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20 February 2024, Volume 46 Issue 1

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Research paper

OPPORTUNITIES BROUGHT BY 3D GEOSCIENCE MODELING FOR EARTHQUAKE NUMERICAL FORECASTING

YAO Qi, LU Ren-qi, SU Peng, WANG Hui, ZHU Ya-ling, WANG Li-wei

2024, 46(1):  1-18.  DOI: 10.3969/j.issn.0253-4967.2024.01.001

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Earthquake prediction and forecasting need to transform from the traditional empirical, qualitative, and semi-quantitative to quantitative. The improvement also calls for multi-disciplinary, highly integrated physical and mechanical simulations rather than only a single discipline. The global development of observation technology and the construction of observation networks have already built a data foundation for earthquake numerical prediction and forecasting to a certain extent. However, the biggest constraint is the difficulty of synthesizing a large amount of observation data and quickly establishing complex numerical models with geological significance for numerical calculation. It is a vital issue restricting experimental research and industry development of earthquake numerical prediction and forecasting. Based on a brief introduction of the concept, development, and research status of earthquake numerical prediction and forecasting, this paper analyzes the difficulties in numerical modeling, which essentially come from the disciplinary differences between active tectonics, structural geology, solid earth, seismology, and numerical simulations. The development of 3D Geoscience Modeling and its application in the earthquake industry can establish a large-scale complex earthquake tectonic model close to the real world with geological significance. It provides a significant opportunity and technical means for developing earthquake numerical prediction and forecasting by solving the problems in numerical modeling. 3D geological modeling has built a bridge for multi-disciplinary geological applications. It can multi-disciplinary data fusion, establish a 3D geological model with geological significance and characteristics in line with geomechanical characteristics, and integrate data, geological model, up to building a numerical model, which advances the efficiency of modeling and simulation. Therefore, the rapid development of 3D geological modeling provides an opportunity to solve the modeling difficulties mentioned above in earthquake numerical prediction. Then, we briefly describe the development of 3D geological modeling technology, its application in the seismic industry, and the construction and application of 3D standard fault models domestically and overseas. Here, we introduced the development and essential contents of the Community Fault Model of Southern California in the United States for the Uniform California earthquake rupture forecast, the New Zealand Community Fault Model from the Institute of Geological and Nuclear Sciences Limited, and the Community Fault Model in Sichuan and Yunnan region in China.

The prospective future of 3D geological modeling and its potential application in earthquake numerical prediction and forecasting makes it a common concern of researchers in earthquake science. The five future modeling trends are the joint modeling of multi-source and multi-precision heterogeneous data, the integrated modeling of the geological model-attribute model-numerical model, flat fault structure modeling, 3D fault structure modeling, data-model-calculation iteration, and mutual driving. Finally, the paper describes the difficulties of applying the 3D geological modeling technique in earthquake numerical prediction and forecasting, including the industry construction, public approval of the 3D Community Fault Model, and the variations of numerical modeling and applications. 3D geological modeling technology can provide more realistic numerical and geometric models for earthquake numerical prediction, forecasting, and related numerical computing fields, reduce construction periods, create fast iterations, and solve modeling difficulties.

3D STRUCTURAL MODELLING OF THE ANNINGHE-ZEMUHE-XIAOJIANG FAULT ZONE IN THE EASTERN BOUNDARY OF SICHUAN-YUNNAN BLOCK USING MULTI-DATA AND IMPLICIT MODELING METHODS

WANG Mao-mao, HU Shun-yang, MA Hao-ran, LIANG Bo-yu, ZHANG Jin-yu, LU Ren-qi

2024, 46(1):  19-34.  DOI: 10.3969/j.issn.0253-4967.2024.01.002

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The Anninghe-Zemuhe-Xiaojiang fault zone is located at the intersection between the Qinghai-Xizang Plateau and the Yangtze block, representing the eastern boundary of the Sichuan-Yunnan block with frequent seismic activities. Its overall kinematic characteristics involve left-lateral strike-slip motion, and the fault structures along its strike are complex, posing significant challenges in accurately characterizing the 3D structural features of deep faults. The main issues include the structural complexity of the fault surfaces, uncertainties in the intersection relationships of fault systems, spatial constraints of blind faults, and the definition of fault surfaces in regions with weak seismic activity. Traditionally, 3D structural modeling for fault geometry heavily relies on high-resolution seismic reflection profiles, 3D seismic data volumes, and borehole data. It defines the geometric shapes of objects with limited nodes in a triangular mesh, and then simulates the topological structure of objects by connecting these nodes. However, obtaining high-resolution seismic reflection data in active tectonic areas like the eastern boundary of the Sichuan-Yunnan block is challenging, and even when available, it is often sparse in space. Alternatively, a large amount of relocated earthquakes and surface fault traces are generally used to create initial three-dimensional models of active faults. However, this approach overlooks the contributions of focal mechanism solutions in constraining the modeling, with more subjectivity in the selection of relocated seismicity, and does not adopt a differentiated weight strategy for various data sources. In this study, a 3D implicit modeling approach, combining deep and shallow geological and geophysical data that are generally available in active tectonic environments, was used to construct a detailed 3D structural model of the Anninghe-Zemuhe-Xiaojiang fault zone at the eastern boundary of the Sichuan-Yunnan block. The modeling process effectively integrated the fault plane constraints provided by focal mechanism solutions with surface fault traces and relocated seismic data, using a multi-iteration process with differentiated weight to increase the accuracy of the fault models. This approach ultimately represented the 3D complex structural features of the eastern boundary of the Sichuan-Yunnan block using multiple data sources. The modeling results show that the Anninghe-Zemuhe fault zone is characterized by a steep strike-slip fault structure with along-strike geometry variations. The Anninghe Fault shows its steepest dip angle in the central segment and gradually becomes gentler to both ends. Meanwhile, the Zemuhe Fault exhibits several asperities that are perpendicular to the direction of fault slip at a depth of 5~15km. By contrast, the north-to-central segment of the Xiaojiang fault zone is more complex. The western branch of the Xiaojiang Fault, which is an east-dipping, left-lateral strike-slip fault, is characterized by a relatively gentle fault plane with an average dip angle of 76° to 78°. The west-dipping segment of the eastern Xiaojiang Fault has a steeper dip with an average angle of 85°. The detailed 3D structural model of active faults constructed through implicit modeling can be used for analyzing fault roughness and fault system studies, which are crucial for understanding the distribution of asperities on fault planes and conducting seismic rupture simulations. Implementing the implicit modeling approach allows for the development of improved fault surface representations that can contribute to Community Fault Models in active tectonic environments, and support fault system modeling, rupture simulations, and regional hazard assessments.

A WEB-BASED PROTOTYPE SYSTEM FOR THE THREE-DIMENSIONAL FAULT MODELS OF THE CHINA SEISMIC EXPERIMENTAL SITE

WU Xi-yan, LU Ren-qi, ZHANG Jin-yu, SUN Xiao, XU Fang, CHEN Gui-hua

2024, 46(1):  35-47.  DOI: 10.3969/j.issn.0253-4967.2024.01.003

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The data of active fault structure and three-dimensional(3D)fault models is essential for seismic risk analysis. With more and more requirement for complex 3D fault models, the demand for data sharing and related research increases dramatically. A web-based display system for three-dimensional fault models would improve data sharing and user experience. Moreover, constructing such a web-based system is also an important issue for data sharing.

The 3D active fault models are built in a data modeling platform, while the web display system is constructed by the geographic information system(GIS)platform. Because the data structure, type, and content between data modeling and GIS platforms are different, the following questions are critical, for example, how to migrate 3D model data from the modeling platform to the GIS platform?and can the migrated data present the right attributions?In this paper we used the Web AppBuilder of ArcGIS 10.6 Enterprise Edition to build a Web prototype system to display 3D fault models of the China Earthquake Science Experimental Field(Sichuan-Yunnan region). The system implemented the basic functions of a 3D Web application and successfully tested the 3D scene display scheme, user interaction mode, and data migration scheme.

The prototype system adopted a local scene, which can easily switch between the above-ground and underground viewing angles of the scene. The scene included 2D fault surface traces, 3D fault models, and earthquakes with or without focal depth. After data fusion, the 3D fault models were classified and displayed with active age, having a good visual fusion effect with 2D fault data. Earthquakes with or without focal depth were displayed in different colors. The earthquakes without focal depth were uniformly displayed at 17km depth according to the average focal depth of the earthquakes with focal depth. So the earthquakes without focal depth can be highly consistent with other elements in the 3D scene.

The user interface interaction mode in the 3D scene of the prototype system was consistent with the common interaction mode of 2D map applications in the following aspects: 1)map browsing; 2)Navigation menu; 3)Geographical inquiry; and 4)Functional interactive tools. The system interface was simple, clear, logical, and unified. Users were easily acquainted with the three-dimensional scene interface according to the two-dimensional map interaction experience. It conformed to the user interface interaction principles of simple, consistent, predictable, and easy feedback.

The prototype system had the basic functions of 3D scene browsing, zooming in and out, 3D object attribute viewing, geographic query, base map switching, layer control, legend, and distance measurement. However, the prototype system needed further development and more complex functions such as data attribute table browsing, space selection, and space query.

This paper presented a data migration scheme from the modeling platform to the GIS platform. The data migration of this scheme can be divided into four steps: data format conversion, coordinate system conversion, 2D and 3D attribute information mapping, and 3D data attribute table construction. After transforming the data format and coordination system from the modeling platform to the GIS platform, 2D and 3D data fusion should be carried out to make 3D data and 2D data have the same attribution. The format conversion and coordinate system conversion steps can be automatically completed in batches. Otherwise, mapping the 2D and 3D attribute information and building the 3D data attribute table need manual handling.

In summary, this paper presents a data migration scheme from the modeling platform to the GIS platform. Practice in reality shows that only after conversing data format and coordination system from the modeling platform, the 2D and 3D data fusion steps are caplable of ensuring a better visual integration of them. The Web-based prototype system of displaying 3D fault models of the China Seismic Experimental Site implements the basic functions of 3D scene application and tests the fused 2D and 3D data visualization. It is friendly and open to users, with a great demonstration significance.

FULL-WAVEFORM INVERSION BASED ON EXPONENTIAL-PHASE COHERENCY MISFIT FUNCTION

LIU Jian-huan, CHEN Jian-ye, ZHAO Ji-hai, Deyan Draganov

2024, 46(1):  48-62.  DOI: 10.3969/j.issn.0253-4967.2024.01.004

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Full waveform inversion(FWI)has emerged as a highly effective approach for obtaining accurate and high-resolution S-wave velocity structure of the shallow subsurface. However, there exist several challenges when applying FWI in the field. The first challenge is the issue of local minima that arises when inaccurate initial models are used, especially when inverting shallow surface seismic data dominated by high-frequency and strongly dispersive surface waves. These local minima are caused by the non-linear misfit function that represents the differences between the measured and simulated data. Moreover, defining an appropriate minimization criterion to reduce the sensitivity of the inversion results to errors in the recorded seismic wave amplitudes is another significant challenge. Amplitude errors can arise from various factors, such as different coupling effects between seismic sources, receivers, and the ground, or variations in the strength of seismic sources excited at different shot locations. If the amplitude information of the recorded seismic wavefield is inaccurate, the reliability of the FWI inversion results will be effected negatively.

To overcome these problems, we propose a novel misfit function that incorporates exponential-phase coherency, thereby eliminating the reliance on amplitude information from the measured and simulated data. This new misfit function is designed to measure the coherency between the measured and simulated data based on exponential phase. It achieves a balance in extracting valuable information from various amplitude components of the recorded seismic wavefield, such as surface waves, reflections, and scattering waves. The method for computing this coherency is inspired by Phase-Weighted Stacking(PWS)method to detect weak but correlated seismic signals. In PWS, the exponential phase of the data is computed to obtain a phase-dependent coherency that is independent of amplitude. This correlation is then used to enhance the stacking of signals with similar instantaneous phases.

By utilizing the adjoint-state method, we efficiently calculate the gradient of the misfit function with respect to the model and conduct a thorough analysis of its shape and characteristics. To demonstrate the robustness of our proposed misfit function against random noise, we perform experiments by using simulated data contaminated with varying levels of noise. The results demonstrate that the misfit function based on exponential-phase coherency remains highly robust and reliable, even in the presence of significant random noise. This robustness is particularly crucial in practical applications where noise contamination is a common challenge.

To evaluate the performance of FWI employing exponential-phase coherency in a real field environment, we employ seismic data collected at an archaeological site as a benchmark dataset. This dataset presents a complex and challenging scenario due to the presence of complex subsurface structures. In addition to the inherent noise in the data, we introduce additional random noise to assess the robustness of our proposed misfit function.

The inversion results obtained with our novel FWI approach exhibit an impressive resemblance to the known velocity structure of the archaeological site. These results are further validated through independent archaeological excavations, which confirms the accuracy and reliability of our imaging technique. The success of using our method in accurately reconstructing subsurface features under challenging field conditions underscores the significant potential of FWI based on exponential-phase coherency to enhance the accuracy and reliability of shallow subsurface imaging in practical scenarios.

APPLICATION PROGRESS OF RECRYSTALLIZED CARBONATE IN THE STUDY OF FAULT AND LANDSLIDE ACTIVITY IN CARBONATE BEDROCK AREA

LIU Chun-ru, YUAN Ren-mao, YIN Gong-ming, JI Hao, WEI Chuan-yi, TIAN Ying-ying, MA Xi, DANG Jia-xiang

2024, 46(1):  63-80.  DOI: 10.3969/j.issn.0253-4967.2024.01.005

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The research on the activity history of seismogenic faults is the basis for the research and prevention of natural disasters such as earthquakes and landslides. Dating has always been the focus and difficulty of the research on the activity history of fault. However, it is difficult to carry out geochronological surveys for faults and landslides evolution in the carbonated areas due to the lack of suitable dating materials, such as the region of south-eastern Tibet where the main lithology is carbonate bedrock. The exposure dating of cosmogenic nuclides is the main method to determine the activity history of fault. But the cosmic nuclides  36Cl and ¹⁴C dating methods still have some limitations, such as the complex generation mode of  36Cl being caused by fission under the action of cosmic rays, neutron capture and meson action, the yield of  36Cl being changed with chemical composition change of dating mineral(the range of 2-171atom/g·a), and so on. More importantly, the rapid rock weathering in the carbonate bedrock area is a big problem. Once exposed, the bedrock will start rapid weathering and erosion and dissolution. Therefore, it is necessary to find new dating materials or dating methods in carbonate bedrock areas, especially in areas with little or no quaternary sediments. When a large landslide moves on the sliding surface of carbonate bedrock, heat is often generated due to high-speed friction, and then the dynamic metamorphism can occur easily on the sliding surface to form recrystallized carbonate, which can be used to determine the active time of faults.

Carbonate is one of the main materials for ESR dating. As early as the 1970s, Ikeya made the first electron spin resonance(ESR)dating study of carbonates using stalactite calcite. After that, many researches on the ESR signal characteristics of carbonate(such as coral, shell, aragonite, stalagmite and etc)were carried out, and the carbonate ESR dating then became one of the main methods in Quaternary chronology and had been widely used. The recrystallized carbonate on the fault friction surface and the sliding surface of the landslide is a newly discovered dating material. Although its main component is calcium carbonate, its origin is different from the carbonate materials commonly used in ESR dating(such as stalagmite, stalactite, etc.), so it is necessary to study its characteristics of ESR dating.

The characteristics of recrystallized carbonate collected from the fault friction surface of Jianchuan section on Lijiang-Xiaojinhe Fault(Yin et al., 2021)and the sliding surface of Qiaojia landslide which is located at the intersection of Xiaojiang Fault and Zemuhe Fault(Liu et al., 2023)have been studied, including microstructure, thermal annealing characteristics, sunlight bleaching characteristics, and compared with the previous dating results of AMS ¹⁴C and OSL on sediments. Yin et al.(2021)and Liu et al.(2023)analyzed and demonstrated the feasibility and reliability of the recrystallized carbonate ESR dating method used in the analysis of bedrock fault and landslide activity in the carbonate bedrock area, and established the recrystallized carbonate ESR dating technology.

Therefore, the ESR dating of recrystallized carbonate is an effective dating technology and can be used widely for the studying of activity history of faults and landslides in carbonate bedrock areas. This paper introduced the latest research progress of recrystallized carbonate ESR dating in the Carbonate rock area of southwest China by Yin et al.(2021)and Liu et al.(2023). In this paper, the requirements for sample collection and the range of dating were proposed which provide technical support for dating of key geological samples for research on fault and landslide activity history, engineering exploration, active structure, and seismic risk assessment in Carbonate rock bedrock area.

ESR DATING OF CALCITE VEINS AND IMPLICATIONS FOR THE ACTIVITY OF THE JIANCHUAN SECTION OF THE LONGPAN-QIAOHOU FAULT ZONE

JI Hao, LIU Chun-ru, WEI Chuan-yi, YANG Hui-li, YIN Gong-ming, CHANG Zu-feng

2024, 46(1):  81-100.  DOI: 10.3969/j.issn.0253-4967.2024.01.006

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The Sichuan-Yunnan region is located in the southeastern part of the Qinghai-Tibet Plateau. Because of the compression and collision dynamics of the Indian Plate and the Eurasian Plate, the tectonic deformation is strong and seismic activities occur frequently. There have been many earthquakes above magnitude 7.0 in history. A series of active fault zones have developed in the region, among which the Sichuan-Yunnan rhombus block bounded by multiple active faults has attracted great research interests in recent years. The Longpan-Qiaohou fault zone is a boundary fault of the Sichuan-Yunnan rhombus block. The fault zone starts from Longpan in the north, passes through Jiuhe, Jianchuan, and Shaxi in the south, and ends at Qiaohou. It is about 120km long and the fault trend is 15°~20°. This fault zone is large in scale and highly active, with frequent seismic activity, complex mechanical properties, and variable movement patterns. The Mesozoic movement was intense. In the early Cenozoic, compression-thrust movement was dominant, and in the late Cenozoic, tension-strike movement was dominant. Since the Holocene, the fault zone has been characterized by left-lateral strike-slip movement with normal faulting properties, and earthquakes of magnitude 5 or above have occurred many times. Therefore, studying the activity of this fault zone is of great significance for the prediction and evaluation of regional strong earthquake risk. Thick calcite veins are well developed on the Henancun Fault of the Jianchuan section of the Longpan-Qiaohou fault zone, providing very valuable materials for fault dating. Calcite veins are coseismic rapid precipitation formed during seismic activity or syntectonic precipitation that filled along fractures after seismic activity. Therefore, their ages represent the latest time at which seismic activity occurred. Previous studies have shown that tensional fissures formed during coseismic events can close in a short period of time(days to months), suggesting that the filling of calcite veins within fault fissures is a relatively rapid process. This paper uses the ESR method to conduct dating study on the calcite veins in the study area. The results show that the ages of the four calcite veins(HNC-ESR01, HNC-ESR02, HNC-ESR03 and HNC-ESR04) are: (7.1±0.8)ka, (7.1±0.9)ka, (7.3±1.7)ka and (6.9±1.5)ka, respectively. The age results are concentrated, and the average age is(7.1±1.3)ka, indicating that the fault was active no later than(7.1±1.3)ka. The age results are consistent within the error range with the second paleoseismic event time revealed by trenching work in the area(between(6 130±30)a BP and(6 320±40)a BP), indicating that the dating of ESR in the fault zone is an effective dating method for the study of active tectonics and paleo-earthquakes. It is an effective chronological method for research, but it can be seen that compared with ¹⁴C and luminescence dating, the error of ESR results is relatively large. For faults with short earthquake recurrence intervals, it is still very challenging to accurately judge their activity. In the follow-up work, it is necessary to further improve the experimental process and reduce experimental errors, including refinement of sample pretreatment, accurate monitoring of irradiation dose, and accurate calculation of dose rate. In addition, by using five fitting functions(LIN, SSE, DSE, EXP+LIN and D_gamma)to calculate the equivalent dose values of calcite vein samples in this study, we found that the SSE function is capable of providing the best fitting effect.

THE LUMINESCENCE SIGNAL CHARACTERISTICS OF PHYTOLITH AND ITS APPLICATION IN DATING

LI Zhao-ning, YIN Jin-hui, YANG Hui-li, SHI Wen-fang, ZHENG Yong-gang

2024, 46(1):  101-116.  DOI: 10.3969/j.issn.0253-4967.2024.01.007

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Phytoliths have shown a lot of advantages in dating due to their widespread distribution, structural stability, and preservation integrity, especially since phytolith carbon was used in radiocarbon dating. However, there is a problem of overestimation of phytolith carbon ¹⁴C ages, which may be due to its structure destruction during pre-treatment processes of the rapid oxidation and high temperature. It is necessary to identify the subtle changes in its structure under high-temperature conditions based on its chemical composition and certain physical properties. As a special amorphous hydrated SiO₂, using their luminescence signals for thermoluminescence(TL)or optically stimulated luminescence(OSL)dating can be directly compared with their ¹⁴C ages even as an alternative dating method, and the luminescence property changes of phytoliths may have a reference value for identifying the physical structure changes. In this paper, modern phytolith samples extracted from dry rice straw were taken as an example to study the stability of OSL and isothermal TL(ITL)signals. We conducted a series of conditional experiments to determine the specific experimental process of OSL and ITL₁₆₅ signals, and discussed the feasibility and reliability of the dose-signal response relationship and test process under different given doses of dose recovery experiments. Moreover, the OSL signal were tested by a conventional single-aliquot regenerative-dose(SAR)protocol which firstly preheated at 200℃/180℃ to remove the instability signal and then blue excited at 125℃. The OSL signal of phytolith sample rapidly decays under blue light and is basically reset within 5s of excitation. The sensitivity corrected OSL signal intensity and regeneration dose can respond one by one and a growth curve can be established, indicating that there is also a trap structure within the phytolith particles that can generate relatively stable OSL signals. However, there are still some problems which need further to be solved such as relatively weak signal intensity and poor sensitivity. The results of bleaching experiment show that the OSL signal intensity of phytolith particles decays exponentially under sunlight, and the OSL signal irradiated by given dose of 85Gy can be bleached about 90%within 100s and bleached completely around 800s, which suggests that the phytolith samples have good sunlight bleaching susceptibility. There is a stable and easily bleached OSL signal in natural phytolith particles, however, the experimental procedure is only suitable for samples with lower given doses(below about 200Gy)and the dose recovery rate of older samples is lower. On the other hand, 425℃ TL peaks were found in the ITL₅₀₀ curves of phytolith at both a higher given dose(850Gy)and a lower given dose(85Gy), but they could not be used for dating because of instability. There was a remarkble and stable TL peak at 165℃ under a higher given dose(850Gy). The relatively stable ITL₁₆₅ signal has the potential to be used in dating research and its experimental procedure is also by SAR method. The dose recovery rate of ITL₁₆₅ signal under different given doses(above about 50Gy)was in the range of 0.8-1.2. Both OSL and ITL₁₆₅ signals have the potential to be used in dating studies but need to be tested with samples of known-age. The characteristic dose D₀ of OSL and ITL₁₆₅ signals of phytoliths are(326.8±19.5)Gy and(504.9±49.9)Gy, which implys that phytoliths have a greater saturation level than quartzes. Meanwhile, the ITL₅₀₀ curve heated at different annealing temperatures has the potential to identify changes in phytolith luminescence properties and physical structure. The structure of phytoliths begins to change at around 300-350℃, and irreversible structural changes have occurred at around 600℃ and gradually become sensitized. This also means that the extraction process of phytoliths using wet ashing rather than dry ashing may destroy the structural integrity of phytoliths and resulting in an overestimation of phytolith carbon AMS ¹⁴C ages. Whether it is by using AMS ¹⁴C dating method or OSL/TL dating method which phytoliths as the main dating material, phytolith particles should not be placed in a high temperature environment above 300℃ at any stage during the experiment in order to avoid irreversible damage to its structure. The luminescence age obtained by the OSL signal and ITL₁₆₅ signal of phytoliths can be compared with the ¹⁴C age to determine whether there exists an overestimation, and if the two ages can be verified, there is no need to use other dating minerals which is of great significance for the dating of precious archaeological materials.

REMOTE SENSING INTEPRETATION OF COSEISMIC LAND-SLIDES TRIGGERED BY 1976 LONGLING M_S7.3 AND M_S7.4 EARTHQUAKES AND THE TECTONIC SIGNIFICANCES

LI Hao-feng, XU Yue-ren, GUO Ya-li, LIU Han, ZHAO Xin-yu, LU Ling-yu, TANG Jia-cheng

2024, 46(1):  117-140.  DOI: 10.3969/j.issn.0253-4967.2024.01.008

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The Longling-Lancang seismotectonic belt in southwestern Yunnan is critical for accurately defining the boundaries of active blocks and evaluating seismic risks. Using pre- and post-earthquake high-spatial-resolution satellite imagery to study strong earthquakes retrospectively proves to be a practical method in such study. Strong earthquakes frequently cause secondary effects such as coseismic landslides, collapses, and debris flows, which lead to considerable loss of life and property. These secondary effects, often as the most dramatic manifestations of an earthquake, show geologic signatures providing evidence of historic or prehistoric seismic activities. The use of satellite imagery captured shortly after historic earthquakes to interpret these secondary effects is particularly beneficial in determining the intensity and influence radius of earthquakes, thereby helping study on seismogenic faults of earthquakes.

On May 29, 1976, two strong earthquakes with M_S7.3 and M_S7.4 occurred in Longling county, southwest China, followed by intense aftershocks. The seismogenic structure of these earthquakes still remains undetermined to present. These earthquakes triggered numerous coseismic landslides in the regolith of the granitic rock mass. The seismic zone, located in subtropical regions, is characterized by high precipitation and dense vegetation. Apart from the ancient landslides in the northwest and southeast, no records of landslides and debris flows persisted in the epicenter zone for a century, making the occurrence of substantial landslides post the main earthquakes unexpected. Currently, these landslides have undergone reshaping by land surface processes and re-vegetation, which makes them indistinguishable in recent remote sensing images. Using Keyhole satellite images with a resolution of 0.6~1.2m offers a useful means to identify the coseismic landslides of the Longling mainshocks. In this study, we employ these images for a comprehensive visual interpretation of the coseismic landslides. To ensure the accuracy and reliability of the results, we used images captured in 1981(the most recent following the earthquakes)to extract coseismic landslides and substantiated them with images from 1974, field investigation photos from 1976, and relevant records. Finally, we have compiled an exhaustive database of coseismic landslides triggered by the 1976 Longling cases.

Our results are summarized as follows: 1)A total of 14 448 landslides were interpreted, encompassing an overall area of 17.2km². The area of individual landslides primarily ranged from several hundreds to one thousand m², and most were superficial slides in the surface regolith with short sliding distances. The regional stratigraphy is complex, with 70.9% of the landslides occurring in the regolith of granitic rock mass, 15.3%in sandstones or siltstones, and a mere 13.8%in other areas such as limestones. Consequently, these landslides were relatively small compared to those in other regions like the Loess Plateau in north China, where the surface sediment is extremely loose. 2)A strong correlation exists between the intense area of coseismic landslides and the earthquake sequence, which tends to migrate from south to north. Notable aftershocks(e.g., M_S6.2 on June 9 and M_S6.6 on July 21)particularly exhibited the general NNW distribution direction of the earthquake sequence and triggered scattered landslides outside the epicenter zone. Through synthesizing field surveys, combining other records and the findings of this study, we believed that the two main earthquakes triggered numerous coseismic landslides, and the continuous strong aftershocks led to the destabilized regolith of the granitic rock mass creeping successively, resulting in subsequent landslides. 3)The concentration areas of coseismic landslides do not match the high-earthquake-intensity areas, instead, they are all located on one side of active faults, which suggests that the seismogenic fault is neither the Longling-Ruili Fault nor the Wanding Fault. The spatial distribution of the landslides suggests that the scope of the surface rupture zone is about 30km. The conjugated strong earthquake ruptures in southwestern Yunnan may limit the spatial scale of single strong earthquakes, so it is crucial to pay more attention to the intersection zone of NE and NW trending active faults when assessing regional strong earthquake risk in the future.

EVIDENCE FOR THE HOLOCENE ACTIVITY OF THE LEIBO FAULT ZONE

ZHANG Guo-xia, SUN Hao-yue, LI Wei, SUN Wen

2024, 46(1):  141-161.  DOI: 10.3969/j.issn.0253-4967.2024.01.009

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The Yingjing-Mabian-Yanjin tectonic zone(YMYTZ)is an important boundary structure between the southeastern margin of the Tibet Plateau and the Sichuan Basin. It consists of several small-scale secondary faults with different strikes and is generally characterized by the intersections of north-northwest oriented longitudinal faults and nearly east-west oriented transverse faults. The YMYTZ is seismically very active in the late Quaternary and hosted several moderate-strong earthquakes, including two M≥7 earthquakes since 1216AD, namely the 1216 Mahu earthquake and the 1974 Daguanbei earthquake. After the Daguanbei earthquake, several M≥6 earthquakes and hundreds of M≥5 earthquakes occurred along the YMYTZ to date, implying it is a newly generated seismotectonic belt. Even so, the activity of each fault is still unclear, bringing out great uncertainty in understanding the current crustal deformation pattern and in evaluating the regional seismic potential. Specifically, although several M≥6 earthquakes have occurred along the Leibo fault zone in the southern segment of the YMYTZ, the late Quaternary activity of the fault zone has not been well determined due to insufficient work as well as subsequent lack of solid evidence. The Leibo fault zone strikes NE-SW and spreads on the southeast flank of the Chenqiangyan-Shanzhagang anticline. It starts at the Huanglang township near the Mahu Lake, cuts through the Jingkou Mountain, Lianhuashi, and Leibo, and extends southwestwards to the vicinity of Lianlajue. The latest investigation shows that the Leibo fault zone consists of four subparallel right-lateral strike-slip faults named F₁—F₄ from the north to the south, respectively. These fault branches together constitute a 43km-long and 10km-wide structural belt. Previous paleoseismic work along the Leibo fault zone found that the faults ruptured the late Pleistocene sedimentary layers with their upward terminations covered by the undeformed Holocene deposits, implying it was active in the late Pleistocene and has not been active since the Holocene. However, the ground surface traces of the Leibo fault zone are the most obvious among the faults in the YMYTZ, and recent seismologic studies show strong seismic activity for the Leibo fault zone, bringing out a controversy about whether it is active in the Holocene or not.

To address the late Quaternary activity of the Leibo fault zone, we conducted detailed indoor deformed geomorphic feature interpretation on remote sensing imageries like 2m-resolution GF-2 imagery and high-resolution imageries on Google Earth, and further mapped the fault traces in the field using an unmanned aerial vehicle(UAV)derived digital orthographs and digital surface models(DSM). Based on the geological and geomorphological surveys, two trenches were excavated at Pengjiashan and Luohangou along the northern(F₂)and southern(F₄)branches of the Leibo fault zone respectively. On the trench walls, surface-rupturing paleoearthquakes were identified for each fault according to criteria for faulting events like cut-and-cover structures, scarps, and colluvial wedges. Subsequently, we collected and dated several radiocarbon samples from the sedimentary layers immediately before and after the rupturing events, and finally carried out stratigraphic sequence calibration using the acquired ages with the OxCal 4.4 program to constrain the timings of the revealed paleoearthquakes.

According to the identification criteria of paleoseismic events, it was revealed 3 paleoearthquakes in the Pengjiashan trench on the northern fault branch(F₂)and another 7 rupturing events in the Luohangou trench along the southern fault branch(F₄). Radiocarbon sample dating constrain the ages of the paleoearthquakes along F₂ to be 21190—20590BC(EP1), 20550—12120BC(EP2), and after 12090BC(EP3), while the latest two paleoseismic events on F₄ occurred 9270—5040BC(EL6)and after 5000BC(EL7). Our paleoseismic studies show that the LFZ has experienced several surface-rupturing earthquakes in the Holocene, verifying it is a Holocene active fault zone. Moreover, the ages of the paleoseismic events revealed on two fault branches do not overlap with each other, suggesting they are different paleoearthquakes so that the fault branches in the Leibo fault zone are independent seismogenic structures. By collecting and analyzing the magnitudes of strike-slipping earthquakes that have generated surface ruptures in western China since 1920, it is believed that the minimum magnitudes of the paleoearthquakes determined on the Leibo fault zone are 6.5. Through the empirical relationships between magnitude and surface rupture length, it is estimated that the LFZ has the capability to produce an earthquake with M≥7.

CRUSTAL FINE VELOCITY STRUCTURE IN THE ERYUAN AREA, YUNNAN FROM DOUBLE-DIFFERENT TOMOGRAPHY

CAO Ying, QIAN Jia-wei, HUANG Jiang-pei, ZHOU Qing-yun

2024, 46(1):  162-183.  DOI: 10.3969/j.issn.0253-4967.2024.01.010

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The Eryuan area is located in the central part of the northwest Yunnan region on the southeastern edge of the Qinghai-Xizang Plateau. The geological structure in the area is complex, including the Weixi-Qiaohou-Weishan Fault and Honghe Fault, as well as the Longpan-Qiaohou Fault and Heqing-Eryuan Fault, which intersect in an “X” shape. The geothermal activity in the area is also active and exhibits strong fault structural characteristics. Earthquake activity is also frequent in the Eryuan area. Since 2013, multiple earthquakes with M_S≥5.0 have occurred on the west side of the Weixi-Qiaohou-Weishan Fault. Given the complex geological and geothermal background and seismic activity in the Eryuan area, we use the seismic travel time data recorded by the Yunnan Regional Seismic Network and the Northwest Yunnan Dense Array from January 1, 2008, to July 20, 2023, V_P/V_S model consistency-constrained DD tomography method to obtain the 3D V_P, V_S, and V_P/V_S models and relocation results in the Eryuan and its surrounding areas. The research results indicate that: 1)based on the relocation results, the dense small seismic cluster develops at the intersection of the Weixi-Qiaohou-Weishan Fault, Honghe Fault, and Heqing-eryuan Fault deserves special attention. Four earthquakes with M_S≥5.0 that have occurred since 2013 are mainly distributed on the west side of the Weixi-Qiaohou-Weishan Fault, with an NNW-SSE trend distribution. The fault system at the western boundary of the Sichuan-Yunnan block is very complex, and its seismic risk deserves our special attention. 2)Based on the tomography results, the widely distributed low-velocity anomalies in the upper crust of the study area may be related to the crustal material migration pathway caused by the escape of the Sichuan-Yunnan diamond block towards SSE. 3)Combining imaging results with geochemical research results for inference, the high V_P/V_S below 10km depth beneath the small seismic cluster at the intersection of the Weixi-Qiaohou-Weishan Fault, Honghe Fault, and Heqing-Eryuan Fault may correspond to hot spring geothermal fluids. Moreover, the circulation process of hot spring fluids in complex fault systems may have penetrated to a depth of 7~10km below the seismic cluster, accompanied by some small earthquakes. However, there is no imaging evidence of fluid within a depth of 5~7km where some earthquakes are densely distributed. The occurrence of these earthquakes may be related to the distribution of brittle rocks, but it cannot be ruled out that fluid may have a potential infiltration effect on them in the future. Based on the results of velocity structure, they are combined with pre-existing seismology research results. It is found that the Eryuan earthquake sequences on March 3 and April 17, 2013 were mainly located within low V_P, low V_S, and low V_P/V_S anomaly. Generally, if there is fluid present, V_S decreases faster than V_P, resulting in high V_P/V_S. So low V_P/V_S indicates that low V_S is not caused by fluids, which may be due to lithology. Therefore, the imaging evidence indicates that there is no fluid in the region where the sequence is located, thus inferring that the occurrence of the sequence is not directly related to the fluid. The M_S5.1 Yangbi earthquake sequence on March 27, 2017, is spatially close to the Eryuan sequence and also has the same velocity structure characteristics, so it may not be directly related to fluids. The mainshock and some aftershocks of the M_S5.1 Yunlong earthquake sequence on May 18, 2016, were mainly located in high V_P, high V_S, and relatively high V_P/V_S regions. High V_S indicates that it is not the fluid that causes relatively high V_P/V_S. It is speculated that there may not be fluid in the region where the mainshock is located, and the fluid did not directly participate in the occurrence process of the sequence.

Research paper

DISCUSSION ON REASON WHY AFTERSHOCKS OF THE 2014 LUDIAN EARTHQUAKE OCCURRED ON THE CONJUGATED FAULTS BASED ON TECTONIC STRESS FIELD

WAN Yong-ge, SONG Ze-yao, GUAN Zhao-xuan, HUANG Rui-qi, GU Pei-yuan, WANG Run-yan

2024, 46(1):  184-200.  DOI: 10.3969/j.issn.0253-4967.2024.01.011

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An earthquake of M_S6.5 occurred in Ludian county, Zhaotong city, Yunnan province, on August 3, 2014. Aftershock activity appears to be on two conjugated near-vertical Zhaotong-Ludian Fault and Baogunao-Xiaohe Fault, which is unusual phenomenon in aftershock distribution. Usually, most earthquakes occur on a single fault or fracture zone with several end-to-end linked faults. Therefore, the 2014 Zhaotong-Ludian earthquake sequence has attracted great attention from geoscientists, and a lot of work has been done in terms of the precise location of aftershocks, the focal mechanisms of the earthquake sequence, co-seismic slip model and the deep structure of the Earth’s crust. In order to determine the fault on which the mainshock occurred and to understand why the aftershocks occurred on the conjugate faults based on the tectonic stress point of view, we used the new developed fuzzy clustering and fault plane fitting method to calculate the geometry parameters of the 2 fault branches. The result shows that the strike and dip angle of the NNW-SSE branch(Baogunao-Xiaohe fault zone)are 336.67° and 88.41°, respectively, and the strike and dip angle of the near EW branch(Zhaotong-Ludian fault zone)are 266.10° and 86.42°, respectively. Both the Zhaotong-Ludian Fault and Baogunao-Xiaohe Fault are nearly vertical faults, which is consistent with the strike-slip stress regime in this area. In order to investigate the relative magnitude of the shear and normal stress on the two faults generated by the regional stress field, 128 focal mechanisms around the 2014 Ludian earthquake were collected, and types of focal mechanism are classified. It is found that most of the focal mechanisms in this area are strike-slip type, accounting for 61.72%, while the sum of other types of focal mechanisms is only 38.28%. By using the collected focal mechanism data, we estimated the stress tensor in this region. The determined stress tensor shows NW-SE compression and NE-SW extension, also a strike-slip stress regime. It reflects that the Qinghai-Tibet plateau was pushed and uplifted by the Indian plate, and the material flowed eastward, which was blocked by the hard blocks of the Sichuan basin and forced to turn southward, showing the extrusion of NW-SE and the tension of NE-SW. By projecting the stress field onto the two faults, it was observed that the relative shear stress on both the Baoguanya-Xiaohe Fault and the Zhaotong-Ludian Fault are large with 0.99 and 0.77, respectively, indicating that both faults are capable of seismic activity. Moreover, the Baogunao-Xiaohe Fault experiences higher relative shear stress compared to the Zhaotong-Ludian Fault, suggesting that seismic activity on the former is stronger. The observed aftershock seismicity on the Baogunao-Xiaohe Fault is stronger than that on the Zhoatong-Ludian Fault illustrated by frequency, magnitude and overall releasing seismic moment v.s. time, which validates the estimated larger shear stress on the Baogunao-Xiaohe Fault.

EFFECTS OF STRIKE-SLIP FAULT GEOMETRIC COMPLEXITY ON EARTHQUAKE RUPTURES PROPAGATION: A REVIEW

WANG Hui, CAO Jian-ling, YAO Qi, WANG Li-wei, ZHU Ya-ling

2024, 46(1):  201-219.  DOI: 10.3969/j.issn.0253-4967.2024.01.012

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Active strike-slip fault usually hosts major earthquakes. Therefore, studies on fault segmentation, which is controlled by geometric complexity, are very important for assessing the maximum magnitude of potential earthquakes. Based on previous literature, we summarized the behavior of earthquakes on strike-slip faults related to fault geometry, segmentation, and cascading rupture from the aspects of field investigation and numerical modeling.

Previous field investigations have shown that geometrically complex sections of a strike-slip fault usually play the role of barriers that can separate earthquake rupture segments and effectively stop the propagation of earthquake rupture. Statistical results based on the limited field investigations of the surface rupture illuminated semi-quantitatively the influence of geometrically complex sections on the rupture behavior of the earthquake. Since not all earthquakes can produce surface rupture zones, the case number of surface ruptures are unlikely to meet statistical requirements in the coming years. In addition, knowledge gained from field investigation is mainly statistical results based on fault geometry and kinematics. They have some consistency but vary greatly, indicating the complexity of seismic rupture modes on strike-slip faults. No simple threshold that can be used as a criterion to refine the capability of earthquake rupture propagation on strike-slip faults with complex geometry. Based on the statistical results of field surveys, geologists have applied the factor, that complex geometricity controls earthquake rupture behavior, in seismic risk assessment. Lacking support from dynamic analysis, it is necessary to develop and integrate physics-based dynamic models to help improve earthquake-rate models and probability models.

Numerical modeling can not only present the earthquake simulation scenario but also provide insights into the fundamental physics of dynamic rupture propagation. The modeling results improve our understanding of how the geometric complexity of the fault influences the dynamic rupture propagation. Different modeling approaches focus on different aspects of this challenging scientific problem, each with unique advantages and disadvantages. 2D modeling is relatively simple. They allow modelers to consider more complex physical processes, variated parameters, and constraints from the field and laboratory observation, etc. They provide a comparative benchmark on rupture dynamics on a strike-slip fault with complex geometry. 3D modeling can provide closer approximations to realistic faults and more direct comparisons to observations. The simulations of one earthquake rupture process may focus on the influence of single/multiple parameters on the rupture process. While the multicycle earthquake simulation can predict spatiotemporal patterns of earthquake ruptures on a strike-slip fault. Both simple 2D modeling and complex 3D modeling have shown that one of the most important factors affecting rupture behavior on strike-slip faults is the fault geometric complexity. In addition, other dynamic factors, such as the initial stress, the properties of the rock medium, and nucleation location, also affect dynamic fracture propagation on strike-slip faults. It indicates that rupture behavior on a certain strike-slip fault has its unique characteristics that are controlled by dynamic factors such as the regional tectonic environment and the properties of the fault itself. The numerical modeling provides a dynamic perspective on the complexity of rupture behaviors on strike-slip faults given by field investigations.

In the China Seismic Science Experimental Site, 3D dynamic modeling supported by fault detection, dense geophysical observations, and high-performance computation will provide new insights into the rupture behavior in the complex multi fault system. That is helpful in determining the maximum magnitude of a potential earthquake.

News flash

GEOLOGICAL DISASTERS AND SURFACE RUPTURES OF JANUARY 23, 2024 M_S7.1 WUSHI EARTHQUAKE, XINJIANG, CHINA

ZHANG Bo-xuan, QIAN Li, LI Tao, CHEN Jie, XU Jian-hong, YAO Yuan, FANG Li-hua, XIE Chao, CHEN Jian-bo, LIU Guan-shen, HU Zong-kai, YANG Wen-xin, ZHANG Jun-long, PANG Wei

2024, 46(1):  220-234.  DOI: 10.3969/j.issn.0253-4967.2024.01.013

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The M_S7.1 earthquake in Wushi, Xinjiang on January 23, 2024, represents the largest earthquake in the Tianshan seismic belt since the 1992 Suusamyr M_S7.3 earthquake in Kyrgyzstan. Preliminary precise aftershock localization and initial field investigations indicate an NE-trending aftershock zone with a length of 62km that is concentrated at the mountain-basin transition area. This event produced geological hazards, including slope instability, rockfalls, rolling stones, and ground fissures, primarily within a 30-kilometer radius around the epicenter. The epicenter, located approximately 7 kilometers north of the precise positioning in this study, witnessed a rapid decrease in geological hazards such as collapses, with no discernible fresh activity observed on the steep fault scarp along the mountainfront. Consequently, it is inferred that the causative fault for this main shock may be an NW-dipping reverse fault, with potential rupture not reaching the surface.

Moreover, a surface rupture zone with a general trend of N60°E, extending approximately 2 kilometers, and displaying a maximum vertical offset of 1m, was identified on the western side of the micro-epicenter at the Qialemati River. This rupture zone predominantly follows the pre-existing fault scarp on higher geomorphic surfaces, indicating that it is not new. Its characteristics are mainly controlled by a southeast-dipping reverse fault, opposite in dip to the causative fault of the main shock. The scale of this 2-kilometer-long surface rupture zone is notably smaller than the aftershock zone of the Wushi M_S7.1 earthquake. Further investigation is warranted to elucidate whether or not the M_S5.7 aftershock and the relationship between the SE-dipping reverse fault responsible for the surface rupture and the NW-dipping causative fault of the main shock produced it.