Table of Content

20 August 2023, Volume 45 Issue 4

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Review

MORPHOLOGIC DATING OF SCARP MORPHOLOGY BASED ON DIFFUSION EQUATION: A REVIEW

XU Jian-hong, CHEN Jie, WEI Zhan-yu, LI Tao

2023, 45(4):  811-832.  DOI: 10.3969/j.issn.0253-4967.2023.04.001

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A scarp is a common step-like landform in nature, which consists of a gently sloping plane connected to the upper and lower geomorphic surfaces of differing elevations. Common scarps include fault scarps, terrace scarps, lake shoreline scarps, shoreline scarps, volcanic ash cinder cones, etc. Scarps are often used as strain markers because of their linear characteristics and are favored in the study of active tectonics. However, it is difficult to directly constrain their ages. Instead, they are usually constrained by the ages of the upper and lower geomorphic surfaces. The scarp developed in loose deposits is controlled by a long process of low-energy degradation after a short collapse. This process can be modeled by the diffusion equation because the process can be considered as a slope process under the transport-limited condition. Under this condition, the slope can provide enough loose material for transport, that is, the material transport capacity is less than the material supply capacity. If process assumptions are sufficiently valid and rate constraints can be calibrated independently, the true age of scarps can be obtained. This method is called morphologic dating. This method has been included in many textbooks published overseas, but there have very little research on this method in China. Both linear and nonlinear models have been developed to describe scarp degradation. Linear diffusion models assume that the diffusion coefficient is a constant, whereas nonlinear transport models generally define the diffusion coefficient as a nonlinear function related to the topographic gradient. Compared to the linear transport models, nonlinear transport models can better explain the phenomenon of rapidly increasing deposition flux as the gradient approaches a critical value. In this paper, we review the study history of scarp degradation analysis and the concept model of scarp degradation. We focus on the establishment of the nonlinear model, the role of the different parameters in profile evolution, determining the best-fit age using a full-scarp nonlinear modeling procedure, and so on. Furthermore, we introduce the model of the nonlinear age chart, including the effect of far-field slope on morphologic dating of scarp-like landforms and two examples of the application of the chart, which shows that this method can correctly evaluate the ages of single-event scarps. Finally, we discuss the extension of the concept and method of the scarp degradation model, the applicability of the model, and repeated fault scarp morphological analysis. For nonlinear diffusion models, in addition to n equal to 2, two parameters (critical gradient (S_c) and diffusion constant (k)) need to be constrained. The critical gradient can be obtained from the young scarps in the study area, which roughly represents the initial state of scarp evolution, typically 0.6 to 0.7(30° to 35°). The diffusion constant needs to be characterized by a known age scarp. The slopes of the upper and lower geomorphic surfaces have an obvious influence on the morphology of a degraded scarp. These discussions indicate that both linear and nonlinear models can be used for the degradation analysis of single-event scarps, but a nonlinear diffusion model is recommended for young single-event scarps. The constant slip rate nonlinear model can be used to simulate the evolution history of<10ka high-slip rate active fault scarp. The multiple-event scarp model requires careful evaluation of the fault location and the amount of displacement per event. There are several assumptions in the scarp topography diffusion modeling, which require practice to verify its reliability. With advances in surveying technology, it is now possible to rapidly obtain high-resolution terrain data over broad areas from which numerous topographic profiles can be efficiently extracted. This provides a broad application prospect for scarp degradation analysis and morphologic dating.

Application of new technique

APPLICATION OF SMALL UNMANNED AERIAL VEHICLE(sUAV)IN THE SELECTION OF SUITABLE SITES IN PALEO-SEISMIC STUDY OF BEDROCK FAULT SURFACES

ZOU Jun-jie, HE Hong-lin, ZHOU Yong-sheng, WEI Zhan-yu, SHI Feng, GENG Shuang, SU Peng, SUN Wen

2023, 45(4):  833-846.  DOI: 10.3969/j.issn.0253-4967.2023.04.002

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Bedrock normal fault scarps, as classical topographic features and geomorphological markers along mountain range fronts, form in consolidated bedrock due to faulting in extensional settings. They generally preserve more complete records of paleo-earthquakes than fault scarps in unconsolidated sediments. With the development of technologies such as fault surface morphology measurement and terrestrial cosmogenic nuclide dating, bedrock fault planes have become a nice object for paleo-earthquake study in bedrock areas. The reconstruction of paleo-seismic history from a bedrock fault scarp in terms of the times, co-seismic slips and ages by a combination of quantitative morphological analysis, TCNs dating and other physical/chemical index has been proven feasible by several previous studies.

However, this success heavily relies on a suitable site selection along the bedrock fault scarp because erosional processes can exhume the bedrock fault surface, and the sedimentary processes can bury the bedrock fault surface. Namely, non-tectonic factors such as gully erosion, sediment burial, and anthropogenic activity make bedrock fault planes difficult to record and preserve paleo-seismic information.

Therefore, to successfully extract paleo-seismic information from the bedrock area, it is necessary to select suitable study points along the bedrock fault scarp in advance. Traditional survey and mapping methods are time-consuming and labor-intensive, and it is difficult to understand bedrock fault scarps. The resolution of satellite images cannot obtain the fine structure of bedrock fault scarps. Small unmanned aerial vehicle(sUAV), combined with Structure-from-Motion(SfM)photogrammetry has emerged over the last decade. It is used as an established workflow in acquiring topographic data by filling the spatial gap between traditional ground-based surveys and satellite remote sensing images. As a low-altitude photogrammetry technology, it can quickly obtain high-precision three-dimensional surface structures of bedrock fault scarps.

In this paper, taking the Majiayao bedrock fault scarp at the northern foot of Liulengshan in Shanxi Rift as an example, the high-precision and three-dimensional topographic data of the bedrock fault was obtained by using sUAV combined with SfM photogrammetry technology. The high-resolution and high-precision images of tectonic landforms can be obtained conveniently and efficiently by sUAV survey. The sUAV-obtained photos can be further processed by the SfM photogrammetry for generating a digital 3D structure of the bedrock fault scarp with true or shaded color.

The non-tectonic factors such as rock collapse, sediment burial, and gully erosion along the bedrock fault scarp are identified by interpreting the 3D model of the bedrock fault scarp. The profile shape characteristics of the erosion, burial and tectonic fault scarps are summarized through fine geomorphological interpretation and fault profile analysis. For the erosion profile, the hanging wall slope is down-concave, showing that the fault surface below the ground surface has been partially exposed. For the bury profile, the hanging wall slope shows an obvious concave-up shape, indicating that the lower part of the bedrock fault surface has been partially buried by the colluvium. For the tectonic profile, the hanging wall slope shows a smooth and stable slope, showing the exhumation of bedrock fault scarp is controlled purely by tectonics. Finally, the study sites suitable for paleo-earthquake study on bedrock fault surfaces were selected, showing the important role of sUAV aerial survey technology in the selection of paleo-earthquake study sites in bedrock areas.

This study illustrates that based on the high-precision three-dimensional surface structure of the bedrock fault plane from sUAV aerial survey, the existence of non-tectonic factors such as gully erosion, sedimentary burial and bedrock collapse can be clearly identified. These non-tectonic sites can be excluded when selecting suitable sites for paleo-earthquake study indoors. The shape analysis of bedrock fault scarp is also helpful to determine whether the bedrock fault surface is modified by surface process and suitable for paleo-seismic study. The sUAV aerial survey can play an important role in paleoseismic research in the bedrock area, which can accurately select the study points suitable for further paleo-seismic work in the bedrock area.

QUANTITATIVE ASSESSMENT OF SEISMIC DAMAGE IN TYPICAL SCENARIOS BASED ON OBLIQUE AERIAL PHOTOGRAPHY: THE CASE OF THE MAERKANG EARTHQUAKE SWARM, SICHUAN, CHINA

XIAO Ben-fu, YUAN Xiao-xiang, CHEN Bo, ZHANG Lu-lu, LIANG Yuan-ling, QI Yu-ping, YANG Lu-yao, LIU Yang

2023, 45(4):  847-863.  DOI: 10.3969/j.issn.0253-4967.2023.04.003

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Post-earthquake disaster information extraction and quantitative evaluation are the foundations of earthquake relief work. The effectiveness of its transmission mode and the accuracy of evaluation results directly affect the efficiency of post-earthquake emergency analysis, emergency response decision-making, and earthquake relief. In recent years, the nationally targeted poverty alleviation, reinforcement of housing facilities in earthquake-prone areas, and the rapid promotion of urbanization have greatly improved the overall seismic protection capability of buildings, and the reference indexes of seismic damage assessment have changed from the previous focus on the damage indexes of the roof of the disaster-bearing body to the comprehensive damage indexes of the roof of the disaster-bearing body, external walls, and subsidiary structures. The traditional two-dimensional damage assessment method based on a quasi-vertical perspective can hardly meet the requirements of an accurate quantitative assessment of earthquake damage at this stage.

With the rapid development of aerial photography technology, the efficiency and accuracy of earthquake hazard information extraction and quantitative earthquake hazard assessment have been greatly improved. The three-dimensional seismic damage scenario visualization model based on UAV oblique photography technology has the advantages of multi-angle, high accuracy, and rich texture, which can accurately reflect the characteristic differences of seismic disasters of disaster-bearing bodies. It can be used to realize multi-dimensional and high-granularity seismic damage information extraction, thus effectively improving the accuracy of a quantitative assessment of single-time and space-time seismic damage, and can provide scientific and technological support for practical quantitative assessment of seismic damage.

On June 10, 2022, a M6.0 magnitude earthquake swarm occurred in Maerkang City, Ngawa Tibetan and Qiang Autonomous Prefecture(also known as Aba Prefecture), Sichuan Province, in which the epicenter locations of three earthquakes of M5.8, M6.0 and M5.2 were located in Caodeng Town, Maerkang City, Aba Prefecture, Sichuan Province, with source depths of 10km, 13km and 15km, respectively. After the swarm, the Sichuan earthquake agency initiated a level Ⅱ emergency response, and the earthquake site working group rushed to the earthquake site with UAV equipment to carry out oblique photography of typical buildings, geological hazards, and other earthquake damage.

In this study, the quantitative evaluation process of typical scenario visualization seismic damage based on oblique photography technology is constructed. Based on the texture, spectrum, shape, position, and combination of UAV remote sensing images, the interpretation signs of scenario visualization seismic damage are established. Taking the Maerkang 6.0 earthquake swarm in Sichuan on June 10, 2022 as an example, the seismic damage information of typical scenario visualization houses in the meizoseismal area is extracted, and the quantitative evaluation of seismic disaster in the meizoseismal area is realized. At the same time, the feasibility and accuracy of the method are verified by the field seismic damage investigation results. The results show that: 1)the typical scenario visualization model constructed based on oblique photography can reflect the earthquake damage information on the top, exterior walls and bottom of buildings, which can intuitively reflect the earthquake damage aftershocks. Compared with the traditional post-earthquake remote sensing images from a quasi-vertical perspective, this model can more effectively extract the basic information and seismic damage information of the building hazard-bearing body. 2)According to the equivalent seismic damage index, the seismic damage degree of 520 houses in 3 types of building structures is quantitatively evaluated. Among these the equivalent seismic damage index of a Tibetan stone-wood structure is 0.60, the equivalent seismic damage index of a brick-concrete structure is 0.44, and the equivalent seismic damage index of a reinforced concrete frame structure is 0.37. The seismic intensity of the study area is determined to be Ⅷ degree(8 degree), which is consistent with the field survey results. The OA values and Kappa coefficients of seismic damage extraction based on visualization and field survey were 92% and 0.87, respectively, while the OA values and Kappa coefficients of seismic damage extraction based on UAV orthophotograph and field survey were 45% and 0.25, respectively, which showed that the seismic damage extraction based on visualization was more accurate than that based on UAV orthophotograph in terms of recognition accuracy and precision. Compared with the quantitative assessment method of seismic damage based on UAV orthophotograph, the quantitative assessment method of earthquake damage based on scenario visualization is more effective in terms of recognition accuracy and precision. The typical scenario-based visualized seismic damage extraction method based on oblique photography technology provides a new idea for high-precision UAV remote sensing data for building damage extraction work, and its extraction results can be used as a reference basis for seismic intensity assessment, earthquake emergency rescue and personnel command and dispatch.

Research paper

THE EMPIRICAL GREEN'S FUNCTION-BASED SIMULATION OF NEAR-FIELD STRONG GROUND MOTIONS OF THE 2021 YANGBI EARTHQUAKE(M_S6.4), YUNNAN, CHINA

WU Hao, IRIKURA Kojiro, LIN Guo-liang

2023, 45(4):  864-879.  DOI: 10.3969/j.issn.0253-4967.2023.04.004

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Both short(0.1s)and long(1~2s) periods ground motions were observed at the near-field strong-motion stations during the mainshock of the 2021 M_S6.4 Yangbi earthquake, Yunnan. As there are only two strong-motion stations observed by the National Strong Motion Observation Network System of China in the near-field, the strong ground motion records at seven intensity stations observed by the Local Earthquake Early Warning Network of Yunnan Province are combined to construct the characterized source model with the empirical Green's function method. The broadband strong ground motions in the near-field are synthesized by using the characterized source model. We firstly select out an M_W4.3 foreshock as the empirical Green's function event. Then, we calculate the observed source spectral ratios at seven stations assuming that the source spectrum obeys to the ω^-2 law. The results show that both the observed source spectral ratios in the frequency of 0~0.2Hz at strong-motion stations and 0~0.5Hz at intensity stations deviate from the ω^-2 law assumption. Thus, we determine the size of the sub-fault by fitting the observed source spectral ratio averaged at two strong-motion stations to the theoretical source spectral ratio in the frequency of 0.2~30.0Hz. Taking two strong-motion stations and four intensity stations as the target stations, we adopt the simulated annealing method to determine the optimal parameters required for ground motion synthesis, such as proportional parameters N and C, the position of rupture starting point within the strong motion generation area, rupture velocity, and rise time, when the misfit function between synthesized and observed ground motions reach the minimum at the target stations. We fix the rupture starting point at the hypocenter of the mainshock, and determine N=4, C=3.69, rupture velocity of 2.5km/s, and rise time of 0.56s. In this study, the characterized source model used for the strong ground motion simulation consists of one strong motion generation area which is 5.4km along the strike direction and 5.4km along the dip direction, and with the stress drop of the strong motion generation area of 12.8MPa. In general, the ground motions synthesized by the characterized source model and those optimal parameters are comparable with the observed ground motions at the target stations. Further, we apply those optimal parameters to synthesize ground motions at other three intensity stations which are not taken as the target stations. In consideration of the different instrumental responses in the strong-motion and intensity stations, we impose different frequency band of bandpass filters on both the observed and synthesized ground motions, i.e. the frequency band is 0.2~30.0Hz for the ground motions at the strong-motion stations, while the frequency band is 0.5~30.0Hz for the ground motions at the intensity stations. The ground motions of the mainshock in the 53YBX strong-motion station are characterized by the large amplitude of the response spectrum around 0.1s which obviously exceeds the level of design response spectrum for rare earthquake, and the pulse(~1s)is observed in the east-west direction. The synthesized ground motions in this study are in good agreement with the above characteristics, except that the amplitude of the synthesized ground motions around 1s is smaller than the observed one, which may be caused by the small amplitude of the empirical Green's function. Moreover, the long-period(~2s)ground motions in the 53DLY strong-motion station are reproduced by the synthesized ground motions. On the other hand, the synthesized ground motions in the frequency band of 0.5~30.0Hz agree well with the observed ground motions in the intensity stations with sufficiently large signal-to-noise ratios. Finally, we examine the relationships between two source parameters and the seismic moments. The strong motion generation area and the flat amplitude of the acceleration source spectrum in the short period range are found to have linear relations with the seismic moment, which is consistent with the empirical scaling relationships. In the future work we will continue to examine the applicability of the empirical scaling relationships for prediction of strong motions by analyzing more earthquakes with different magnitudes in China.

NEW PROGRESS IN PALEOEARTHQUAKE STUDIES OF THE JIANGSU SEGMENT OF THE ANQIU-JUXIAN FAULT IN THE TANLU FAULT ZONE

ZHANG Hao, LI Li-mei, JIANG Xin, ZHANG Dong, XU Han-gang

2023, 45(4):  880-895.  DOI: 10.3969/j.issn.0253-4967.2023.04.005

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Paleoseismology is a discipline that studies prehistoric earthquakes or earthquakes that occurred before instrumental records using geological and geomorphological methods, mainly by trench excavation and Quaternary chronology. It focuses on the time and intensity distribution of large earthquakes, to reveal the recurrence characteristics of large earthquakes and provide basic data for evaluating the probability of future earthquakes. The Tanlu fault zone is the most active fault zone in eastern China. The Jiangsu section of the Tanlu fault zone is mainly composed of five branch faults, which are strongly active in the Pleistocene. Among them, the Anqiu-Juxian Fault continued to be active until the Holocene, which is the seismogenic fault of the Tancheng 8½ earthquake in 1668. The Xinyi-Sihong section is likely to generate strong earthquakes in the future in the south-central section of the Tanlu fault zone.

The total length of the Jiangsu section of the Anqiu-Juxian Fault is about 170km, with an overall strike of 5°~15°, extending southwards from the north Maling Mountain to the Chonggang Mountain. The geomorphic features are distributed from north to south by the alternation of the bedrock mountain and the sedimentary basin. The Anqiu-Juxian Fault shows a single exposed fault on one side of the bedrock mountain, extending to the basin into two branches in the east and west, of which the east branch is the active late Pleistocene Fault and the west branch is the Holocene active fault. The Jiangsu section of the Anqiu-Juxian Fault is dominated by dextral strike-slip and has both dip and thrust components.

Lots of research have been done on the ancient earthquakes of the Anqiu-Juxian Fault. The trenches are mostly located in Maling Mountain, Zhangshan Mountain and Chonggang Mountain, which are in the state of uplift and denudation. The Holocene is very thin, and the dating method is mostly optical luminescence. The identification of ancient earthquake events is less since the Holocene, with the accuracy of ancient earthquake time not high and the ancient earthquake sequence not complete. According to the topographic and geomorphological characteristics of the Jiangsu section of the Anqiu-Juxian Fault, three trenches were excavated along the Anqiu-Juxian Fault, of which two were in exposed areas and one in a buried area. Three trenches completely revealed the Holocene sedimentary strata in the Jiangsu section of the Anqiu-Juxian Fault, in which MLTC2 revealed the early Holocene strata, MLTC1 revealed the middle Holocene strata, and HSTC revealed the late Holocene strata. The determination of the age of earthquake events is one of the most uncertain factors in the study of paleoearthquakes and is the main indicator of the recurrence period of paleoearthquakes. At present, most of the paleoearthquake events studied have occurred since the late Pleistocene, and the accuracy of ¹⁴C dating is the highest. A total of 13 ¹⁴C samples were collected from the trenches. Combined with the paleoearthquake events and time revealed by previous trenches, it is concluded that there have been three paleoearthquake events in the Jiangsu section of the Anqiu-Juxian Fault since the Holocene, with theelapsed time of ~3000aBP, ~6000aBP and ~11000aBP, and the coseismic vertical offset are all nearly 1m.

The 1668 Tancheng M8½ earthquake showed signs of surface ruptures in the exposed area of the Xinyi section of the Anqiu-Juxian Fault, accompanied by a large amount of sandblasting and water gushing in the buried area. Dense fissures and sand veins are observed in the late Holocene strata overlying the fault, indicating the impact of the 1668 Tancheng earthquake. More representative chronological data are needed as to whether the 1668 Tancheng earthquake ruptured Suqian City.

COMPARISON OF THE EFFECTS OF EARTHQUAKE-TRIGGERED LANDSLIDE EMERGENCY HAZARD ASSESSMENT MODELS: A CASE STUDY OF THE LUSHAN EARTHQUAKE WITH M_W5.8 ON JUNE 1, 2022

MA Si-yuan, XU Chong, CHEN Xiao-li

2023, 45(4):  896-913.  DOI: 10.3969/j.issn.0253-4967.2023.04.006

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Earthquake-induced landslides, as an important secondary geological disaster, typically occurring during or shortly after an earthquake, have the characteristics of large quantity and scale, wide distribution, complex mechanism, serious casualties and economic losses, and long-duration post-earthquake effect. Rapidly and accurately obtaining the spatial distribution and potential hazard assessment of coseismic landslide following an earthquake is critical for emergency rescue and resettlement planning. Currently, the most commonly-used coseismic landslide hazard assessment methods include the data-driven machine learning methods and the Newmark method based on mechanics mechanism. The 2022 M_W5.8 Lushan earthquake provides a valuable window for us to carry out rapid emergence assessment of earthquake-induced landslides with different evaluation models. In this study, a new generation of China's earthquake landslide hazard model(hereinafter referred to as Xu₂₀₁₉ model)and a simplified Newmark model are used to carry out the rapid landslide assessment of Lushan event. The Xu₂₀₁₉ model selects 9 earthquake-induced landslide inventories around China as training samples and uses a total of 13 influencing factors such as elevation, relative elevation, slope angle, and aspect, and etc. to generate a near real-time evaluation model for coseismic landslides based on the LR method. The model can rapidly assess coseismic landslides towards a single earthquake event according to the actual PGA distribution. For Newmark model, the cumulative displacement(Dn)is calculated by the critical acceleration(a_c)and PGA maps. For the landslide inventory of this earthquake event, we completed the landslide inventory covering the entire affected area based on high-resolution optical satellite images(Planet)with 3m resolution acquired on 6 July 2022. Based on the coseismic landslide inventory including 2 352 landslides with an area of 5.51km², the accuracy and applicability of the two models are estimated. The results show that the landslide area calculated based on Xu₂₀₁₉ model is 5.07km², which is very close to the actual landslide area, and the predicted area calculated based on Newmark model reaches 21.3km². From the perspective of the spatial distribution of the prediction results, the distribution of the predicted high failure probabilities of the two models is roughly same, with the high probability values mainly located on the left side of the seismogenic fault. However, the difference lies in the low probability predictions of the northwest region of Baoxing county by the Xu₂₀₁₉ model. A zoomed-in view of a specific area comparing the spatial distribution of predicted landslide probabilities with the landslide abundance area shows that most actual landslide are concentrated in the medium to high failure probability areas predicted by the Xu₂₀₁₉ model, with only a few sporadic events occurring in the low probability zone. On the other hand, the Newmark model primarily identifies high instability probability regions in steep slope areas, which correspond closely to the actual landslide and collapse occurrences. However, the predicted hazard level of the northwest region i.e. the landslide highly developed area is obviously low by Xu₂₀₁₉ model, while the prediction result based on Newmark model for the southwest region is obviously overestimated. In terms of the LR model, the prediction results are very close to the actual landslide distribution, and the majority of the landslides are essentially located in areas with a high failure probability, indicating that the model has a relatively high prediction accuracy. The ROC curve is used to assess the model's accuracy. The results suggest that the model based on Xu₂₀₁₉ outperforms the Newmark model, with a prediction accuracy of 0.77, while the prediction accuracy of the Newmark model is 0.74. Overall, both two models have good practicability in the rapid evaluation of cosesimic landslide. However, the Newmark model needs multi parameter input, and these parameters themselves and the way of human acquisition are uncertain, which results in that the model evaluation is greatly affected by subjectivity.

THE ANALYSIS OF THE STARTING YEAR AND THE COM-PLETENESS OF SEISMIC RECORDS FOR EARTHQUAKES WITH MAGNITUDES M≥7 IN THE BORDER REGION OF SICHUAN, YUNNAN AND TIBET

ZHOU Jie-yuan, ZHOU Qing, RAN Hong-liu

2023, 45(4):  914-935.  DOI: 10.3969/j.issn.0253-4967.2023.04.007

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Earthquake catalog is the foundational data for analyzing seismic activity, assessing seismic hazard, and studying earthquake prediction. The majority of historical earthquake records are sourced from historical documents, with a significant portion of these records found in local gazetteers. Compiling historical literature is an essential way in analyzing seismic activity because historical accounts of earthquakes often provide more detailed and accurate information than geological data. Among these sources, analyzing relevant content in local gazetteers, such as the historical development of local governance, military garrison, official records, and descriptions of disasters and auspicious events, plays a crucial role in seismic activity research. This article aims to acquire historical earthquake records by consulting local gazetteers, folk books, and other historical sources containing natural, social, and political records. These records serve as historical foundations for analyzing the completeness of seismic data records.

The border region between Sichuan, Yunnan, and Tibet is located in the northwest secondary block of the Sichuan-Yunnan block, which is one of the areas with frequent strong earthquakes in China. The Xianshuihe fault zone and the Jinshajiang fault zone are the northeastern and northwestern boundary faults of the Sichuan-Yunnan block, respectively. They are large-scale and highly active fault zones formed due to the eastward escape of the Tibetan plateau caused by the relative movement between the Indian and Eurasian plates. Previous studies on active tectonics have shown that major earthquakes with magnitudes of 8 and above, as well as over 80% of strong earthquakes with magnitudes of 7, mainly occur in the boundary zones of active blocks with intense structural deformation and high stress accumulation. Moreover, the known active faults in the study area, such as the Batang fault and Litang fault, are also major faults that significantly have influence on the occurrence strong earthquakes. The Sichuan-Yunnan-Tibet adjacent region is home to significant infrastructure, including the Sichuan-Tibet railway and hydropower stations. Analyzing the completeness of earthquake data in the border region of Sichuan, Yunnan, and Tibet can contribute to the assessment of fault hazards and the analysis of regional seismic activity trends. This, in turn, can help minimize the damage caused by earthquakes to critical infrastructure and further enhance the safety and security of people’s lives and properties.

This study reviewed the local gazetteers of 44 counties in the border region between Sichuan, Yunnan, and Tibet, and summarized the establishment and historical evolution of each county. Based on the analysis of the road evolution from Sichuan to Tibet and from Yunnan to Tibet, we examined the significant roles of important transportation hubs and nodes, such as stations, pond flood, and grain platforms, in regarding of recording earthquakes. Combining various historical sources and previous research on the completeness of earthquake data in the region, we conducted a comprehensive analysis to determine the probable starting years for the availability of seismic records of magnitude 7 and above in the Xianshuihe area and the three parallel rivers area. Additionally, based on the data of the length and short axis of isoseismal lines from 88 earthquakes, an elliptical model was used to derive the seismic intensity attenuation relationship for the Sichuan-Yunnan block. By placing the fitted isoseismal lines of magnitude 6 and 7 earthquakes in the study area, we analyzed their impact range, providing a spatial dimension basis for the completeness analysis of seismic data.

This article provides a comprehensive analysis and demonstration of the complete starting years of seismic data in the border region between Sichuan, Yunnan, and Tibet from both temporal and spatial perspectives. The results indicate that due to the establishment of grain stations and Tangxun along the Sichuan-Tibet road, as well as the appointment of officials, several counties in the Xianshuihe area, including Kangding, Luhuo, Garzê, Litang, and Yajiang, were developed between 1719 and 1736. At the same time, there are relatively abundant historical documents related to earthquakes in the Xianshuihe area. Local chronicles, reports from governors and resident ministers, written records in Tibetan temples, and accounts from lamas have documented earthquake surveys, disaster assessments, and relief efforts. By combining these historical sources with the analysis of intensity attenuation relationships in the Sichuan-Yunnan block, the affected areas of earthquakes with magnitudes 6 and 7 can be determined that the period from 1719 to 1736 marks the starting years with complete M≥7 earthquake data in the Xianshuihe area. The towns of Batang, Mangkang, and Changdu in the three parallel rivers area are also significant nodes and hubs along the road to Tibet. They were established with administrative institutions and granaries between 1719 and 1728, and the road network extensively covered Tangxun in the region. In considering the seismic records and historical sources in the three parallel rivers area, as well as referencing the recording capabilities of granaries, administrative institutions, and Tangxun in the Xianshuihe area, and estimating the potential recorded seismic magnitudes based on the intensity attenuation relationships of the Sichuan-Yunnan block, it can be suggested that the period from 1719 to 1728 is a possible starting point for complete earthquake data with M≥7 in the three parallel rivers area. In areas farther away from the road to Tibet, such as Jiangda, Gongjue, Baiyu, Xinlong, and the northern regions of Batang and Litang, as well as the large contiguous regions of Derong, Xiangcheng, Daocheng, and Jiulong, the eastern boundary is the Xianshuihe fault zone, while the area between the two zones is divided by the northeast-oriented Batang fault. Previous seismic geological investigations have found that within the aforementioned regions, the influence of the Jinshajiang fault zone extends along the Batang-Derong-Benzilan line. In remote areas away from the road and with sparse population, the possibility of individual earthquakes with magnitudes above 7 occurring but being missed cannot be ruled out. However, in other areas not located on active fault zones, it can be considered unlikely to experience earthquakes with magnitudes above 7. Based on the analysis of the data, the starting years of earthquakes with a magnitude of 6 and above should be the same as those of earthquakes with a magnitude of 7 and above. However, according to the analysis of the average occurrence rate of earthquakes per year, there is a significant lack of records for earthquakes of magnitude 6 and above. This may be due to the sparsely populated and vast nature of the Tibetan region during historical times, limited administrative capabilities of officials, and lack of earthquake historical records and documents. Therefore, it is not possible to determine the exact starting year for complete data on earthquakes of magnitude 6, which would be the same as for earthquakes of magnitude 7 and above.

CRUSTAL DENSITY STRUCTURE OF THE EASTERN TIBETAN PLATEAU AND ITS GEODYNAMIC IMPLICATIONS

LI Dan-dan, TANG Xin-gong, XIONG Zhi-tao

2023, 45(4):  936-951.  DOI: 10.3969/j.issn.0253-4967.2023.04.008

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The continuous collision and convergence between the Indian and Eurasian plates have caused strong uplift and deformation within the Tibetan plateau and the surrounding areas. The eastern Tibetan plateau, as an important channel for the eastward and south-eastward expansion of plateau materials, is an critical area for understanding the interaction between the Tibetan plateau and the eastern tectonic blocks and for understanding the eastward escape of plateau deep materials, which is of great significance for studying the uplift and deformation mechanism of the Tibetan plateau. A large number of studies on the eastern Tibetan plateau have provided an important basis for revealing the uplift mechanism of this region. However, its complex geology makes it difficult in understanding the uplift mechanism from the single geophysical interpretation. The gravity field reflects the density properties of the subsurface material, and can be related to the wave velocity properties of the seismic data by certain translation relationships. In addition, gravity data can improve the crustal model of the area not adequately covered by seismic data, which can not only provide the three-dimensional crustal density structure of the area, but also reflect the relationship between the spatial distribution of earthquakes and the crustal structure from a gravity perspective. In this paper, based on the previous research results, we selected field survey gravity data of nine intersecting lines and used the deep seismic reflection as the constraint to invert the density interface depth distribution of each line by using human-computer interaction mode, and then used the kriging interpolation method to obtain the three-dimensional Moho depth and basement depth in the area, and then we obtained the sediment thickness by analyzing the difference between the topography and the basement depth. The inversion results show that the overall trend of Moho depth is deep in the west and shallow in the east, with the deepest depth in the west being 61km and the shallowest in the east being about 40km. There is a large difference between the two sides of the arc belt formed by the Longmenshan-Anninghe-Xiaojinhe fault, with the northwest side of the arc belt basically above 52km, among which the Moho depth is about 58km in the Bayankara block and the northern part of the Chuan-Dian rhombus block, and about 53km in the Chuan-Dian rhombus block and the southern part of the Indo-China block. The Moho depth is about 42km in the Sichuan Basin on the east side of the arc belt, which constitutes a gradient zone of Moho depth around the Tibetan plateau. There also exists a depressional zone of Moho in the Bayankara block, which may be related to the eastward flow of plateau material and the blockage of Sichuan Basin, so that part of the asthenosphere material accumulates and squeezes, thus forming a relatively thicker crust and the sinking of Moho. Part of the eastward overflowing asthenosphere material turns to the south and south-east direction, resulting in the thickness of the crust in the southwest of the Chuan-Dian rhombus block is greater than the east and west sides. At the same time, the late Paleozoic mantle column activity led to the uplift of the lithosphere and the intrusion of high-density material into the lithosphere, which also blocked the southward flow of material from part of the Tibetan plateau. From the inverted sediment thickness, the sediment on the eastern Tibetan plateau is relatively thicker in the center of several tectonic blocks, up to 7km thick, while the sediment at the edges of the blocks is relatively thinner, and even bedrock is exposed in some areas. Combined with the spatial distribution characteristics of earthquakes in this area, the Moho depth and sediment thickness distribution in the eastern Tibetan plateau are strongly correlated with the distribution of earthquakes in this area, which has important reference value for future earthquake prediction.

Research paper

CHARACTERISTICS AND RELATIONSHIP BETWEEN GRAVITY VARIATION AND CRUSTAL VERTICAL DEFORMATION BEFORE AND AFTER CHANGNING EARTHQUAKE

WANG Jia-pei, TAN Hong-bo, LI Zhong-ya, LIU Shao-ming, ZHANG Yi, HAO Hong-tao, HU Min-zhang, SHEN Chong-yang

2023, 45(4):  952-969.  DOI: 10.3969/j.issn.0253-4967.2023.04.009

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The M_S6.0 Changning earthquake in Sichuan Province caused heavy casualties and property losses. Many scholars have carried out a lot of research on the tectonic background and seismogenic mechanism of the earthquake. However, whether the triggering mechanism of the earthquake is related to fluid injection remains controversial, and further comprehensive analysis should be conducted based on multidisciplinary observation data. The preparation and occurrence of large earthquakes are often closely related to crustal deformation and mass density changes. The comprehensive study of these two methods is conducive to capturing the real crustal dynamics information. To study the process of material migration in the earth's crust before and after the earthquake and its triggering mechanism, the characteristics of gravity changes before and after the earthquake, the crustal vertical deformation data and their relationship were analyzed by using the gravity and GNSS data in Sichuan.

The characteristics of regional gravity change before and after the earthquake show that the epicenter of the Changning earthquake is located in the gradient zone of positive and negative gravity change anomalies and presents obvious reverse before and after the earthquake. According to the summary of previous earthquake cases, this region may have entered the short- and medium-term stages of earthquake preparation. The wavelet multi-scale decomposition method is used to process the regional gravity change characteristics of two periods before and after the earthquake, and the gravity change characteristics of different depths and scales are obtained. The results show that in the shallow part of the seismic region, the local variation characteristics of the dispersion are obvious, mainly distributed in the boundary of the active fault zone and block. The trend change characteristics are significant in the deep part, and the gravity gradient zone associated with the earthquake is found. In theory, there is an approximate ratio relationship between surface gravity change and crustal vertical deformation, and different ratio coefficients contain different geophysical meanings. The relationship between gravity changes and crustal vertical deformation at four sites around the epicenter is extracted. The results are inconsistent with the approximate law and show different characteristics, including subsurface material migration information at each location.

Based on the above research results, the process of material migration in the crust before and after the Changning earthquake and its triggering mechanism are comprehensively analyzed and discussed. The results show that the long-term driving force of the Changning earthquake is the result of the deep migration of materials from the Qinghai-Tibetan plateau to the southern boundary of the Sichuan Basin. In the shallow surface of the epicenter and the surrounding area, there may be some cavities or airbags and other Spaces, and the loss and filling phenomenon of gas, liquid, or high-density substances may occur before and after the earthquake. Combined with the previous research results on the seismogenic mechanism of the Changning earthquake, it is indicated that the factors such as salt mining, gas production and wastewater reinjection in this area may be one of the reasons for the triggering of the Changning earthquake.

3D P-WAVE VELOCITY STRUCTURE OF CRUST IN FUJIAN AREA AND ITS TECTONIC IMPLICATIONS

LI Qiang, WU Jian-ping

2023, 45(4):  970-986.  DOI: 10.3969/j.issn.0253-4967.2023.04.010

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The Fujian area is located tectonically at the southeastern margin of the South China continent, which consists of three sub-blocks, the northwest Fujian block, the southwest Fujian block and the east Fujian block. This region is the forefront of the interaction between the Eurasian plate and the Philippine Sea plate. Geologically, the Fujian area has undergone a complex tectonic evolution process, and the huge intrusive-volcanic rocks formed by multi-stage tectonic changes were widely exposed in this region. Since the inversion of the crustal three-dimensional P-wave velocity structure was important for understanding the tectonic evolution process and the deep seismogenic environment in the region, a lot of research work has been carried out in Fujian area, including seismic body wave tomography, ambient noise surface wave tomography and artificial seismic profiles. Although some important features of the crustal velocity structure in this region had been obtained by natural seismic body wave or ambient noise surface wave imaging, the grid lateral resolution was relatively poor(generally above 0.5° horizontally), which made it difficult to constrain effectively the detailed features of the fault zone velocity structure in this region. For example, the Fu'an-Nanjingfault zone, as an important fault zone in the region, which controlled the magmatic intrusion activities before the Mesozoic, the features of its deep velocity structure have been rarely revealed. Although the resolution of artificial seismic profiles was high, it covered a relatively limited detection range in this region.

In this paper, 3203 natural local earthquakes were selected using the observation reports of Fujian seismic network from 1999 to 2021 and integrating some data from neighboring provinces, which includes both 76423 absolute arrival time data and 389021 P-wave relative arrival time data from131 seismic stations. The test results of checkboard showed that the northwest Fujian block had poor recovery at all depths due to the limited internal seismic ray coverage, most areas of the southwest Fujian block had good recovery at all depths, and the east Fujian block could been recovered at all depths except for its northern region which had poor recovery at 0km, 25km and 30km depth. Under this resolution condition, the three-dimensional crustal P-wave fine velocity structure in Fujian region was obtained. The arrival time residual conforms to a Gaussian distribution before and after the inversion. The travel time residuals of the seismic phases were mainly distributed in the range of -1.5 to 1.5s before the inversion, and these travel time residuals of the seismic phases were mainly distributed in the range of -0.5 to 0.5s after this inversion. The travel time residuals were reduced significantly and were more concentrated around 0. Using the velocity structure obtained from the inversion and combining with the geological structure and geophysical field characteristics of this region, the tectonic implications which may be related to these features of velocity structure in the region were discussed. The main results are as follows:

(1)In the near-surface shallow layer, the P-wave low-velocity feature is mainly correlated better with the NW-trending faults, such as the Nanri island fault, Meizhou bay fault, Yong'an-Jinjiang fault and Jiulong river fault. This may be related to the relatively young activity age and more fragmented shallow parts of the NW-trending faults. The lateral variation of velocity is small in the middle and upper crust at 5km and 10km depths relative to other depths, but there is a relatively high velocity zone of P velocity in northeastern Fujian area.

(2)The P-wave velocity structure shows generally a relatively low velocity feature at 15~25km depth within the southwest Fujian block, especially in the south of the Yong'an-Jinjiang fault zone. Although the range distribution of this low velocity anomaly is relatively large, the magnitude of the anomaly is not large, and the upper crust and the bottom of the lower crust in the southwest Fujian block do not show this anomalous feature. On the other hand, the magnetotelluric sounding of the middle and lower crust of this block shows a high resistivity and the receiver function shows a low Poisson's ratio, this suggests that the low-velocity feature of this block is not caused by partial melt or ductile shear zone, but may be mainly caused by the more quartz-rich composition of the regional crust.

(3)There exist two P-wave low velocity anomalies in the middle-lower crust of the East Fujian block, which are below the two high thermal anomalous area of the geothermal heat flow in this region. It may suggest that the formation of these two relative low velocity anomalies may be related to the transformation of the coastal area into an extensional environment and the upwelling of deep mantle materials caused by the high-angle retraction of the Paleo-Pacific plate in the late Yanshanian period.

(4)The P-wave velocity features show that the velocity at the two sides of the Fuan-Nanjing fault zone is different obviously in the middle and lower crustal depths. This may imply the Fu'an-Nanjing fault has a certain control on the distribution of crustal velocity structure in the region, which is consistent with its deep characteristics of cutting the Moho interface which reflected by the Bourg gravity anomaly and aeromagnetic anomaly, which further confirms that it is a major deep fault zone in the region.

Original article

PRECISE LOCATION AND SEISMOGENIC STRUCTURE OF THE 2022 LUSHAN M_S6.1 EARTHQUAKE

FU Ying, HU Bin, ZHAO Min, LONG Feng

2023, 45(4):  987-1005.  DOI: 10.3969/j.issn.0253-4967.2023.04.011

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On June 1, 2022, a M_S6.1 earthquake occurred in Lushan, Sichuan Province, western China, which is approximately 10km from the Lushan M_S7.0 event on April 20, 2013. To understand if the earthquake has the same seismogenic structure as the Lushan M_S7.0, we relocated the event in the Lushan area using the multi-stage locating method based on the seismic phase arrival data of the Sichuan Seismic Network from April 20, 2013, to July 1, 2022. A total of 6992 M_L≥1.0 earthquakes were acquired, with a relative locating error of 0.5km and 0.7km in the horizontal and vertical directions, respectively, with a travel time residual(RMS)of 0.18s. The results show that the M_S6.1 event is located at 102.943°E, 30.382°N with an initial-rupture focal depth of 15.6km, lying on the NW side of the 2013 Lushan M_S7.0 event. The sub-surface rupture length of the long and short axis is 10 and 8km, measured from the dense aftershock area in NE-SW and NW-SE directions, respectively. The NE-SW profile in the Lushan area shows that the depth of Lushan M_S7.0 earthquake in 2013 was about 15km, similar to that of Lushan M_S6.1 and M_S4.5 on June 1, 2022. The M_S6.1 earthquake sequence, located at the NE end of the long axis, shows no evidence to break through the rupture termination point of the Lushan M_S7.0 earthquake and enters the Dayi seismic gap, which is bounded by the 2008 Wenchuan M_S8.0 and 2013 Lushan M_S7.0 aftershock regions. The short-axis profile shows that the M_S6.1 earthquake sequence occurred on a new back-thrust fault in the pre-existing seismogenic structure of the 2013 Lushan M_S7.0. The new structure dips SE and ruptures in a slight arc protruding into the NW, parallel to the northern segment of the seismogenic structure of the 2013 Lushan M_S7.0 earthquake with a horizontal distance of about 5km. The new and old structures connect at the detachment base to the main segment of the 2013 Lushan M_S7.0 earthquake.

We also inverted the focal mechanism of the Lushan M_S6.1 earthquake using the CAP(Cut and Paste)method. The result indicates that the centroid depth of the M_W5.7 main event is 14km which is very close to the initial-ruptured depth of 15km calculated by the phase arrival times. The best double couple parameters are 221°/40°/105° for nodal plane Ⅰ and 22°/52°/78° for nodal plane Ⅱ. The parameters are in order of the strike, dip, and rake angles. Combined with the realization of the NE-striking, SE-dipping seismogenic structure characteristics determined by the accurate locating of the earthquake sequence, it can be quickly confirmed that the nodal plane Ⅱ is the fault plane.

Based on the accurate locating results, focal mechanism solutions, and geodynamic background of the focal area, it is inferred that the seismogenic structure of the Lushan M_S6.1 earthquake is induced by the thrust dislocation of a NE-SW trending and SE inclining thrust fault in the southern section of Longmenshan fault zone. Finally, we discussed the relationship between M_S7.0 and M_S6.1 in the Lushan area. The two could be considered a unique sequence: the mainshock and the maximum aftershock, respectively, regarding spatial relationship and tectonic correlation. However, the time interval of these two earthquakes significantly overextends the statistical relationship between the principal earthquake and the maximum aftershock. Furthermore, considering the effects of the Coulomb stress change produced by the earthquakes repeated at the end of the Dayi gap, Lushan earthquake further enhanced the stress level in the Dayi seismic gap located in its northern segment.

RELOCATION OF THE 2022 M_S6.0 MAERKANG EARTHQUAKE SWARM IN SICHUAN PROVINCE AND ITS SEISMIC FAULT ANALYSIS

XU Ying-cai, GUO Xiang-yun

2023, 45(4):  1006-1024.  DOI: 10.3969/j.issn.0253-4967.2023.04.012

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The 2022 M_S6.0 Maerkang earthquake swarm in Sichuan Province is the first rare strong swarm activity with high frequency, concentrated spatial and temporal distribution, strong explosive and strong magnitude in Maerkang area in the eastern segment of Bayan Har block in China seismic network records. It is also another significantly strong earthquake event in Bayan Har block after the M_S7.4 Maduo earthquake on May 22, 2021. The M_S6.0 Maerkang earthquake on June 10, 2022 not only broke the 33-year record without M_S≥6.0 earthquakes within 100km of the epicenter, but also broke the historical record without M_S≥6.0 earthquakes within 50km of the epicenter. The earthquake swarm is mainly located in the nearly “T” shaped conjugate fault structure area composed of the NW strike Maerkang fault and NE strike Longriba fault in the Bayan Har block. This area is a relatively rare region for moderate and strong earthquakes in the history. Therefore, it is of great significance to analyze and discuss the possible seismogenic faults of the Maerkang strong earthquake sequence for the study of seismogenic structures and the risk of strong earthquakes in the weak seismic region of Bayan Har block.

The earthquake swarm was relocated by double-difference method, and focal mechanisms and centriod depths of M_S≥3.6 earthquakes were calculated by using gCAP inversion method. Then the relationship between the stress system in the Malkang area and these earthquake focal mechanisms was analyzed, and fault plane was fitted by using relocation results. Maerkang earthquake swarm is mainly distributed along NW direction, and the initial rupture depth is 9.8km on average. Depth profiles show that earthquakes are mainly concentrated at depth between 0km to 15km. The most earthquakes of early-stage occurred in 48 hours. The mid-stage and late-stage earthquakes are located less than 15km in depth and move to the northwest of the epicenters. Initial rupture depth of the largest M_S6.0 earthquake is 12.5km, which is almost at the bottom of the dense area. The focal mechanism of M_S6.0 earthquake is 150° in strike, 79° in dip, and 7° in rake on nodal plane Ⅰ, and 59° in strike, 83° in dip, and 169° in rake on nodal plane Ⅱ, with the centroid depth of 9km. Other focal mechanisms of M_S≥3.6 earthquake are strike-slip types. Dips of nodal plane of focal mechanism range from 71° to 86°, and there exist different dip directions for one strike of every nodal plane. All azimuths of P axis are in NWW direction, and the plunges are nearly horizontal. The focal mechanisms of M_S≥3.6 earthquakes show that the tectonic environment is very favorable for NE or NW strike faults to generate the strike-slip movement. Centriod depths range from 5 to 9km, which are lower than the average depth of 9.8km of relocation, indicating that these earthquakes mainly ruptured from deep to shallow. The relative shear stress of the NW nodal plane are significantly greater than that of the NE nodal plane, and the normal stress of the NW nodal plane was smaller than that of the NE nodal plane, indicating more possibility of strike-slip dislocation on the NW nodal plane. The fault plane fitting results reveal that there are obviously two nearly parallel and nearly NW strike earthquake belts in the epicenter area. Fitted fault plane parameters of the belt in the north branch show the strike 333°, the dip 88°, the slide -22°, and the belt in the south branch show the strike 331°, dip 88°, and slide -23°. It is indicated that the fault properties of these two earthquake belts are basically the same, revealing that most of earthquake activities of the swarm may be controlled by at least two parallel structures near the Maerkang fault with the NW strike, dip 88° and left-lateral strike-slip. Combined with the existing regional geological structure, it is inferred that the Maerkang earthquake swarm may be induced by the NW and NE strike conjugate faults, and the NW strike faults control most of the earthquake activities.

Research paper

STUDY ON HETEROGENEITY OF THE STRESS FIELD IN THE YANGBI EARTHQUAKE FAULT ZONE

WAN Yong-ge, WANG Yu-ru, JIN Zhi-tong

2023, 45(4):  1025-1040.  DOI: 10.3969/j.issn.0253-4967.2023.04.013

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The fine crustal stress field plays an important role in geodynamics. The 2021 Yangbi earthquake sequence occurred in an area with densely deployed seismic stations. Before the mainshock, there occurred multiple 3-4 magnitude earthquakes. The mainshock was followed by strong aftershocks, M_S5.0 and M_S5.2, occurring 7 and 36 minutes later respectively. The earthquake sequence is a typical example of a “foreshock-mainshock-aftershock” earthquake sequence. The abundant seismic data of the 2021 Yangbi earthquake sequence provide many seismic focal mechanisms for the fine stress field analysis in the study region.

To study the relationship of the stress field, fault structure, and earthquake dynamics in the Yangbi earthquake source region, the central focal mechanism solution algorithm is selected for the earthquakes with several focal mechanisms to ensure the accuracy of the focal mechanism data, and 93 precisely determined focal mechanism data are determined. The overall stress field in the source region is determined as a compressive stress axis of nearly NS direction and extensional stress axis of nearly EW direction. Then, to reveal the heterogeneity of the stress field in the source region, according to the location of the earthquake sequence, the focal mechanism solutions are divided into 6 regions by using the moving window strategy and obtain the stress field in each sub-region. To verify the inversion results are not caused by the selection of a specific partition mode, we used two different partition methods to discuss the stress field inversion experiments: 1)change the number of sub-regions from 6 to 8, the number of focal mechanisms in each subregion is still 23, and moving the 15 focal mechanisms in each iteration; 2)the number of the sub-region is still 6, change the number of focal mechanism to 28 in each subregion. It can be found that although the different partition strategies are changed, the characteristics of the obtained stress field will not change. Finally, the earthquake dynamics revealed in the heterogeneous stress area are analyzed.

The results show that the compressive stress axis changed from NNW-SSE direction in the northwest of the Yangbi earthquake focal area to NNE-SSW direction in the southwest region, with the rotation angle of 23°; And the stress shape factor in the northwest part of the rupture zone is always larger than that in the southeast region. Combined with the geodynamics studies of crustal motion map, tomography from seismic data, hydrographic net distribution, and topography of the study region, it is speculated that the change of the stress field in the northwest and the southeast is caused by the combined action of the blocked southward movement of the material in the northern part of the fracture area and the NNE extension in the shallow part of the study area due to the low angle NNE subduction of the Indo-Burma arc. The horse-tail-like fault distribution in the southeast of the Yangbi earthquake fault zone and the mountain and river alignment around the Yangbi earthquake are consistent with the predicted stress deflection and stress shape factor change. These studies are of significance for understanding the characteristics of fault activity and earthquake dynamics in study regions.