE SUB-STRAND OF THE NORTHERN QINLING FAULT ZONE

YANG Chen-yi, LI Xiao-ni, FENG Xi-jie, HUANG Yin-di, PEI Gen-di

2023, 45(2):  464-483.  DOI: 10.3969/j.issn.0253-4967.2023.02.010

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The northern Qinling fault zone is an important active structure in the southern margin of the Weihe Graben Basin, containing many branch faults, of which the near EW striking Taochuan-Huxian Fault is located on the northern side of the fault zone, and the eastern segment is buried in the Weihe Graben Basin. Shallow seismic exploration has been carried out on the middle part of the buried segment of this fault, and the fault inferred to be a late Pleistocene fault with normal strike-slip movement, but the age and rate of the latest activity have not been determined. By conducting new shallow seismic and drilling joint exploration, we further study the shallow structure, the geometric distribution, the latest activity era and the slip rate in the Quaternary in the two segments of the Taochuan-Huxian Fault. The profile of shallow seismic exploration line TB1 reveals that the west segment of the Taochuan-Huxian Fault with NEE trend can extend at least 20km westward from Taochuan Town. The main fault plane dips to N, and the normal-slip movement has faulted the Quaternary bottom boundary and the underlying crystalline basement in the Taibai Basin. The vertical offset of the Quaternary bottom boundary is about 300m, and the remnants of the old thrust structure are still preserved in the fault zone. The shallow seismic reflection lines ZZ1 and YX1-2 reveal the location of the eastern Taochuan-Huxian Fault with the EW striking buried in the Quaternary of the Weihe Graben Basin in Zhouzhi and Huxian. The main fault plane dips to N, and the fault zone is represented by a fault depression zone of about 6km wide and a stepped structure of about 4km wide respectively. The fault up-breakpoints on both profiles offset the bottom boundary of the Holocene in the Weihe Graben Basin. The drilling joint profile exploration applied at Tanjiazhai in Zhouzhi County and Xiashimasi in Meixian County show that the Taochuan-Huxian Fault is distributed in the junction of the southern Weihe Granben Basin and the Qinling Mountains, where the Holocene marker layer S₀ has been vertically offset by 4~5m, yielding an average vertical slip rate of 0.4~1.3mm/a. Combined with the results of shallow seismic surveys, it is well demonstrated that the eastern segment of the Taochuan-Huxian Fault(buried in the Weihe Graben Basin)shows Holocene activity, and it is significantly more active than the western segment(the Taibai Basin segment). This may be due to the fact that the eastern segment has been incorporated into the Weihe Graben Basin and has become part of the primary active tectonic zone on the block boundary, while the western segment has not been incorporated. Spatially, the eastern segment of the Taochuan-Huixian Fault is subparallel to the middle-eastern segment of the North Qinling Fault, which is capable of generating strong earthquakes of magnitude 7 or higher. As an important branch of the North Qinling Fault, the Taochuan-Huixian Fault may also be under the same strong seismic background. These two faults probably jointly control the important active boundary of the southern margin of the Weihe Graben Basin. Future research in seismology and geology of these two faults should be strengthened, including their interrelationships at depth, their roles in vertical and horizontal movement distribution, and their seismogenic capacity and potential seismic hazard. In particular, the activity of the Taochuan-Huoxian Fault since the late Quaternary has only recently received attention, and the level of seismo-geological research on the fault is generally low. In this paper, we conducted preliminary studies on the location, shallow tectonic structure, activity segmentation, latest activity and Holocene vertical slip rate of this fault. Future research on the seismogenic structure of the Taochuan-Huoxian Fault needs to be strengthened in order to deepen and improve the understanding of the fault activity and to provide a basis for analyzing the seismic hazard of this fault.

SHALLOW STRUCTURE AND LATE QUATERNARY ACTIVITIES OF BRANCH FAULTS ON THE NORTHERN SIDE OF THE WEINAN TABLELAND IN THE SOUTHEASTERN MARGIN OF THE WEIHE BASIN

LI Xiao-ni, YANG Chen-yi, LI Gao-yang, FENG Xi-jie, HUANG Yin-di, LI Chen-xia, LI Miao, PEI Gen-di, WANG Wan-he

2023, 45(2):  484-499.  DOI: 10.3969/j.issn.0253-4967.2023.02.011

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The Weinan Tableland Piedmont fault is an important near-EW-trending Holocene active fault in the southeastern margin of the Weihe Basin, which is closely related to the occurrence of the 1556 Huaxian M8 earthquake. The northern branch of the fault, the northern branch fault in front of the Weinan tableland, passes through the urban area of Weinan. Therefore, finding out the distribution, shallow structure, late Quaternary activity, and seismic capacities of the northern branch fault are of great significance for local earthquake prevention and reduction. The Weihua fault zone, which is composed of F₁ and F₂ faults, generally strikes near east-west and has a gentle wave shape on the plane. It is a group of active normal faults rising in the south and descending in the north belt one. The Wei-Hua fault zone can be divided into two segments, east and west, and according to its spatial location and geometric distribution, strike change and the difference in geology and landforms on both sides. The eastern section is distributed in front of Huashan Mountain and is called Huashan Piedmont Fault(F₂); the western section is distributed in Piedmont of Weinan tableland and is called Weinan Piedmont Fault(F₁). There is a large sub-parallel branch fault about 2km to the north of the Piedmont Weinan tableland fault(F₁)in the west section, which is called the branch fault on the north side of the Piedmont Weinan tableland. It is also the boundary fault between the Weinan tabland and the Gushi Sag. The Weinan tableland Piedmont Fault(F₁)starts from the Weinan Xihekou in the west and extends eastwards through the Fenghe River to Mayukou, Huaxian County, with a length of about 54km; it strikes NWW from the Mayukou to Chishui River, and nearly EW from the Chishui River to the Fenghe River, the west of the Minhe River is NE to NEE, and it is mostly distributed in the form of broken lines or oblique rows. The fault plane dips northward with a dip angle of 60°~70°. The latest activity of the fault is manifested in the latest terraces and alluvial-pluvial fans faulting the Holocene strata, river valleys, and gullies; along the main fault, and a series of stepped normal faults on the north and south sides, a Holocene steep ridge belt with a width of between tens of meters and hundreds of meters, the Holocene strata are vertically faulted by 6~7m, and the vertical slip rate since the Late Pleistocene is about 0.29mm/a. In this paper, the shallow location and structural characteristics of the branch fault on the north side of the front of the Weinan tableland are determined through the combined profile detection of shallow seismic exploration and drilling, and evidence of the new activity of the fault is provided. The shallow seismic exploration results of the four survey lines all reveal the existence of a branch fault on the northern side of the front of the Weinan tableland, as well as the distribution location and cross-sectional structural characteristics of the fault new understanding. The results show that the branch fault on the north side of the Weinan Tableland Piedmont fault is a parallel branch of the main fault in front of the Weinan tabland. The branch fault on the north side of the front of the Weinan tableland is located at the front edge of the second-level terrace of the Weihe River in front of the Weinan tableland. The south end of the road, the mouth of the river, Zhangbaozi, and the outside of the north gate, have a length of at least 22km. The main section of the fault is inclined to the north, with a dip angle of about 70°~80° and a break distance of 6~20m at the upper breaking point, so it is a normal fault. Mainly concealed active faults, which have at least faulted the strata from the Middle Pleistocene to the late Pleistocene in the upward direction. In the four seismic sections, it appears as a normal fault zone with a width of 200~1 800m, including the main and secondary normal faults. Stepped structures and small grabens; secondary faults also fault up at least the Late Pleistocene strata. The combined geological profile of the Chongye Road borehole revealed that the main fault on the north side of the Weinan tableland had been faulted with many landmark strata of the Late Quaternary, and the latest fault occurred after 19ka; the average vertical activity rate since the middle of the Late Pleistocene between 0.07~0.26mm/a. Combined with phenomena such as fault ridges developed along the surface of the fault, it is judged that the fault was active in the Holocene. The branch fault on the north side of the front of the Weinan tableland has had strong activity since the late Quaternary, which means that the fault, as one of the branches of the southeastern boundary zone of the Weihe fault basin-the Weihua fault zone-obviously bears part of the deformation of the belt At the same time, the fault is located in the historically strong earthquake-prone area of the southeastern boundary of the Weihe fault basin, and it cannot be ruled out that it once participated in the rupture of the 1556 Huaxian M8 earthquake. Considering that the branch fault on the north side of the Weinan tableland passes through the urban area of Weinan, its potential seismic hazard and hazard are urgent research topics.

THE RESEARCH ON RELOCATION AND FAULT PLANE SOLUTION AND GEOMETRIC MEANING OF THE MADUO M7.4 EARTHQUAKE ON 22 MAY 2021

LIU Bai-yun, ZHAO Li, LIU Yun-yun, WANG Wen-cai, ZHANG Wei-dong

2023, 45(2):  500-516.  DOI: 10.3969/j.issn.0253-4967.2023.02.012

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At 2:04 on May 22, 2021, an earthquake of M7.4 occurred in Maduo County, Golog Prefecture, Qinghai Province, with the focal depth of 17 kilometers, the epicenter at 34.59°N and 98.34°E. This earthquake was the largest after the Wenchuan earthquake in China. The epicenter of the earthquake is 38km away from Maduo county seat and 385km from Xining, the provincial capital. The earthquake caused some houses to collapse and some damage to roads in the epicenter. But due to the sparse population in the epicenter area, the earthquake did not cause casualties.

Seismologist believe that the earthquake is the result of the continuous activity of the boundary fault of the Bayankala block, which is geographically located in the north of the Qinghai-Tibet Plateau and is the hub for the transformation of the direction of the crustal movement of the plateau. In recent years, many destructive earthquakes occurred inside the block. This earthquake is another strong earthquake after the M7.1 Yushu earthquake in Qinghai in 2010. According to the analysis of this earthquake briefing, the fault zone that induced this earthquake is speculated to be the Maduo-Gande fault zone or the Kunlun Mountains Pass-Jiangcuo fault zone.

In order to find out which fault is the seismogenic structure and the distribution of the seismogenic structure of this earthquake, we relocated the dense earthquakes by double-difference method based on the data of 1357 aftershocks in the Maduo M7.4 earthquake area recorded by 72 fixed stations of the digital seismic network of Gansu and its adjacent seismic network and 12 portable seismographic stations during the May 22 to May 27, and obtained the source parameters for 1289 earthquakes. The accurately located small earthquakes distribute along both sides of the Kunlun Mountains Pass-Jiangcuo Fault, which is NNW-trending obviously. It shows that the seismogenic structure of this earthquake is the Kunlun Mountains Pass-Jiangcuo Fault, rather than the Maduo Gande Fault as considered previously by some scholars. This is consistent with the research results of surface fracture zone, magnetotelluric detection, InSAR coseismic deformation and relocation of other aftershocks. Most earthquakes distribute at the depth range of 0~15km of the crust after the relocation, and the result shows that the focal depths are more concentrated. The relocation also shows that the east and west ends of the main fault have bifurcations. It may be that the complex stress distribution triggered two new branch faults during the occurrence of the great earthquake, and the overall fault shows a “tree-type” structure. The west branch trends 306°and intersects the main fault at 21°. The east branch is nearly EW trending and connected with the east section of the main fault.

Generally, the earthquakes are closely related to active tectonics, large earthquakes and its aftershocks usually occur on fault zones with obvious activity. The distribution of small earthquakes is related to the complex underground stress state and the complex structure of the fault zone. We can inverse the shapes and positions of the fault planes using spatial distribution of hypocenters of mainshock and the corresponding aftershocks, according to the principle that clustered earthquakes occur near the faults. Six rectangular regions are selected according to the distribution characteristics of relocated aftershocks and by reference to the distribution of geological faults and earthquake rupture zones. We obtained the detailed parameters of fault plane in each region by using the simulated annealing algorithm and the Gauss-Newton algorithm according to the source information after the relocation in 6 rectangular areas. On this condition, rake angle of the fault plane is further inferred from regional tectonic stress parameters. The results show that the main fault is a large, high dip angle, sinistral strike-slip fault with thrust component, striking 285°~290° and about 146km long. It extends from Tanggema Township of Maduo in the southeast(34.49°N, 98.91°E)to Gazejialong Township in the northwest(34.81°N, 97.54°E). The movement characteristics of the newly generated western segment 2 show dextral strike slip and thrust, which is diametrically opposite to that of the main fault. This shows the complexity of the earthquake rupture process, and further research is needed on the tectonic mechanics and deep structures that produce this special rupture.

Compared with the focal mechanism solutions obtained by domestic and foreign authorities, the fault plane parameters obtained in this paper are similar to them, indicating that our conclusions are reliable. Besides, the spatial distribution of inverted fault plane is basically identical to that of the rupture zone derived from post-earthquake investigation in the earthquake area.

THE 3-D SHALLOW VELOCITY STRUCTURE OF THE MIDDLE-NORTH SECTION OF THE DONGPU DEPRESSION DERIVED FROM DENSE ARRAY OBSERVATIONS OF AMBIENT NOISE

ZHOU Ming, DUAN Yong-hong, TAN Yu-juan, QIU Yong

2023, 45(2):  517-535.  DOI: 10.3969/j.issn.0253-4967.2023.02.013

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Dongpu depression is located at the junction of Henan and Shandong in the south of Bohai Bay Basin in eastern China. It is an early Tertiary faulted basin with NNE strike, with thick sedimentation. It is adjacent to Luxi uplift in the East and Luxi uplift in the West. There are mainly three major faults in the area: Lanliao fault, Changyuan fault, and Yellow River fault. Lanliao fault is a major fault that controls the boundary between the Dongpu depression and the Luxi uplift. Changyuan fault is the boundary between the Dongpu depression and the Neihuang uplift. Yellow River fault is a secondary fault in the Dongpu depression. Dongpu depression controlled by these three fault zones has formed a structural form of “two depressions and one uplift”. To understand better the distribution of faults and velocity structure in the Middle-North Section of the Dongpu depression, from March 26 to April 22, 2018, the Geophysical Exploration Center, China Earthquake Administration set up a short-period dense seismic array consisting of 412 short-period seismometers in the middle-north section of the Dongpu depression, the Luxi Uplift the Neihuang Uplift. The array range is about 50km×45km, the station spacing is 1.3~2.5km, and the station spacing around the array is 4.5km. In the array, there is also a linear array with a length of about 50km, with a station spacing is about 500m, and 98 stations, which are distributed near vertical fractures. Based on noise cross-correlation technology, cross-correlations of vertical component ambient noise data of different station pairs are computed in 1-day segments and stacked. Clear fundamental-mode Rayleigh waves are observed from 0.5s to 5s period. Then we use the direct surface wave tomographic method with period-dependent ray tracing and a wavelet-based sparsity constrained to invert phase dispersion travel-time data simultaneously for 3-D shear-wave velocity structure. The shear-wave velocity model results from 0.5km to 3.5km depths are consistent with the known geologic features and reveal strong shallow crustal heterogeneity. The results follows: 1)the velocity of the Middle-North Section of Dongpu depression in the study area is low, the velocity of the Neihuang uplift and Luxi uplift on both sides are high, and the shear velocity variation between uplift and depression continues to about 3.5km. 2)The boundary between high and low velocity coincides with the boundary of depression and uplift, and is also consistent with Lanliao Fault and Changyuan Fault, indicating that the caprock deposition in the Dongpu depression is controlled by the Lanliao fault and Changyuan fault. 3)The Cenozoic sedimentary structure of the Dongpu depression is mainly controlled by Lanliao fault. The 1~3.5km depression shows obvious low velocity characteristics, indicating that the Paleogene Lanliao fault activity has a strong impact on the sedimentary characteristics of the middle-north section of the Dongpu depression; the velocity difference between the depression and uplift of 0~1km decreases, the Neogene and quaternary Lanliao fault activities become weaker, and the sedimentary structures in this period are less affected by the Lanliao fault. Although the velocity of the Dongpu depression is generally low, the depression also shows some heterogeneity: the sedimentary structure of the northern section is not only controlled by the Lanliao fault, At the same time, it also received that the control of the secondary fault in the depression presents “W” shape, which disappears in the middle section, indicating that the Cenozoic sedimentary structure of Dongpu depression is mainly controlled by the Lanliao fault, and the Paleogene Lanliao fault activity has a strong impact, with obvious segmentation characteristics, resulting in the existence of multiple sedimentary centers in Dongpu depression, thus making the velocity structure in the Dongpu depression present non-uniformity. 4)The characteristics of the Lanliao fault in the middle-north section of the Dongpu depression are shown as an SEE trend, and the dip angle of the Lanliao fault in the north section is significantly steeper, indicating that there are differences in the activity characteristics of Lanliao fault in the study area. The Shijiazhuang-Mazhai-Liuta fault is a branch fault of the Changyuan fault extending northward, with a strike of NNE and a dip of E or SEE. From the velocity distribution feature image, it can be seen that it is significantly different in the north-central section of the Dongpu depression. From the velocity distribution image, it can be seen that it is significantly different in the north-central section of the Dongpu depression, with a gradual steep dip from south to north, and then gradually slowing down. This feature is consistent with the different structural characteristics of each branch fault of the Changyuan fault at a different section.

DEEP STRUCTURE IN THE MIDDLE AND SOUTH AREA OF NORTH CHINA DEPRESSION AND THE VICINITY BY MAGNETOTELLURICS

WANG Ming-liang, ZHANG Yang, XU Shun-qiang, XU Zhi-ping

2023, 45(2):  536-552.  DOI: 10.3969/j.issn.0253-4967.2023.02.014

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In order to study the deep electrical structure and its formation mechanism of different tectonic units in the central and southern part of the North China Depression, especially the southern North China Depression with NW-NNW trending tectonic lines and the northern North China Depression with NE-NNE trending tectonic lines, as well as the deep tectonic background of three destructive earthquakes in the study area, a 110km long magnetotelluric(MT)sounding profile across the main structural units in the study area was deployed to study the deep fine electrical structure in the central and southern part of the North China Depression by using high-density and broadband MT method with the support of the active fault exploration project in Kaifeng City. The profile is NS-trending as a whole, starting from Tongxu County of Kaifeng City in the south, passing through Xiangfu District of Kaifeng City, Fengqiu County and Changyuan County of Xinxiang City, and terminating at Banpodian Township of Huaxian County of Anyang City in the north, with a total length of 110km and an average point distance of about 3km. The observation points in the survey area of Kaifeng City are dense, with a point distance of 2km. From south to north, the whole profile crosses two first-order tectonic units of the southern North China Block and the northern North China Block and four second-order tectonic units of the Taikang Uplift, the Kaifeng Depression, the Dongpu Depression and the Neihuang Uplift. In MT data processing, in addition to the remote reference and robust techniques, the multi-point and multi-frequency tensor decomposition was employed to determine the regional electric strike, and the NLCG 2D inversion was performed on TE and TM data. And finally, the deep electrical structure is obtained.

The result shows that, with Xinxiang-Shangqiu Fault as the boundary, the deep electrical structure on its north is relatively simple than that on the south. The electrical structure of Neihuang Uplift and Dongpu Depression in the northern North China Depression is relatively simple, and its resistivity structure is characterized by vertical segmentation and divided into low resistivity and high resistivity zones corresponding to the crust of the area consisting of sedimentary cover and crystalline basement of hard block with good basement integrity. At the same time, the high resistivity zone is very thick, which could represent the unified crystallization basement in the North China Block region. The deep electrical structure of the tectonic units in the southern North China Depression on the south of Xinxiang-Shangqiu Fault is relatively complex, showing a three-layer structure of low-high-low resistivity in the vertical direction and alternating high and low resistivity in the horizontal direction. For example, the resistivity of the crust below the Taikang Uplift shows a low-high-low three-layer structure, that is, a low resistivity zone above the depth of 2km, a high resistivity zone at the depth between 2km and 15km, and a low resistivity zone below 15km. This may be related to the mutual subduction and collision between the Yangtze plate and the southern margin of the North China plate, while the northern North China Depression is less affected by the Yangtze plate and Qinling-Dabie orogeny due to the control of the boundary Xinxiang-Shangqiu Fault.

The two destructive earthquakes of 1342 and 1918 in Tongxu, Henan Province are located in the intersection area of high and low resistivity zones beneath the Taikang Uplift, and the Fengqiu earthquake of 1737 in Henan Province is located near the gradient zone of high and low resistivity in the crust.

Research paper

DETERMINATION OF GRAVITY VALUES FOR MULANSHAN CALIBRATION BASELINE AND ANALYSIS OF GRAVITY CHANGE

WANG Jian, ZHANG Xin-lin, TAN Hong-bo, HU Min-zhang, WU Gui-ju, LI Zhong-ya, ZHANG Ming-hui

2023, 45(2):  553-569.  DOI: 10.3969/j.issn.0253-4967.2023.02.015

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Using the gravity observation data of Mulanshan short gravity baseline field in 2018 and 2022, we established a high-precision short gravity baseline field of Mulanshan based on the relative gravity joint measurement method under the control of absolute gravity. We also analyzed and discussed the accurate calibration of the monomial coefficient of the relative gravimeter during the construction of the gravity short baseline field, the distribution of gravity values in the gravity baseline field of Mulanshan and the contribution of various environmental factors in the gravity variation results, these results show that:

(1)Maximum gravity segment difference of Mulanshan calibration baseline is 102.176mGal from G01 to G03 stations, and the average accuracy of gravity value of each measuring station reaches 4.8μGal. The geological structure of the Mulanshan baseline is stable, and the gravity change of measuring stations is not obvious. From 2018 to 2022, the gravity variation range of measuring stations was 5.9~12.8μGal, with an average of 9.5μGal, and the average uncertainty was ±5.7μGal. The gravity field mainly showed a positive change. The variation range of gravity in each measurement section is -4.8~6.9μGal, with an average of(1.8±8.6)μGal. The change of the surrounding environment has a certain impact on the gravity field, and the contribution of the new buildings near the G01 and G02 to the gravity change is 3.6μGal and -0.51μGal, respectively. These gravity changes of measuring stations in the IOS and Mulanshan baseline caused by vertical surface movement are(2.17±0.44)μGal and(1.67±0.45)μGal. The gravity effect caused by the change of surface water storage is(1.07±0.84)μGal, which cannot be ignored. Compared with observation results, the gravity change of each measuring station and section after correction is reduced, and the average gravity change values are reduced by 38.2% and 50.8%, respectively. The corrected gravity change results are more accurate. Due to the cumulative effect of errors in the correction process, the uncertainty of gravity change results after correction increases accordingly, and the uncertainty of gravity change results of measuring station and measuring section increases by 2.5% and 2.8%compared with observation results, respectively. Combined with the gravity change results of the measuring station and the measuring section, we can effectively extract abnormal information in gravity dynamic change results.

(2)There are differences in monomial coefficients of different gravity sections of the relative gravimeter. The results of CG-6 and CG-5 relative gravimeters are relatively consistent, and there is no systematic deviation between the two gravimeters. The difference in the monomial coefficient between the Wuhan-Yichang section(sub-section)and the Wuhan-Lücongpo section(total section)is 4.809‰, which has a great influence on the gravity observation results. The monomial coefficient needs to be accurately measured. The difference of the monomial coefficient in the sub-section is negatively correlated with the proportion of the gravity segment difference in the sub-section to the total section; the monomial coefficient of the total section is a weighted average result of each sub-section, and the proportion of gravity segment difference in sub-section to total section is the corresponding weight factor. Accurate calibration of the monomial coefficient of the relative gravimeter is a technical guarantee to obtaining high-precision gravity observation results. The gravity segment difference of sub-segments cannot cover the gravity range of the measurement area due to smaller segment difference, which will lead to the extrapolation of the monomial coefficient, so it cannot effectively calibrate the monomial coefficient of the relative gravimeter applicable to the whole measurement area. The total section can cover the gravity range of the measurement area, and the monomial coefficient is the ratio between the segment difference measured by the relative gravimeter and the known segment difference, and its calibration accuracy is inversely proportional to the gravity segment difference, so when using the total section as a reference for calibration of the monomial coefficient of the relative gravimeter, accuracy of the calibration can be guaranteed and precision of the calibration can be improved, so calibration result of the monomial coefficient using the total section is more accurate. The existing widely used relative gravimeters(such as LCR, CG-5, BURRIS, CG-6, and so on)have time-varying characteristics of the monomial coefficient, weakening the errors caused by changes of the monomial coefficient is essential to improve the accuracy of observations, and corresponding calibration is required before each period of gravity observation. The monomial coefficient of the relative gravimeters needs to be calibrated using a large segment difference, and the segment difference(or the accumulated segment difference)should be greater than 300mGal.

Review

APPLICATIONS AND ADVANCES FOR THE COSEISMIC DEFORMA-TION OBSERVATIONS, EARTHQUAKE EMERGENCY RESPONSE AND SEISMOGENIC STRUCTURE INVESTIGATION USING INSAR

ZHAO De-zheng, QU Chun-yan, ZHANG Gui-fang, GONG Wen-yu, SHAN Xin-jian, ZHU Chuan-hua, ZHANG Guo-hong, SONG Xiao-gang

2023, 45(2):  570-592.  DOI: 10.3969/j.issn.0253-4967.2023.02.016

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With the recent development of geodetic observation theory, the increasing satellite platforms and the progress of related technology, InSAR is emerging as a new data source and useful tool for remotely-based geodetic observations. More importantly, InSAR observations play an increasingly irreplaceable role in the field of coseismic deformation observations, earthquake emergency responses, earthquake hazard evaluation and seismogenic structure research. Particularly, InSAR is the most commonly used tool in coseismic deformation measurements on the Qinghai-Tibetan plateau or other global seismic zones, where GPS data are sparse or inaccessible in some cases. Specifically, InSAR measurements help us to respond in time after disastrous earthquakes and provide valuable information associated with how the surface of the crust deforms due to large earthquakes. In the area of scientific research, InSAR provides products of surface deformation observations and serves as model constraints kinematically or dynamically in identifying the buried faults, studying the characteristics of seismogenic faults, obtaining three-dimensional displacements, and investigating the relationship between earthquakes and tectonic structures. InSAR observations and its deformation products have the technical advantages of large spatial scale, high precision and in-time, compared to other geodetic measurements. Consequently, InSAR has the ability to provide scientific and technological support for earthquake emergency observations, and meeting the practical needs of earthquake disaster reduction on the Qinghai-Tibetan plateau.

In this review, we mostly limit our focus to the application of InSAR technology in earthquake cycle deformation monitoring in different structural settings on the Qinghai-Tibetan plateau. We also summarize the InSAR-based studies on fault kinematics and seismogenic structures related to some noted earthquakes on the Qinghai-Tibetan plateau. We highlight how the applications of InSAR data can greatly promote earthquake science and can be used as routine observations in some important areas. Then proceed to discuss the cutting-edge development trend and some new challenges of InSAR technology, which are frequently discussed and investigated, but not well resolved, in recent applications. The endeavors in increasing the precision of small-magnitude deformation measurements and expanding the InSAR data volumes can make the scientific objectives of earthquake disaster reduction on the Qinghai-Tibetan plateau and its surrounding areas feasible and reliable. To better understand how InSAR observations have changed the way we study earthquakes, we summarize the development, commercialization, insights, and existing challenges associated with InSAR coseismic deformation measurements and application in recent two decades.