Table of Content

20 February 2023, Volume 45 Issue 1

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A CENTENNIAL PUZZLE OF THE EVOLUTION OF THE YANGTZE RIVER: RETROSPECTION AND PROGRESSES

GUO Ru-jun, WEI Chuan-yi, LI Chang-an, ZHANG Yu-fen, LI Ya-wei, SUN Xi-lin, ZHANG Zeng-jie, LENG Yong-hui, SU Jian-chao, LI Guo-nai, LÜ Ling-yun, CHEN Xu, DING Zhi-qiang

2023, 45(1):  1-28.  DOI: 10.3969/j.issn.0253-4967.2023.01.001

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The evolution history of the great rivers is one of the most important subjects in earth science, especially, the capture events and changes of great rivers which originate from the inner area of the Qinghai-Tibetan plateau and flow into the ocean are hot problems for geomorphology and geology. The Yangtze River is a representative river link with the Qinghai-Tibetan plateau and the Pacific Ocean, formation of the Yangtze River is considered an important mark ofthe Chinese landscape formation and the establishment of the modern geomorphic pattern of the East Asia. The evolution of the Yangtze River is closely linked to the uplift of the Qinghai-Tibetan plateau and the birth of the margin seas and monsoon evolution. In this study, we concluded the main debates on the evolution of the Yangtze River for more than one century, and the progresses of provenance analysis applied to the continental and sea basins of the Yangtze River in the past two decades. We collected the provenance analysis results from typical sedimentary depositions in the Yangtze River catchment, including the Xigeda Formation in the Panzhihua-Xichang area of the upper reaches, Cenozoic sedimentary of the Jianchuan Basin which is near the First Bend of Shigu, Gravel Layers in the middle and lower reaches, borehole sediment of the Jianghan Basin and Yangtze River Delta, and sediment of the marginal sea basins(Yinggehai Basin, Taiwan Island). We conclude that: 1)the debates on the evolution of the Yangtze River are still focused on two questions: when the Three Gorges was formed and whether south flowed off the palaeo-Jinsha River in the First Bend of the Shigu, but the debates have extended to the palaeo-drainage model in East Asia during the Cenozoic period, geomorphic formation history and exhumation-deposition process of the SE Tibet, high elevation-low relief surface formation in the SE margin of the Tibet and many important issues. 2)There is no consensus regarding the formation time and process of the Three Gorges and the First Bend, the formation time, process, and mechanism of the Yangtze River are still vigorously debated. There are mainly two views on the Miocene and early-middle Pleistocene for the formation time of the Yangtze River and mainly three paleo models of the upper Yangtze, south flow, east flow, and southeast flow. The provenance of gravel layers in the middle and lower reaches of the Yangtze River and boreholes sediment in the Jianghan Basin have complex source regions. Because of the extreme stability and multiple recycle of the detrital zircons, it is difficult to distinguish the provenance signals of the upper reaches of the Yangtze River effectively from the modern and Cenozoic sediment in basins based on the detrital zircon U-Pb age, whether the “Yangtze Gravel at Nanjin” represents the age of the Yangtze River is still strongly debated. There is still no agreement on the initial signal of the sediment of the upper Yangtze River from the boreholes record in the Jianghan Basin and the Yangtze River Delta. The boreholes deposition age is also controversial. The provenance implications of the Cenozoic sediment of the Jianchuan Basin and the Xigeda Formation for the south flow(east flow)of the Jinsha River are widely debated. The marginal sea sediment provenance signals that constrain the evolution model between the Yangtze and the Red River are also controversial. 3)There is a big difference between the drainage catchment of the paleo-Yangtze and modern Yangtze, in the provenance analysis of the sedimentary basins of the Yangtze River, suggesting constrain provenance area by multi-mineral and multi-index and strengthen the comparison between the continental and marginal sea basins. The evolution history of the Yangtze River will be reconstructed more comprehensively from the perspective of geomorphology, tectonic evolution, sedimentary paleogeography and climate change.

EMPIRICAL QUANTITATIVE ANALYSIS OF STRENGTH AND SEISMOGENIC DEPTHS FOR THE BRITTLE-DUCTILE TRANSITION OF CONTINENTAL FAULT ZONE

LEI Hui-ru, ZHOU Yong-sheng

2023, 45(1):  29-48.  DOI: 10.3969/j.issn.0253-4967.2023.01.002

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The strength properties of fault rocks at shearing rates spanning the transition from crystal-plastic flow to frictional slip play a central role in determining the distribution of crustal stress, strain, and seismicity in a tectonically active region. Since the end of the 20^th century, many experimental and modelling works have been conducted to elucidate the variation of the strength profile and mechanism of brittle-ductile transition(BDT)with temperature, pressure, and sliding rate. We review the substantial progress made in understanding the physical mechanisms involved in lithospheric deformation and refining constitutive equations that describe these processes. The main conclusions obtained from this study are as follows:

(1)The mechanical data and microstructure of friction and creep experiments indicated the transition from brittle to plastic deformation with the increasing crust depth, which not only controls the ultimate strength of the crustal profile but also limits the lower limit of the seismogenic zone. Moreover, based on the variation of rock characteristics, temperature, normal stress and sliding rate, the brittle-ductile transition zone distributes at different depths in the crust. The strength profile consisting of friction law and flow law is widely used to describe the strength and seismicity of the continental crust. However, this profile model is oversimplified in the BDT zone because this area involves a broad region of semi-brittle behavior in which cataclastic and ductile processes occur. At the same time, the model also lacks characterization of the transient dynamic properties of faults. Rate-and-state friction(RSF)law stipulates that the occurrence of slip instabilities(i.e. earthquake)can be linked with the velocity dependence of friction. Therefore, the RSF equations, when applied to the kilometer-scale of fault zones, models incorporation RSF equations can reproduce several important seismological observations, including earthquake nucleation and rupture, earthquake afterslip, and aftershock duration. However, these key microphysical processes of fault gouge evolution are unknown to this model.

(2)During numerical model-fitting experimental observations, the Friction-to-flow constitutive law merges crustal strength profiles of the lithosphere and rate dependency fault models used for earthquake modelling on a unified basis, which is better than controlling the boundary of BDT using the Mohr-Coulomb criterion, Von Mises criterion and Goetze’s criterion. The Friction-to-flow constitutive law can predict the steady-state and transient behavior of the fault, including the response of shear stress, sliding rate, normal stress, and temperature, in addition to simulating the transition of fault sliding stability from velocity-weakening to velocity-strengthening. It also solved seismic cycles of a fault across the lithosphere with the law using a 2-D spectral boundary integral equation method, revealing dynamic rupture extending into the aseismic zone and rich evolution of interseismic creep, including slow slip before earthquakes. However, these constitutive models do not base on microphysical behavior. Furthermore, at low to intermediate temperatures, the ductile rheology of most crystalline materials are different from those at high temperatures.

(3)A recent microphysical model, which treats fault rock deformation as controlled by competition between rate-sensitive(diffusional or crystal-plastic)deformation of individual grains and rate-insensitive sliding interactions between grains(granular flow), predicts both transitions well, called the CNS model. Unlike the numerical model, this model quantitatively reproduces a wide range of(transition)frictional behaviors using input parameters with direct physical meaning, which is closer to the natural strength of the fault. This mechanism-based model can reproduce RSF-like behavior in microstructurally verifiable processes and state variables. However, the major challenge in the CNS model lies in capturing the dynamics of micro- and nanostructure formation in sheared fault rock and considering the different processes of rock deformation mechanisms.

Since it is microphysically based, we believe the modelling approach can provide an improved framework for extrapolating friction data to natural conditions.

GEOLOGICAL DEFORMATION OF THE TUOLI FAULT IN THE WEST JUNGGAR SINCE THE LATE QUATERNARY

YUAN Hao-dong, LI An, HUANG Wei-liang, HU Zong-kai, ZUO Yu-qi, YANG Xiao-ping

2023, 45(1):  49-66.  DOI: 10.3969/j.issn.0253-4967.2023.01.003

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In the Cenozoic, under the influence of the collision of the India-Eurasia plate and the northward pushing after that, deformation occurred in the interior of the continent, and the crustal deformation is mainly absorbed by the thickening of the crust and the strike-slip movement of the fault. The GPS velocity field shows that the area north of Tianshan absorbs the shortening with a rate of~2mm/a. How the shortening with these rates is absorbed is a topic worthy of study. The West Junggar, located to the north of the Tianshan Mountains and developed with the inclined parallel strike-slip fault system is an important area of crustal shortening. The inclined parallel strike-slip fault system includes the east Tacheng Fault, Tuoli Fault and Daerbute Fault. Hence, the structural deformation of the Tuoli Fault in the late Quaternary is significant for understanding the structural deformation and crustal shortening absorption mode in the north of Tianshan Mountains.

In this study, two branches were found extending along the Tuoli Fault in the direction of NE based on remote sensing image interpretation. Field investigation to the two branch faults shows that many marker landforms were dislocated in the study area, including gullies and terrace riser. The two faults cross through the terraces developed in the Kapusheke River and the Tiesibahan River in this area, forming offset terrace riser. Because the terrace riser is in the retained bank of the river, the upper-layer terrace model is used to calculate the fault’s slip rate. The gullies are mainly distributed on the T3 terrace of the Kapushek River on the west branch fault. The horizontal dislocation of these gullies ranges from 10m to 37.5m, and the largest horizontal dislocation is located in the No. 8 gully, which is (37.5-4.1/+2.7)m. Since the actual value of the fault movement rate must be greater than the rate obtained by the sub-gully offset, we choose the maximum offset of the gully on the landform surface in calculating the slip rate. We used OSL(Optical Stimulated Luminescence)to date the age of the landform and used UAV(Unmanned Aerial Vehicle)photogrammetry technology to extract high-precision DEM of the study area. Then, we calculate the movement rate of the Tuoli Fault since the late Quaternary from the dislocations and the age of landmark landforms such as gullies and terraces. The results show that the Tuoli Fault comprises two branch faults in the east and the west, both of which are left-lateral horizontal strike-slip. The east branch fault produced a (89±31)m and (39±13)m horizontal dislocation on the T3 and T2 terrace of the Kapusheke River, respectively. Combined with the (52.9±5.1)ka of the T3 terrace age and (23.4±1.5)ka of the T2 terrace age, the horizontal slip-rate of (1.7±0.8)mm/a is calculated for the eastern branch fault. The western branch fault produced a horizontal dislocation of (34.0±6.8)m on the T2 terrace of the Tiesibahan River and 37.5⁽-4.1/+4.1)m of the gully on the T3 terrace of the Kapusheke River. Combined with (18.8±1.3)ka of the T2 terrace age, we obtained a sinistral slip rate of 1.8(+0.5/-1.3)mm/a for the western branch fault. The sinistral slip rate of two branch faults of the Tuoli Fault is similar to the sinistral slip rate of the east Tacheng Fault in the previous research results. This study result indicates that these parallel left-lateral strike-slip faults in the West Junggar area conform to the characteristics of the bookshelf faults structural model, and most of the compression shortening in the West Junggar area is absorbed by the parallel strike-slip movement of the fault system. So this fault system has played an important role in controlling the NS shortening of the crust in this region.

THE QUATERNARY NORMAL FAULTING AND RECENT CO-SEISMIC SURFACE RUPTURE AND RELATED SEISMOLOGICAL SIGNIFICANCE ALONG THE ARU CO GRABEN SYSTEM IN NORTHERN NGARI, TIBET

WU Zhong-hai, Baima Duoji, YE Qiang, HAN Shuai, SHI Ya-ran, Nima Ciren, GAO Yang

2023, 45(1):  67-91.  DOI: 10.3969/j.issn.0253-4967.2023.01.004

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The Qinghai-Tibetan plateau, with an average altitude of about 5^{}000m, is one of the most intense regions of intraplate deformation in the globe during the Quaternary. However, the very weak field investigation of active faults and incomplete historical earthquake data in the northern Qinghai-Tibet Plateau limit the in-depth understanding of the deformation mechanism of active tectonics and the characteristics of related strong earthquakes in the Qinghai-Tibetan plateau. Based on the comprehensive geological, remote sensing, and seismic data, the active faults in northern Ngari are interpreted in detail, and the Quaternary activity of the normal faults along the western boundary of Kunchuke Co graben in the southern section of the Aru Co graben system, the newly discovered co-seismic surface ruptures, its magnitude and seismogenic time are analyzed. The newly active fault images show that high-density active fault system dominated by the near east-west extension deformation was developed in the north Ngari. The Quaternary active fault system mainly includes near north-south normal faults and the conjugated strike-slip faults composed of the NW and NE strike-slip faults. The density of the normal faults is significantly higher than that of the strike-slip faults in the region. Based on the comprehensive analysis of the Aruko graben system and the latest co-seismic surface rupture along the western boundary of Kunchuke Co Graben. We present two main conclusions. 1)The Aru Co graben system, with a total length of 210 to 220km, is one of the largest extensional fault depression structures in northern Ngari. The graben system contains four secondary graben and half-graben distributed in left-step echelon distribution from south to north and shows obvious segmented activity characteristics. Meima Co-Aru Co graben is the most intense extensional deformation section along the Aru Co graben system during the Quaternary period. The left echelon pattern of the secondary graben in the graben system indicates that there is a right-lateral shear deformation component along the NW-trending graben system in the region. 2)The newly discovered co-seismic surface ruptures along the boundary fault of the western margin of Kunchuke Co Graben in the southern section of the Aru Co graben are typical normal fault-type ruptures. The surface rupture is distributed along the NNW-trending, with an outcrop length of nearly 400m, a maximum vertical displacement of about 0.8m, and an average vertical displacement of about 0.30.4m. Comprehensive historical earthquake records, the freshness of co-seismic surface ruptures, and the magnitude results based on the classic “surface displacement and magnitude” statistical formula, we concluded that the Kunchuke Co surface rupture should be a result of the 1955 M_W6.5 earthquake event, which epicenter of the instrument was located in eastern Nawu Co of Gègyai county, with a focal depth of 35km and small length and displacement. The deep focal depth is a major cause of lead to the co-seismic surface rupture is obviously small-scale. This small-scale surface rupture event on active faults suggests that irregular or random local fault rupture behavior should be paid attention to in the study of the earthquake recurrence model of active faults.

TYPICAL REMOTE SENSING IMAGES OF HOLOCENE FAULTS IN GAIZE PAN(I45)IN TIBET

BAI Qilegeer, SHEN Jun, XIAO Chun, DAI Xun-ye

2023, 45(1):  92-110.  DOI: 10.3969/j.issn.0253-4967.2023.01.005

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Active faults refer to faults that have been active since the late Quaternary(100000~12 0^{}000 years)which are the culprits of large earthquakes. They can be divided into Holocene faults and Late Pleistocene faults. The Holocene fault is the active fault that has displaced on or near the surface in the past 10000 years. The Active faults may cause seismic surface dislocation in the future, which will damage the project crossing the active fault. It is necessary to take measures to avoid or resist the fault. Therefore, finding out the distributions of active faults are the prerequisite for reducing earthquake disaster losses and disaster risks.

We undertook the compilation of the 1︰1000000 seismotectonic map of Tibet in the first national comprehensive risk survey of natural disasters. The preparation of a seismotectonic map is to conduct detailed investigation and research on active faults within the research scope, including large-scale active faults with a strong earthquake-generating capacity, as well as small-scale and highly active faults. The Qinghai-Tibetan plateau is a typical strong earthquake-prone area with wide distribution, high frequency, high intensity and shallow source of seismicity. This study introduces the Holocene active faults in the modified scale(I45)of 1︰1000000 international standard topographic map.

We use Satellite remote sensing images to determine the locations of the faults, identify their characteristics, and assess the ages of their latest activity and quantitative parameters such as intensity. Satellite remote sensing interpretation is the most important method to study active faults. This is especially true in the Qinghai-Tibetan plateau region, where active fault traces are clear and lack overlying Quaternary layers. High-resolution satellite remote sensing images can capture various tectonic and geomorphological phenomena formed by fault activity.

In the study area, we interpreted Six Holocene active faults by using high-resolution satellite images, including the MargaiCaka fault, the Riganpeicuo fault, the Yibuchaka graben, the Qingwahu fault, the Dongcha fault, and the central part of Qixiangcuo fault. When analyzing each fault, typical images with evidence of active faults are intercepted, and the typical remote-sensing image features of active faults are summarized. It is clear that the typical remote sensing images of active faults are the remote sensing images which can reflect the dislocation of late Quaternary strata, geological bodies and geomorphic surfaces(unit).

The latest active age, slipping senses and active intensity of above active faults in the area, as well as the overall tectonic pattern and seismic capacity of active structures in the area are discussed. The MargaiCaka fault in the north of the study area and the Riganpeicuo fault, the Qixiangcuo fault in the south are large-scale left-lateral strike-slip faults of NEE trending and have the capability of generating earthquakes of about magnitude 7.5. The NEE-trending Yibuchaka graben, the Qingwahu fault, and the NW-trending Dongcha fault in the central of the map unit have the capability of generating earthquakes of about magnitude 7. The above-mentioned faults reflect a special dynamic environment in which the area is squeezed in the north-south direction, and a V-shaped conjugate fault formed, making the plateau squeezed out to the east.

NEW EVIDENCE OF THE PALEOEARTHQUAKE RUPTURE IN THE NORTH GAIZHOU-ANSHAN SEGMENT OF THE JINZHOU FAULT

LI An, WAN Bo, WANG Xiao-xian, JI Hao-min, SUO Rui

2023, 45(1):  111-126.  DOI: 10.3969/j.issn.0253-4967.2023.01.006

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The Haicheng M_S7.3 earthquake is the first successfully predicted earthquake in China, which saved a large number of lives and avoided property losses. However, the investigation after the earthquake did not find a continuous surface rupture zone, and only some ground fissures and sandblasting were found in the epicenter area. The isoseismal line of this earthquake shows obvious conjugate characteristics. Which fault is the seismogenic structure of the Haicheng earthquake has always been controversial. According to the focal mechanism and distribution of ground fissures, some scholars suggested the seismic structure is the Haicheng River Fault with a strike of NWW. However, other scholars suggested the Jinzhou Fault has a larger scale and controls the geomorphic boundary. Jinzhou Fault is also a major seismic structure distributed in the west of Liaodong Peninsula, with a strike of NENNE and a length of 280km. The north Gaizhou-Anshan segment of the Jinzhou Fault is conjugated with the Haicheng River Fault. Both of them are likely to be the seismogenic structure of the Haicheng earthquake, or both ruptured in the Haicheng earthquake. Based on remote sensing image interpretations, four sites of the fault scarps, including the Yujiagou, Houwudao, Dongjiagou, and Tashan sites, were distinguished and verified in situ. And using micro geomorphology measurement and paleoseismic trench excavation in the Huluyu site of the north Gaizhou-Anshan segment of the Jinzhou Fault which is conjugated with the Haicheng River Fault, this paper obtains the following understandings: The Jinzhou Fault extends from the northeast of the Dashiqiao City to the south of the Anshan City. There are prominent NE-trending fault scarps, which were formed in the late Pleistocene and Holocene, on geomorphic surfaces of the basin mountain transition zone. Due to farming and building, fault scarps are not preserved well, and the distribution of the fault scarp is discontinuous. The height of fault scarps is mostly 1~2m, up to 3m at most. The paleoseismic trench was excavated in the Huluyu village, south of Haicheng City. The paleoseismic trench revealed a ~²0m wide bedrock fracture zone in the north Gaizhou-Anshan City segment of the Jinzhou Fault. Three Late Pleistocene to Holocene strata(U3 to U5)overlie the bedrock fracture zone. Five fault planes(F₁ to F₅)are revealed in the trench. The fault F₁ recorded the newest paleoearthquake event and the Fault F₂ recorded the earlier one. In summary, according to the cover-cut relationship between strata and faults, at least two paleoseismic events occurred from the Late Pleistocene((37.6±2.2)ka)to the Holocene. The newer one occurred in the Holocene(after(11.7±0.8)ka, probably 400~500a before present). However, because of the thin Holocene strata, we cannot distinguish more paleoearthquakes in the trench. Therefore, it is still doubtful whether the north Ganzhou-Anshan segment of the Jinzhou Fault ruptured in the Haicheng earthquake in 1975. However, the confident conclusion is that the north Gaizhou-Anshan City segment of the Jinzhou Fault is an active fault in the latest Late Pleistocene to Holocene.

NEW ACTIVITY CHARACTERISTICS IN THE CHISHAN SECTION OF TAN-LU FAULT ZONE IN HOLOCENE

ZHENG Hai-gang, YAO Da-quan, ZHAO Peng, YANG Yuan-yuan, HUANG Jin-shui

2023, 45(1):  127-138.  DOI: 10.3969/j.issn.0253-4967.2023.01.007

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The Chishan section of Tan-Lu fault zone is located in Sixian County, northern Anhui Province. Research on the characteristics of Quaternary fault activity of this section began in the 1990s, which includes microgeomorphology survey, trench excavation, dating sample collection and measurement, and so on. Through these studies, many valuable data and results were accumulated, which laid a good foundation for the current research. Based on the field geological survey and previous studies, two geological trenches were excavated, which are named trench XJ1 and XJ2 respectively. Among them, very rich remains of ancient earthquakes were found in trench XJ1 and analyzed as major contents in this paper, and few relics of ancient earthquake were found in trench XJ2, which are not involved in this paper.

In the trench XJ1, ten strata units were revealed, labeled as U₁ to U₁₀ from old to young, respectively. Layer U₁ is the Cretaceous sandstone with a thickness about 0.5~1.0m, lying on the bottom of the west wall of the trench. Layer U₂ is yellowish brown clay with a thickness of 1~2.5m, located at the bottom of the eastern side of trench profile. One OSL sample is collected in the middle of this layer with an age more than 150k {\mathrm{a}}^{}BP, which indicates the layer was deposited before the Mid Pleistocene. Layer U₃ is purple clay-sand, which is wide at the bottom around 6.5m and narrow at the top around 2.5m, and the top extends about 7m continuously from west to east. Layer U₄ is motley gravel with a thickness about 2.0~2.5m, which is below layer U₉ and above layer U₄ on the west side of the trench wall. Layer U₅ is gravel containing a lot of clay and a few of sandstone clumps, wide at the top about 3m and narrow at the bottom about 2m. Layer U₆ is light green gravel containing some sand and clay, thick in the west about 0.8m and thin in the east about 0.2m, extending around 7m discontinuously from west to east. Layer U₇ is grayish white gravel with sand and clay, thick in the west around 1.0m and thin in the east around 0.2m, extending about 5m continuously from west to east. Layer U₈ is yellow clay with a thickness of 0.5~2.0m, located below layer U₉ and above U₇. One peat sample was taken from the top of the layer and the age of this sample is 21.57~21.22k {\mathrm{a}}^{}BP measured by Beta Analytic Inc in the United States, which indicates this layer was deposited in Late Epipleistocene. Layer U₉ is black clay with a thickness of 0.5~1.5m, which is located above Layer U₄, U₅, U₇ and U₈ and is the latest disturbed layer in the trench. One peat sample was taken from the bottom of this layer and the age of this sample is 11.10~10.75k {\mathrm{a}}^{}BP measured by Beta Analytic Inc in the United States, which indicates this layer was deposited in the early Holocene. Layer U₁₀ is the cultivation layer with a thickness of 0.2~0.5m, located on the topmost of the trench wall.

Three faults were revealed in these layers, named as F₁ and F₂ and F₃ respectively from east to west. Three paleoseismic events were identified, which are labeled as E₁ and E₂ and E₃ respectively from old to new. The E₁ represents a thrust activity of fault F₁. After the deposition of layers U₅, U₃ and U₂ finished, the hanging wall U₅ of fault F₁ thrust upward above the footwall U₈, and the soft layer U₃ in between was squeezed and rubbed upward, forming lenticles in the layer, which indicates the movement direction of the hanging wall of F₁ is thrust upward. A compressional overfall scarp was formed by this event, then the layer U₆ was deposited on the east side of the scarp, whose age is not measured. But the dating of layer U₂ beneath the fault F₁ yields an age before Mid Pleistocene, which constrains the lower limit age of E₁ to be after Mid Pleistocene. The E₂ represents a thrust faulting of fault F₂. After the deposition of layer U₆, a new thrust faulting occurred on fault F₂, which cut through layer U₅ and formed a thrust fault scarp. Later, U₇ and U₈ were deposited on the east of the scarp. The layer U₇ is gravel, whose age is not measured, but the layer U₈ is dated as the Late Epipleistocene, which constrains the upper limit age of events E₁ and E₂ to be after Late Epipleistocene. The E₃ represents a strike-slip normal faulting of Fault F₃, which faulted the layer U₃. According to the age of the layer U₃, we can constrain the lower limit age of E₃ to be the Early Holocene, which indicates that the Chishan section of the Tan-Lu fault zone is still active after the Early Holocene.

To sum up, two geological trenches were excavated at the Chishan section of Tan-Lu fault zone, named as trench XJ1 and XJ2 respectively, and three main faults were revealed on the wall of trench XJ1, named as F₁, F₂ and F₃ from east to west, and three paleoseismic events were identified, which are labeled as E₁ and E₂ and E₃ respectively from old to new. The latest ancient seismic event faulted the Early Holocene layer, indicating the Chishan section of the Tan-Lu fault zone is still active after the Late Holocene, and the latest activity is of strike-slip normal faulting, which provides new evidence for the presence of Holocene activity of this fault section and new information for long-term seismic risk assessment in this area.

A PRELIMINARY STUDY OF THE SHALLOW EXPLORATION AND QUATERNARY ACTIVITIES OF THE FENGQIU SEGMENT OF THE XINXIANG-SHANGQIU FAULT

TIAN Yi-ming, YANG Zhuo-xin, WANG Zhi-shuo, SHI Jin-hu, ZHANG Yang, TAN Ya-li, ZHANG Jian-zhi, SONG Wei, JI Tong-yu

2023, 45(1):  139-152.  DOI: 10.3969/j.issn.0253-4967.2023.01.008

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Xinxiang-Shangqiu Fault starts from Yuhekou in the west and extends eastward into Anhui Province through Xinxiang, Yanjin, Fengqiu, Lankao, Minquan, Shangqiu and Xiayi, with a total length of about 400km and a general strike of NWW. It is a regional concealed fault in Henan Province and a boundary fault between northern North China depression and southern North China depression.

This study focuses on the Fengqiu section of Xinxiang-Shangqiu Fault, which is the boundary structure between the Kaifeng sag, Neihuang uplift and Dongpu sag. Controlled by the NE-NEE trending Changyuan Fault and Yellow River Fault at its east and west end, this fault section has a length of about 30km and controls the Mesozoic to early Cenozoic sedimentation in the Kaifeng sag and the south side of Dongpu sag.

In this paper, the shallow structural characteristics and Quaternary activities of Fengqiu section of the Xinxiang-Shangqiu Fault are revealed by the combination of reflection seismic exploration and drilling detection. Two shallow seismic exploration profiles and one composite drilling geological section are arranged across the fault.

The results of shallow seismic exploration show that the Fengqiu section of Xinxiang-Shangqiu Fault is NWW trending. It is a north-dipping normal fault accompanied by several nearly parallel normal faults, and the fault is still active since the Quaternary.

In the composite drilling geological section at Yaowu, the latest faulted stratum is a clay layer between borehole YW5 and YW7, and the buried depth of the upper breakpoint is between 57.00~61.50m. Combined with the dating results of the collected samples, it is comprehensively judged that the latest activity age of Fengqiu section is the middle of late Pleistocene. Since the middle of late Pleistocene, the whole region is in a relatively stable tectonic period. It is verified that the comprehensive detection method of shallow seismic exploration with drilling can effectively find out the accurate location of hidden faults.

The zone with strong vertical differential movement is often the zone where earthquakes occur. The vertical differential movement between Kaifeng sag and Neihuang uplift is very strong, and the difference reaches nearly 1^{}000 meters since Neogene. Moreover, the structural pattern of the main strong earthquakes in the North China Plain is characterized by zoning in NE direction and segmentation in NW direction, especially at the intersections of NWW-trending faults and NE-trending faults. The Xinxiang-Shangqiu Fault intersects with a series of NE-NEE trending faults, including Tangdong, Changyuan, Yellow River and Liaolan faults from west to east. The Fengqiu section is at the intersection with the Changyuan Fault and the Yellow River Fault, and is located in the Fengqiu M6.5 potential seismic source area of the North China plain seismic belt. The intersection of two groups of Quaternary active faults is a favorable place for the preparation and generation of moderate and strong earthquakes. Therefore, the research results provide seismological basis for the site selection of major engineering projects, urban planning and construction in this area, and have reference value for discussing the geodynamic issues such as deep and shallow structural relationship and structural evolution of Xinxiang-Shangqiu Fault.

STABILITY ANALYSIS OF THE BAIGE LANDSLIDE USING D-INSAR AND PFC2D MODELING

JIN Li-zhou, WANG Ying, CHANG Wen-bin, TIAN Ying-ying, YUAN Ren-mao

2023, 45(1):  153-171.  DOI: 10.3969/j.issn.0253-4967.2023.01.009

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At 4:00am on October 11, 2018, under the influence of heavy and continuous rainfall, a large-scale rocky landslide occurred in the Baige village of Bolo Town, Jiangda County, Tibet Autonomous Region, which is located at the upper reach of the Jinsha River. During its sliding, the landslide body is cut out from the upper part of the high and steep slope and falls rapidly, and the lower rock mass is continuously scraped, which increases the volume remarkably. With the disintegration of the landslide mass, the landslide mass is transformed into a fast and remote debris flow sliding. The massive debris flow materials rapidly flowed down to block the Jinsha River, forming a barrier dam. Then the lake rose and flooded many roads. At 5:00pm on the October 12^th, the barrier dam was overtopped and gradually washed by the river to form a drainage channel. At 9:00am on the 13^th, the dam was completely flushed open, accomplishing the flood discharge and relieving the danger caused by the landslide. At 5:00pm on November 3, 2018, the trailing edge of the Baige landslide experienced a sliding rupture, which led to the debris flow, at a high speed, piled up the dam from the first landslide, and blocked the Jinsha River again. The height of the second barrier dam was 50m higher than the first one, forming a larger barrier lake. After the landslide occurred, the water level of the upper reaches of the barrier lake continued to rise, and Jiangda County, Boro Town, Baiyu County Jinsha Town and other towns on the upper reaches of the Jinsha River were flooded. After the second floodwater released, a large scale flood occurred in Jinsha River, which caused the flooding of cities and towns in the middle and lower reaches in Sichuan, Yunnan and other riverside areas, and destructed roads and bridges, posing a great threat to the lives and property of people and the safety of infrastructure such as hydropower stations. The water level of the dammed lake was lowered by artificially constructing a diversion channel to eliminate the danger of dam break and avoid the occurrence of greater flood hazards. On the basis of field investigation on the landslide site, it is found that after the first landslide, three potential unstable rock masses were found at the trailing edge and both sides of the landslide. According to radar monitoring, three potential unstable rock masses at the trailing edge of the landslide are still continuously deformed, with obvious activity, and there is a risk of blocking the Jinsha River again. The author was monitoring constantly the unstable rock of the trailing edge of the Baige landslide for 7.5 days adopting D-InSAR. The surveillance results indicate that there is a slight sliding on the upper side of the landslide and there are four major deformation regions on the upper edge of the landslide. Besides, four measuring data points, selected within the four major deformation areas, show that the deformation value is 200mm and the deformation rate on the landslide top reaches 300mm/day, which suggests that the current landslide is still not stable and there is the risk of blocking the Jinsha River by the landslide. This paper, using PFC2D, simulates the stability of unstable rock on the trailing edge of landslide under the influence of gravity, torrential rain, and earthquake and analyzes the landslide’s stability scientifically in terms of simulation results. The simulation results show that the slope only deforms slowly under static action, without obvious destabilizing sliding. The initial deformation of the slope is basically consistent with the results of radar monitoring displacement, indicating that the sliding body of the slope still has a sliding trend under static action, and is not stable. Under the action of heavy rainfall, with the increase of time step, the deformation and displacement of slope is also increasing. In the process of operation, tensile cracks gradually appear in the slope, and continue to develop until it is cut through, and instability failure occurs. The ground motion is input from the bottom of the slope model in the form of velocity. When the model is running, tensile cracks first occur at the back edge of the slope on the right side. As the shear failure occurs in the middle of the slope and the tensile crack at the back edge goes through, the whole slope becomes unstable and fails. But on the whole, it’s basically stable. The simulation results show that the unstable rock in the trailing edge of the landslide will still lose stability under the inducing factors such as heavy rainfall and earthquake. It’s necessary to take appropriate engineering measures such as slope cutting to control the unstable rock, and the real-time monitoring and early warning system should be set up to eliminate the hidden danger caused by the slide of unstable rock blocking the Jinsha River again in time. At the same time, this paper also provides reference significance for further understanding the development and evolution process, as well as the deformation failure mechanism of landslide and debris flow in alpine regions. It also provides theoretical guidance for emergency measures and disaster prevention and mitigation after a disaster happens.

INVERSION OF INDOSINIAN SURFACE IN CHANGNING, SICHUAN BASED ON PARTICLE SWARM OPTIMIZATION ALGORITHM

LIU Yi-jun, YANG Guang-liang, WANG Jia-pei, TAN Hong-bo, ZHOU Huai-bin, SHEN Chong-yang

2023, 45(1):  172-189.  DOI: 10.3969/j.issn.0253-4967.2023.01.010

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The gravity inversion results of three-dimensional density interface are often not unique, which brings some difficulties to further scientific research. The classical particle swarm optimization algorithm has a higher global extremum search ability, faster inversion speed in computing high-dimensional nonlinear inversion problems, and the final solution is independent of the initial model compared with traditional inversion density interface algorithms such as L-M, Tikhonov regularization, Gauss-Newton method, etc. However, in classical particle swarm optimization, the initial model setting and parameter selection are not perfect. Therefore, this paper further enhances the algorithm based on the classical particle swarm optimization algorithm, referring to the previous optimization ideas. The test results of various models show that the optimized particle swarm optimization algorithm has a stable ability to search for the optimal global solution, and the depth error is smaller. In addition, if we adopt parallel computing, the inversion speed can be effectively improved.

We obtained the Indosinian density interface depth model of the Changning area by inversion using multiple measured high-density gravity profile data based on the improved algorithm. The overall scope of the survey area is small and diamond-shaped, including the complete Changning-Shuanghe anticline and some surrounding synclines. The inversion results show that the Indosinian density interface generally presents the characteristics of uplift in the middle and depressions around it, and the depth range is 0.3~3.3km, which is basically consistent with the inversion results of the drilling data and previous gravity data, and the details are more prominent. It can better express its structural characteristics. The depression degree of the interface on the right side is significantly larger than that on the left side. The uplift part corresponds to the Changning-Shuanghe complex large anticline, and the depth varies from 0.3km to 1.9km. The core of the anticline is exposed to the surface by uplifting and erosion of the tectonic movement. The inversion result provides essential information for studying the seismotectonic environment and is also a vital reference for studying the multi-layer density interface model.

Density interface fluctuation is the product and sign of a specific area under the action of multi-stage tectonic movement, which plays an essential role in studying basin basement, regional structure, and deep structural fluctuation. It provides critical information for the analysis of the origin of earthquakes. Therefore, we analyzed the structural characteristics of this area and its relationship with earthquakes combined with the undulating morphology of the Indosinian surface. Earthquakes in the Changning area are concentrated on the north and south sides of the large anticline. The seismic distribution pattern and focal parameters on both sides are obviously different. The main reason for this phenomenon is that there are significant differences in the causes of earthquakes. The Indosinian surface in the north wing of the anticline is steeper than that in the south wing. The location of the strip distributed shallow earthquakes in the north wing is highly related to the fluctuation of the Indosinian surface, and they mainly occur at the places where the Indosinian surface fluctuates violently. The local density changes drastically, and the earthquakes’ occurrence is greatly affected by hidden faults. The clumped distributed shallow earthquakes in the south wing occur at locations where there is an apparent depression on the Indosinian surface, which may be caused by shale gas exploitation, and the earthquakes are more affected by local stress changes. Deep earthquakes may be closely related to the revival of basement faults. There may still be seismic risk in the northeast wing of the large anticline in the future.

In general, the optimized particle swarm algorithm has achieved good results in both model testing and practical applications. In order to further improve the accuracy of the inversion results, we will focus on improving the applicability of the algorithm in various situations and the ways of adding multiple constraint information. More detailed geophysical research should be carried out in this area, which will help to better understand its crustal structure, earthquake mechanism, geological structure, and the development of earthquake prevention and disaster reduction.

CRUSTAL S-WAVE VELOCITY STRUCTURE BENEATH THE XIAO-JIANG FAULT ZONE AND ADJACENT REGIONS REVEALED BY TWO-STEP INVERSION METHOD OF RECEIVER FUNCTIONS

YANG Jian-wen, JIN Ming-pei, CHA Wen-jian, ZHANG Tian-ji, YE Beng

2023, 45(1):  190-207.  DOI: 10.3969/j.issn.0253-4967.2023.01.011

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In the past few decades, a large number of geophysical explorations were carried out in the Xiaojiang fault zone and adjacent areas, mainly including GPS, seismic geology, fluid geochemistry, seismicity, historical earthquakes and coseismic displacement of large earthquakes, etc. The results of these studies helped us have a better understanding of the fault structure characteristics, movement attributes, seismogenic environment and dynamic mechanism of the Xiaojiang fault zone. In terms of deep structure, the existing researches are limited by factors such as the density of observation stations, and most studies focused on the structural background on the regional scale, and few are specifically on this fault zone. The implementation of Phase I of the China Earthquake Science Array(ChinArray)detection project provides a good data basis for the study of the fault structure in Yunnan. It is of great practical significance for earthquake prevention and disaster mitigation to carry out deep structural detection of the Xiaojiang fault zone and clarify the fine crustal structure of the fault and its adjacent areas.

S-wave velocity is an important parameter to determine the crustal structure, physical state difference and tectonic evolution process. Extracting the P-wave receiver function from teleseismic body-wave waveform data and inverting it is one of the important methods to obtain the crustal S-wave velocity structure at present. The traditional receiver function S-wave velocity structure inversion relies heavily on the selection of the initial model, which results in strong non-uniqueness inversion results. The two-step inversion method, which takes into account the low and high frequency receiver functions at the same time, effectively suppresses the dependence of the inversion process on the initial model, and improves the reliability of the inversion results.

Based on the three-component waveform data of 238 teleseismic events with epicentral distances ranging from 30°~90° and magnitude M≥5.8 recorded by 48 broadband seismic stations in the Xiaojiang fault zone and adjacent areas from September 2, 2011 to January 16, 2014, this paper calculates the low-frequency(α=1.0)and high-frequency(α=2.5)radial P receiver functions, respectively. Then, on this basis, the S-wave velocity structure below each station is inverted using the two-step inversion method and Bootstrap resampling technique and the deep crustal structure of the Xiaojiang fault zone and its adjacent areas is studied. The following conclusions are drawn:=

(1)The crustal S-wave velocity in the study area is obviously non-uniform in both lateral and vertical directions. The overall distribution is as follows: In the near surface, there is a low-velocity layer about 2~4km thick, which may be related to the distribution of shallow sedimentary rocks or Cenozoic soft overburden; The S-wave velocity in the middle and upper crust is alternately distributed with high and low velocity; There is an obvious low-velocity layer in the depth range of 20~35km, mainly intermittently distributed in the Sichuan-Yunnan diamond block west of the Xiaojiang Fault and the Indosinian block south of the Honghe Fault; Besides, there is also local distribution near the Shizong-Mile Fault.

(2)The low-velocity layer in the middle and north segments of the Xiaojiang fault zone are relatively developed, and it is most prominent in the middle segment, with a maximum thickness of about 28km. There is an obvious high-velocity zone in the depth range of 15~25km in the southern segment.

(3)The Poisson’s ratio in the study area is generally low(average 0.24), unevenly distributed, and has drastic lateral changes. The Poisson’s ratio in the Xiaojiang fault zone generally has a segmental feature of higher in the northern segment, the southern segment coming second, and lower in the middle segment. The corresponding relationship between the distribution of low velocity in the crust and Poisson’s ratio in the study area is not obvious, and most of the low velocity layers seem to lack the conditions for partial melting. The differences and inconsistencies in the geophysical results indicate that the deformation evolution mechanism and physical properties of the low velocity layers in the crust are relatively complex.

CHARACTERISTICS OF TECTONIC STRESS FIELD AROUND THE YANGBI M_S6.4 EARTHQUAKE AND ITS SURROUNDING AREAS ON MAY 21, 2021 AND ITS GEODYNAMIC IMPLICATION

FAN Wen-jie

2023, 45(1):  208-230.  DOI: 10.3969/j.issn.0253-4967.2023.01.012

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According to the determination of China Seismological Network, at 21:48:34 Beijing time on May 21, 2021, an M_S6.4 earthquake occurred in Yangbi County, Dali Bai Autonomous Prefecture, Yunnan Province. The epicenter(25.67°N, 99.87°E)is located on the southwest boundary of the Sichuan-Yunnan rhombic block, with a focal depth of 8km. On the basis of the survey results of the Yunnan Eartquake Agency, the highest intensity in the earthquake area is VIII degree, and the long axis of the isoseismic line is NW-striking. We calculated the focal mechanism solutions of 11 M_S≥4.0 events of May 21, 2021, Yangbi earthquake by the CAP method. Combined with the collected focal mechanism solutions of the surrounding historical earthquakes, we inverted the tectonic stress field around the epicenter and its adjacent areas and simulated the relative stress value and the distribution of the focal mechanism solution in the Yangbi area. We analysis and studies the characteristics of tectonic stress field variation in the study area and its relationship with earthquakes, dynamic significance and seismogenic mechanism. The results show that: 1)The Yangbi earthquake sequence and the focal mechanism solutions of historical earthquakes near the epicenter are mostly of strike-slip type, followed by the normal-fault type. 2)The Yangbi earthquake sequence is mainly distributed in the depth range of 4~8km. The depth of earthquakes in the study area is mostly above 15km underground, and they mainly occur in the brittle upper crust. The tectonic stress field is strike-slip type in the Yangbi source area, and the maximum principal stress is the NNW-SSE direction, which is basically consistent with the known regional tectonic stress field. The regional inversion results show that the maximum principal stress axis(σ₁ axis)of the Sichuan-Yunnan rhombic block in the northeastern part of the study area is the NNW-SSE direction, and the minimum principal stress axis(σ₃ axis)is the NEE-SWW direction. In the Lanping-Simao block, the σ₁ axis becomes nearly NS, and the σ₃ axis is close to EW. For the Tengchong and Baoshan blocks in the southwest part, the σ₁ axis rotates in the NNE-SSW direction, and the σ₃ axis is in the NWW-SEE direction. Judging from the uncertainty range under the 95%confidence level of the maximum principal stress azimuth, the variation range of the inversion results of most grid points in the study area is within 20°, indicating that the inversion results are relatively stable. In general, the orientation of the maximum principal stress axis and the minimum principal stress axis show a clockwise rotation trend from northeast to southwest. And the maximum principal stress axis direction distribution is similar to the GPS horizontal velocity field and other stress data results. The stress orientation inside the Sichuan-Yunnan block, Tengchong and Baoshan blocks is relatively consistent. But the stress changes in the block boundary zone, showing a certain difference, which may be caused by the difference in the dynamic action, movement mode and speed of the block. The regional tectonic stress field is affected by the interaction between different blocks. 3)The R-value increases gradually from northwest to southeast in the study area. And the increase in the R-value shows that the intermediate principal stress is mostly characterized by tensile stress, indicating that the compressive stress required for material migration decreases relatively. Combined with the geological tectonic background of the study area, it is considered that the movement speed of the block(material)gradually slows down, which is consistent with the surface deformation observations. According to the simulation results of the relative shear stress and relative normal stress of the Yangbi earthquake, the NW-oriented nodal plane of the Yangbi earthquake is the seismogenic fault plane of the earthquake. Combined with the seismic relocation and tectonic stress field inversion results, We analyzed the seismic mechanism of the Yangbi earthquake as follows: Under the NNW-nearly NS-trending tectonic stress, the NW-trending subvertical faults intersect with the regional principal compressive stress at a small angle, resulting in right-slip shearing along the optimal release nodal plane, and finally rupture leads to the occurrence of earthquakes. The Yangbi earthquake is located in the southwestern boundary zone of the Sichuan-Yunnan rhombic block. In recent years, the 2013 Eryuan M_S5.5 and M_S5.0 earthquakes and the 2017 Yangbi M_S5.1 earthquakes were occurred along this boundary zone. The seismic activities here are very significant. The occurrence of moderate to strong earthquakes near the Sichuan-Yunnan rhombic block may indicate that the main active faults at the block boundary have accumulated a high level of elastic strain energy. In addition, the Yangbi earthquake manifested as a right-lateral strike-slip seismic activity on the NW-trending fault plane, and it is located southwest of the Sichuan-Yunnan rhombic block. It just shows that the Sichuan-Yunnan rhombic blockhas a SE-direction translational motion, which is consistent with the kinematic model of the Qinghai-Tibetan plateau material escaping eastward in the form of a rigid block, accompanied by a certain clockwise rotation, which is consistent with the long-term tectonic movement direction of the block. The Yangbi earthquake occurred under the dynamic background that the Sichuan-Yunnan rhombic block is continuously squeezed by the Qinghai-Tibetan plateau material. These research results have reference significance for understanding the seismic background and dynamic mechanism of the Yangbi earthquake.

THE CHARACTERISTICS AND SEISMOGENIC STRUCTURE ANALYSIS OF THE 2021 YANGBI M_S6.4 EARTHQUAKE SEQUENCE, YUNNAN

ZHANG Ke, WANG Xin, YANG Hong-ying, WANG Yue, XU Yan, LI Jing

2023, 45(1):  231-251.  DOI: 10.3969/j.issn.0253-4967.2023.01.013

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An M_S6.4 earthquake occurred in Yangbi county, Dali Prefecture, Yunnan on May 21, 2021. It is the biggest earthquake in the region during past 40 years, and its epicenter is located in the southwest boundary of the Sichuan-Yunnan rhomboid block. The type of this earthquake is of a typical “fore-main-residual” type, and cause no surface rupture, its aftershock sequence was not distributed along any known fault in the vicinity. There have been several research results which are on the seismogenic structure of this earthquake that occurred in Yangbi county, but it is also necessary to use a different type and source of data, methods and perspectives thinking angles to verify these results and supply new understandings. In this paper, based on the Yangbi sequence(M_L≥2.0)digital waveform recording and its earthquake phase data recorded by Yunnan Seismic Network between May 18, 2021 and June 13, 2021, the Yangbi sequence is relocated by HypoDD double-difference method and the spatiotemporal Yangbi sequence is also analyzed. The focal mechanism solution and centroid depth of the larger earthquakes in the sequence is obtained by the Cut & Paste(CAP)method. The results indicate that the Yangbi earthquake is distributed along the NW-SE direction as a whole, and its extension length is about 34km. The foreshock sequence has an obvious spatiotemporal migration and has round-trip activity characteristics, while the aftershock sequence has irregular spatiotemporal migration characteristics. The depth range of the aftershocks is mainly between 4km and 13km, and there were a few aftershocks whose depth are below 4km, which is reflecting that this series of earthquakes occurred in the shallow layer of the upper crust, and the rupture of the main earthquake may not extend to the surface. The trend of the belt of the aftershock is generally from the direction NW to SE, which has the obvious spatial segmentation: the aftershocks, which are located in the northwest of the main earthquake epicenter, are rare and relatively concentrated, while the aftershocks, which are located in the southeast, are dense and the width of the aftershock zone becomes larger; The foreshock sequence occurred in the southeast side of the epicenter of the main earthquake, which basically overlapped with the location of the dense segment of aftershocks, indicating that the sparse aftershocks in the northwest side of the main earthquake should belong to the triggering type, while the main earthquake rupture may belong to the unilateral rupture type extending from the epicenter to the SE direction. Besides, its fracture length is about 37km and its downdip width is about 16km. The depth cross-section of the foreshock sequence indicates that the focal depth of the sequence earthquake is generally deep in the southwest and shallow in the northeast, and the fault rupture surface is inclined to SW, with a large dip angle. While the depth cross-section of the aftershock zone shows that the main earthquake rupture is obviously segmented: the NW segment of the sequence has a simple structure, which is there existed one earthquake cluster, while the SE segment is relatively complex, which is there probably composed of two high-dip faults with SW inclination. The centroid depth of the 29 M_S≥3.0 events in the Yangbi sequence, mainly range from 3km to 13km, and their focal mechanism solutions are mostly of right-handed strike-slip type with a nodal plane of high dip Angle in NW-SE direction, and possess a certain normal fault component. In the NW segment of the sequence, the focal properties are mainly dextral strike-slip, and a few earthquakes which have positive fault components shows that there is a NW trending earthquake cluster with a SW inclination. Although the SE segment is still dominated by strike-slip faults, there are more positive faults, of which are two NW trending faults with the SW inclination. This difference reflects that the SE segment is likely to bifurcate and develop into two faults. The main shock is a right-handed strike-slip rupture, the source parameters of fault plane Ⅰ are strike 139°, dip 78° and slip angle -¹64°, and the source parameters of fault plane Ⅱ are strike 45°, dip 74°, and slip angle -12°. The centroid depth of this main shock is 5.2km, which is close to the predominant focal depth of 8.9km obtained by repositioning, indicating that the earthquake occurred in the upper crust, and the depth of seismic activity in the earthquake area is shallow. According to the spatial and temporal distribution characteristics of relocated sequence, combined with the focal mechanism solutions of theYangbi series in Yunnan in May 2021, it is indicated that both the Yangbi earthquake sequence and the source fault plane Ⅰ of main shock are NW-SE trending, which is in good agreement with the middle section of the Weixi-Qiaohou-Weishan fault(the closest to the epicentre). In addition, the focal mechanism solution of the sequence earthquakes is consistent with the properties of the Weixi-Qiaohou-Weishan fault, both of which are right-lateral strike-slip type. We conclude that the seismogenic structure of the Yangbi earthquake may be correlated with the Weixi-Qiaohou-Weishan fault, but the epicentre distribution of the sequence earthquakes is different from that of the Weixi-Qiaohou-Weishan fault. It is confirmed that in this fault, the seismogenic structure of this earthquake is a right-lateral strike-slip secondary fault with a steep dip toward SW on the west side of the southern section. Besides, in this fault, there is another NW trending branch fault in the SE section. In addition, combined with the results of the existing regional tectonic stress field in the focal area, it is believed that the earthquake should be caused by a right-handed strike-slip activity in the focal area which is under the force of NNW-SSE direction.

GRAVITY PERTURBATION AND CHARACTERISTICS OF TEM-PORAL AND SPATIAL CHANGES OF BACKGROUND NOISE BE-FORE EARTHQUAKE: THE EXAMPLE OF MADUO M_S7.4 AND YANGBI M_S6.4 EARTHQUAKES

GOU Jia-ning, LIU Zi-wei, JIANG Ying, ZHANG Xiao-tong

2023, 45(1):  252-268.  DOI: 10.3969/j.issn.0253-4967.2023.01.014

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The Yangbi M_S6.4 and Maduo M_S7.4 earthquake occurred successively on May 21~22, 2021 in Dali, Yunnan Province and Guoluo, Qinghai Province of China. The earthquakes caused deformation of boundaries with density difference and changed the density of rocks around fault due to volumetric strains, thus, disturbing the earth’s gravity field. The Earth’s time-varying gravity field contains rich information about distribution and migration of materials in the Earth system and provides very important constraints for the structural and kinematics characteristics, the interaction of various layers and coupling mechanisms of the solid Earth. Therefore, gravity observation means a lot for earthquake monitoring.

The background noise is the continuous high-frequency oscillation on the observation instruments placed on the surface of the earth, such as seismometers and gravimeters, which is affected by many factors such as oceans, atmosphere, wind fields, earthquakes, human activities, etc. The background noise of the gravimeter may vary in different times and locations. However, temporal and spatial variation of background noise before and after an earthquake has not been studied yet. The existing research has observed gravitational disturbance signals before major earthquakes, but it is difficult to capture them directly from the original gravity data without pretreatment.

Permutation entropy(PE)can characterize the randomness of the signal or detect signal mutation, and has been widely used in biomedicine, finance, mechanical vibration time series. In this study, we use PE to detect gravitational disturbance signal from raw data and study the background noise changes before and after the earthquake.

In this paper, 1Hz sampling records from 15 continuous tidal gravimeters(including the types from PET/gPhone to OSG)of Continuous Gravity Network of China, with spanning from 1th Jan to 30th June, 2021, were obtained and analyzed. Firstly, A bandpass filter(0.1~0.18Hz)was employed to extract gravity disturbance, after removing firstly the earth tides and atmospheric effects with the DDW model and a barometric admittance(-0.3×10^-8m·s^-2/mbar). The short time Fourier transform was used to determine the time-frequency characteristics of non-tidal gravity signals. Then, we calculated the PE and background noise magnitude(SNM)in seismic frequency band(200~600s)of the records. The results show that: 1)All kinds of gravimeters can record high-precision solid earth tide, and respond well to high-frequency fluctuation signals caused by most earthquakes above magnitude 6 in the world. 2)There was a set of gravity disturbance signals recorded by most stations on May 15~18th, and there were two other sets of disturbance signals at coastal stations. The disturbance amplitude was ±(10~100)μGal, and there was no evidence to show that the smaller the epicenter distance, the larger the disturbance amplitude. The large disturbance amplitude of coastal stations and the other two groups of disturbances may be related to sea wave pulsation or local rainfall. 3)The PE value of the original gravity record basically oscillated near a high value, in which the spring gravimeter was close to 1, and the superconducting gravimeter was 0.7. From this point of view, we find that the signal-to-noise ratio of superconducting gravimeter is higher than spring gravimeter; Without any preprocessing procedure on original gravity records, PE could effectively explore the abrupt-change signal in the records. The continuous and significant drop of PE corresponded to perturbation signal before the earthquake, and the instantaneous downward pulse of PE agreed with the tremor signal caused by the seismic wave. 4)We find that the background noise had a clear upward trend occurring two months(early March)before the earthquake; The spatial distribution of SNM indicates that the background noise of the gravimeter located in the northern Qinghai-Tibet Plateau and the Bayan Har block has a significant increase before the earthquake. 5)We preliminarily speculate that the preseismic gravity disturbance is the low-frequency tremor generated by the slow slip of seismogenic fault, and the increase in background noise before the earthquake may be related to the increased activity of seismogenic tectonic block.

COMPREHENSIVE ANALYSIS OF GRAVITY AND CRUSTAL DEFORMATION OF JIASHI M_S6.4 EARTHQUAKE IN 2020

ZHU Zhi-guo, ZHU Yi-qing, WANG Dong-zhen, HUSAN Irxat

2023, 45(1):  269-285.  DOI: 10.3969/j.issn.0253-4967.2023.01.015

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The intracontinental orogeny of Tianshan Mountains has led to strong tectonic activity in the southern Tianshan seismic belt, and strong earthquakes occur frequently at the junction of basins and mountains. On January 19, 2020, an M_S6.4 earthquake occurred in Jiashi County, Xinjiang Uygur Autonomous Region. This earthquake is of north-dipping thrust motion with a small amount of strike-slip component. The seismogenic structure of the earthquake is the Keping Fault on the southernmost edge of the Keping Tage thrust nappe in the South Tianshan Mountains. The Keping Fault has a total length of about 460km. The overall strike of the fault is near EW, dipping northwest, and the dip angle is more than 45°. It is the boundary fault between Tianshan Mountains and Tarim Basin in Southwest China. The Jiashi M_S6.4 earthquake is an important earthquake, which broke the 17 years quiet period of M_S6.0 earthquakes since 2003. It is of great value for study on the evolution of regional tectonic deformation. The process of earthquake preparation will be accompanied by changes in geophysical field. Comprehensive analysis of geophysical observation data before and after earthquakes has great practical significance for geodynamics research and identification of earthquake precursor anomaly information. The epicenter of Jiashi earthquake is located in the monitoring area of the “Kashi-Jiashi” mobile gravity observation network and the “Crustal Movement Observation Network of China” GNSS observation. In this paper, the mobile gravity and GNSS observation data from 2015 to 2021 are selected. The mobile gravity observation network uses absolute gravity point constraint to carry out the classical adjustment calculation of the whole network, and then obtains the gravity field change image; The GNSS data are solved by GAMIT/GLOBK software to obtain the movement rate of the study area, and the horizontal apparent strain field distribution is obtained by means of the partial derivative relationship between displacement and strain. By analyzing the dynamic change characteristics of gravity field, velocity field and strain field in the seismogenic area, the relationship between the change of gravity field and GNSS deformation field and the seismogenic process of Jiashi M_S6.4 earthquake is comprehensively discussed. The results show that: 1)The time-varying images of the gravity field on the time scale of one year better reflect the evolution process of the regional gravity field system near the Jiashi M_S6.4 earthquake. The “0” contour of gravity change and its four quadrant characteristics provide useful references for earthquake prediction. 2)The cumulative change of gravity field reflects that the change of regional gravity field before the Jiashi M_S6.4 earthquake was controlled by regional large faults. A series of medium and strong earthquakes in the study area occurred in the process of gravity reverse change. The characteristics of high gradient belt of gravity change are closely related to earthquake occurrence. According to the gravity cumulative change and time-varying image, it is speculated that this tectonic activity may have started in 2018. 3)Strong earthquakes are likely to occur on tectonic active fault zones with significant gravity changes. The Keping earthquake of M=6.4 in 2020 occurred in the area where the high gradient zone of gravity changes turns, which is also the transition zone of surface compressibility changes. The variation process of regional strain field corresponds to the positive and negative cumulative changes of the regional gravity field.

THE COSEISMIC RUPTURE MODEL AND STRESS CHANGE OF THE 2022 MENYUAN M_W6.7 EARTHQUAKE

YU Shu-yuan, HUANG Xian-liang, ZHENG Hai-gang, LI Ling-li, LUO Jia-ji, DING Juan, FAN Xiao-ran

2023, 45(1):  286-303.  DOI: 10.3969/j.issn.0253-4967.2023.01.016

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On January 8, 2022, an earthquake of M_W6.7 occurred in Menyuan County, Qinghai Province. The epicenter of the earthquake is located in the middle eastern section of the Qilian Mountains seismic belt on the northeast edge of the Qinghai-Tibet Plateau. Under the northward push by the Qinghai-Tibet Plateau plate, and the push and subduction in the northeast direction of the Qilian Mountains, and also blocked by the Alxa block and affected by the push in the southwest direction of the Longshou Mountains uplift area at its front edge, a ramp structural pattern of compressive depressions was formed in the Hexi Corridor basins area. As a result, most of the active faults in this area are mainly NW-trending, and the active features are mostly characterized by compressional thrusting and strike slip. This paper reconstructs the coseismic deformation field of Menyuan earthquake through the European Space Agency Sentinel-1A C-band radar satellite data and D-InSAR technology, determines the geometric characteristics of the seismogenic fault through the inversion method of optimum fault slip distribution, and determines the seismogenic fault of this earthquake. The results show that the deformation range of the radar line of sight is -0.42~0.7m for the ascending track deformation field and -0.63~0.72m for the descending track deformation field, and the maximum deformation locates in the Lenglongling section. The data of ascending and descending tracks show that there are two obvious deformation regions with a butterfly-like stripe pattern. The sign of LOS deformation variable observed in InSAR deformation field of ascending and descending orbits in the same area is opposite. Combined with the flight direction of ascending and descending satellites, it is determined that the motion of seismogenic fault is mainly left-lateral strike slip. Among them, the Lenglongling Fault and Tolaishan Fault pass through the fracture surface revealed by InSAR deformation field, which means that the above fault is highly likely to be the seismogenic fault of the Menyuan earthquake in 2022. At the same time, the SE-trending Lenglongling Fault on the east side passes through the fracture surface, with a surface fracture length of about 20km. The EW-trending Tolaishan Fault on the west side also passes through the fracture surface, with a fracture length of about 5km. And then, according to the field geological survey results of this earthquake, taking the InSAR coseismic deformation field data as constraints and based on Okada elastic dislocation model, the geometric structure of the seismogenic fault and the fine slip distribution characteristics of the fracture surface are determined. The inversion results reveal that there are two slip regions, of which the slip is mainly concentrated in the Lenglongling fault section, with a maximum left-lateral slip of 3.66m and a maximum slip depth of 5km. There is also a maximum sinistral slip of 1.95m occurring at a depth of 5km in the Tolaishan Fault. It is inferred that the seismogenic fault is the western section of Lenglongling Fault which also ruptured the Tolaishan Fault on its west.

On this basis, Coulomb33 software is used to calculate the static Coulomb stress changes generated by the Menyuan earthquake at different depths(5km, 10km, 15km and 20km). The Coulomb stress change image within 300km of the epicenter shows a typical four-quadrant distribution characteristic. There are four fan-shaped stress increase and decrease areas at 5km underground. The area with the largest increase in stress is near the Beiyuan Fault of Tuole Mountain in the west of the epicenter of Menyuan earthquake. The increase of stress is over 0.03MPa, greater than the trigger threshold of 0.01MPa. The stress increase coverage area in the south of the epicenter further expanded, with a stress increase of more than 0.03MPa, inducing many aftershocks distributed linearly in the NWW direction along the epicenter. According to the overall analysis, most of the subsequent earthquakes in Menyuan occur at a depth of 10~12km, which is in good agreement with the stress increase area at the corresponding depth. At the same time, for the NW-SE area and NE-SW end of the rupture in the epicenter, the area with ΔCFS≥0.01MPa is worthy of attention for the subsequent risk.

Finally, based on the GPS velocity field relative to the Ordos block, it is analyzed that the Lenglongling area moves in the NE direction relative to the Ordos block as a whole, the GPS velocity vector north of the Lenglongling Fault decreases, and the movement direction turns to NNW. Using GPS velocity field to calculate the principal strain rate, shear strain rate, surface strain rate and principal compressive stress in Lenglongling area, it is shown that there is a significant high value area of surface strain in Lenglongling area. The principal strain rate is NE compression, and the peak value of shear strain rate is located on the north side of Lenglongling Fault and the east section of Minle-Damaying Fault. Its strain accumulation indicates that the area is still in a high stress state, and the seismic activity may continue to be strong in the future. The regional surface strain rates show obvious compression characteristics, and the principal strain rates show NE-SW compression and NW-SE extension. Overall, the source area of the Menyuan earthquake is still under the push in the NNE direction of the eastern Himalaya syntaxis of the Indian plate. It can also be seen that the Menyuan earthquake occurred in the high value area of the maximum shear strain rate and the compression area of the surface strain. The occurrence of the Menyuan earthquake in the Lenglongling area of the North Qilian Mountains on the northeast edge of the Qinghai-Tibet Plateau and its current high stress accumulation indicate that the Lenglongling Fault may still be active today.

On this basis, the seismogenic structural characteristics and seismogenic relationships of the two Menyuan earthquakes in 2016 and 2022 are further discussed. The 2016 Menyuan earthquake is located on the extension line of the Minle-Damaying Fault. The seismogenic fault is a SW-trending thrust fault. The fault extends in NW-SE direction on the surface along the front of the mountain, and its deep part may converge to the detachment layer at the bottom of the Qilian Mountains together with the Lenglongling Fault. The fault has the potential to generate destructive earthquakes. The 2016 M_W5.9 Menyuan earthquake and the 2022 M_W6.7 Menyuan earthquake have different seismogenic mechanisms, but the seismogenic faults all belong to the North Qilian Mountains active fault zone, most of which control the boundary of the Neogene basins. Both earthquakes are the local adjustment of stress accumulation in the region as a whole, and the expression of the northeastward pushing of the Qinghai Tibet Plateau. Some scholars believe that Lenglongling fault zone, Jinqianghe Fault, Maomaoshan fault zone and Laohushan Fault jointly constitute the “Tianzhu earthquake gap”. The occurrence of three Menyuan earthquakes in 1986, 2016 and 2022 has drawn continuous attention to the fault activity and seismic risk of this area.