Table of Content

20 April 2024, Volume 46 Issue 2

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CONTENTS

2024, 46(2):  0-0. 

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Review

FRICTIONAL PROPERTIES OF SERPENTINE MINERALS UNDER HYDROTHERMAL CONDITIONS

LIU Shi-min, ZHANG Lei, HE Chang-rong

2024, 46(2):  235-250.  DOI: 10.3969/j.issn.0253-4967.2024.02.001

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Serpentine minerals are among the minerals commonly found in the Earth’s subduction zones, and their unique physicochemical properties have a significant impact on subducting geodynamics. Friction experimental studies of serpentine minerals are essential to gain a deep understanding of the frictional sliding stability of serpentine-containing faults in subduction zones as well as explaining the complicated misalignment behavior of faults in subduction zone. Previous laboratory research has produced an abundance of results, and this work addresses two main aspects: the stable states of occurrence and interconversion relationships of serpentine minerals, and the parameters affecting the frictional strength and sliding stability of serpentine minerals. First of all, studies on the stable endowment state of serpentine minerals and the interconversion relationship show that different types of serpentines diaplay different stable phases under different conditions. Chrysotile and lizardite are stable at low temperatures, and the stability fields of both chrysotile and lizardite roughly overlap, but chrysotile is in a substable state. Antigorite is stable at high temperature conditions, such as subduction zone mantle wedges containing high pore fluid pressure conditions, and undergoes a transition from lizardite to antigorite with increasing temperature. Secondly, studies on the factors controlling the frictional strength and sliding stability of serpentine minerals have shown that temperature, pore fluid, and the effective normal stress are all critical factors, for example, an increase in temperature can significantly increase the frictional strength of lizardite and chrysotile. In addition, the friction strength of serpentine minerals shows an obvious pressure dependence, and it was found through previous experimental studies that the friction strength of chrysotile exhibits a high-pressure sensitivity, and that the friction strength of antigorite gradually increases with increasing temperature under low fluid pressure conditions, showing an obvious temperature strengthening phenomenon. In contrast, the change in frictional strength of antigorite with temperature under high-pressure fluid pressure conditions is diametrically opposed to the results of low-pore fluid pressure conditions, which shows a clear temperature weakening phenomenon. Previous studies have also found that antigorite-undergoes a dehydration reaction with increasing temperature under lower fluid pressure conditions, and then exhibits unstable velocity weakening phenomenon, while antigorite exhibits velocity weakening phenomenon under low shear deformation rate under high-pressure fluid conditions. By analyzing the variation of friction-slip stability of antigorite with the shear slip rate can help us to better explain the phenomenon of subduction-zone slow-slip. Overall, experimental studies of the friction of serpentine minerals provide a key experimental basis for a deep understanding of subduction zone geologic processes. The results of these studies are scientifically important for predicting earthquakes and explaining the evolution of the Earth’s internal tectonics and subduction zones, providing strong support for research and practice in the field of geosciences.

GEOMORPHIC DATING OF SCARPS AND ITS APPLICATION TO ACTIVE TECTONICS AND GEOMORPHOLOGY

PANG Zhen-hui, XU Hao-ting, SHI Xu-hua, GE Jin, LI Feng

2024, 46(2):  251-276.  DOI: 10.3969/j.issn.0253-4967.2024.02.002

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Scarps are typical geomorphic features of tectonics, climatic changes, and erosion processes. On one hand, interpreting geological information encoded in scarps allows for the quantitative constraint of the kinematic and dynamic mechanisms of the active structures. On the other hand, studying the evolution processes of scarps contribute to a better understanding of the couplings among tectonics, erosion, and climate during geomorphic evolution processes. In regions characterized by adverse geological conditions, limited accessibility, and logistical challenges hindering researchers from reaching certain areas, traditional dating methods such as radiocarbon dating, luminescence dating, and cosmogenic nuclide dating often face difficulties in determining the age of scarps. The geomorphic dating method of scarps, however, offers a promising avenue to address the scarcity of chronological samples in research areas where either sample availability is limited or conventional dating techniques are impractical. This paper provides a concise summary of the theoretical evolution of geomorphic dating of scarps. Emphasis is placed on elucidating the slope evolution processes, transport models, and associated computational methodologies integral to this approach. Additionally, the specific applications of these methods in active tectonics and geomorphology are highlighted, accompanied by a case study showcasing their practical implementation.

The theoretical foundation of geomorphic dating of scarps posits that the evolution of scarps during stable erosion stages can be simulated through models describing the evolution of slope surfaces over time. In practical dating applications, it is essential to determine the theoretical models and computational methods for the evolution of scarps. This necessitates the integration of measured profiles of the scarp to establish boundary and initial conditions, facilitating the determination of the geomorphic age of the studied scarps. On one hand, the related slope evolution model mainly involves processes such as bedrock weathering, sediment transport, and tectonic uplift. Previous studies have proposed dozens of quantitative slope evolution models and geomorphic transport functions(e.g., local linear, local nonlinear, non-local, etc.)based on various slope processes, theoretical assumptions, and numerical simulations. In various transport equations, compared to earlier local linear models, later local nonlinear transport models proposed based on experimental simulations and physical derivations exhibit higher fitting accuracy for real slope evolution. In the past decade, some scientists have proposed nonlocal transport models because of the limitations of traditional transport models, and have applied them in research. This nonlocal model assumes that the distance of sediment movement within a given area follows a probability distribution, thus allowing the simulation of long-distance slope processes over short periods. Additionally, many other transport models have been derived from specific slope processes, such as biotic disturbance and dry ravel. The solution methods for the aforementioned models vary as well. For instance, the analytical solution of a local linear diffusion transport model can be relatively easily obtained, while local nonlinear models and nonlocal models can only be numerically solved through specific approaches. On the other hand, the measured topographic profiles of the studied scarps can be used to determine the practical parameters of slope evolution models, including the present-day morphology of the scarps and their ages since their initial formation. In practical applications, various methods have emerged for the geomorphic dating of scarps, generally classified into two types based on the approach to fitting model calculations with actual topographic profiles: the mid-point slope method and the full slope method. The mid-point slope method uses the mid-point gradient value as the fitting morphological feature, representing an early method for dating scarps, mostly combined with linear diffusion transport functions and requiring numerous profiles for statistical analysis. Due to its low data utilization and limited spatiotemporal precision in statistical methods, the mid-point slope method has gradually been replaced by the full slope method. The full slope method involves fitting the overall shape of actual profile curves using model solutions. With the continuous improvement of observation techniques in the field of Earth sciences and the deepening research on related theories, the application scope of scarps geomorphic dating methods is no longer limited to the study of terraces and simple fault scarp evolution processes but has expanded to more complex geological environments, providing more precise constraints on their formation and evolution history.

For method application, we systematically present the progress in scarp geomorphic dating research across various geomorphic settings(such as river and coastal terraces, lake shorelines, alluvial fans, marine terraces, and extraterrestrial planets). It employs the geomorphic dating of the northeastern Pamir fault scarp as a case study to further explore and anticipate the developmental trajectory of geomorphic dating of scarps within the field of tectonic geomorphology.

APPLICATION OF DEEP LEARNING IN ACTIVE TECTONICS AND GEOMORPHOLOGY

LIU Xin, WANG Shi-rou, SHI Xu-hua, SU Cheng, LU Chen-yan, QIAN Xiao-yuan, SUN Qiao-yang, DENG Hong-dan, YANG Rong, CHENG Xiao-gan

2024, 46(2):  277-296.  DOI: 10.3969/j.issn.0253-4967.2024.02.003

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The research on active tectonics and geomorphology involves extensive sub-topics, including the kinematics of crustal movements, the processes underlying the evolution of landforms, and the associated dynamic mechanisms. These sub-topics are intricately connected with the interactions between the Earth’s endogenic and exogenic processes. In the contemporary realm of the Earth system science, research in active tectonics and geomorphology has become a hot topic for interdisciplinary study. The advancement in big data research coupled with the progressive developments in deep learning technologies has furnished this field of study with a voluminous array of data sources and the requisite analytical tools for technical analysis. In recent years, the application of big data and deep learning technologies in this research field has yielded a series of outstanding results, fostering new research directions, and ushering the discipline into a new phase. In this paper we synthesize existing research to outline the data sources pertinent to the study of active tectonics and geomorphology, including field geological survey, unmanned aerial vehicle (UAV)-based photography, aerial photography, and remote sensing observations. Then, we discuss in-depth examination of the recent innovations progresses in deep learning algorithms, including but not limited to convolutional neural networks(CNNs), deep Gaussian processes, and autoencoders. This article further elaborates on innovative applications of deep learning in the study of active tectonics and geomorphology. These include the identification of changes in glacier extent, monitoring volcanic activity and deformation, recognizing river systems, precise surveillance of landslide events, as well as observations of lithospheric deformation co-seismic surface ruptures.

Based on the summary of prior studies, this paper showcases a distinct application instance. By employing convolutional neural networks(CNNs)within the realm of deep learning image analysis and utilizing UAV-obtained high-resolution images, we undertake the automated detection of structural fractures in granite rocks in Meizhou island, in the southeast of Fujian province, China. In fault damage zones, structural rock fractures are widely developed, and the study of their orientation, system, and secondary characteristics is of great importance for determining their mechanisms of development and the multi-phase tectonic activity events in the region. Under conventional methodologies, the study of structural fractures in rocks is time-consuming and requires considerable manual effort in conducting exhaustive field surveys and detailed interpretation of cartographic representations. However, the application of deep learning can greatly enhance the efficiency of cartographic work. This application case has improved the classic deep learning framework by developing a CNN model specifically designed for the extraction of complex features and multi-scale rock fractures. This model achieved rapid identification of over 9 000 fractures with varied shapes and complex distributions within 55 minutes, attaining an accuracy of 85% and a recall rate of 89%. These findings demonstrate that deep learning significantly enhances operational efficiency in comparison to manual statistical methods for the automated identification of rock structural fractures, while also maintaining exceptional accuracy in fracture detection. Based on the results identified by deep learning, it can be clearly observed that two sets of fractures, oriented NE and NW, develop on the granite outcrops in the study area. According to previous research and the cross-cutting relationships of the fractures, it is known that NE-oriented fractures formed earlier than NW-oriented fractures, corresponding respectively to the Indosinian Movement and the expansion movement of the South China Sea in the tectonic history of South China. Through the automated extraction of deep learning models, the workload of manual mapping can be greatly reduced, yielding results consistent with actual geomorphological phenomena.

MECHANISM DIFFERENCES BETWEEN SEVERAL TYPICAL PYROCLASTIC ROCKS AND THEIR VOLCANISM SIGNIFICANCE

WEI Hai-quan, CHEN Zheng-quan, LIU Yong-shun, BAI Zhi-da

2024, 46(2):  297-311.  DOI: 10.3969/j.issn.0253-4967.2024.02.004

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Pyroclastic rock is the most direct object of physical volcanology and the most important topic of identifying the volcanic explosive fragmentation processes. Some particular species of pyroclastic rocks and equivalents can indicate key characteristics of the volcanism process, which is the basis to estimate the eruptive risks. Volcanic hazard is potential risk related to volcanic eruption, and it is one of the most important types of disasters that human beings face in nature. Volcanic disasters are directly related to the types of volcanic eruptions, among which explosive volcanic eruptions can cause the deadly intensive volcanic risks. The direct product of explosive volcanic eruption is to form various pyroclastic rocks, which represent the different types and intensities of volcanic hazards caused by the eruption process. The primary pyroclasts and secondary fragments reflect the difference of volcanic surface processes during eruptive or intermittent periods, while the distinguish of magmatic, phreatomagmatic and phreatic eruptive deposits marks the systematic development of modern volcanology, which is the leading work in the study of volcanic hazards. 1)Pyroclastic rocks are formed directly by transporting, accumulating and diagenesis of the expelled materials during the eruption. They usually consist of the primary materials such as broken magma, accidental fragments trapped by the volcanic conduit, as well as the epiclasts captured by the volcanic fluid flowing on the surface. Pyroclastic rock, as a direct product of explosive volcanism, has naturally becomes the most important research object in volcanology. The volcanic tephra laminae preserved by fine airfall volcanic ash in basins has been attracted attention because of their good isochron and environmental indication, and the associated rocks may need to be distinguished from different types of volcanic sedimentation such as bedded tuff, sedimentary tuff and tuffaceous mudstone. The autoclastic breccia produced by lava emplacement and the hyaloclatite formed by the quenching of lava under water represent fragmentation that is closely related to the lava flow, rather than those from explosive volcanism. 2)Pyroclast is mainly the product of explosive volcanism, but it can contain a certain amount of normal sedimentation and a small amount of rock fragment near the volcanic channel and the magma chamber roof. Pyroclats are generally defined as the direct products of explosive eruption behavior, while volcaniclastics are formed by volcanic degradation such as slope displacement, avalanche, lahar, and the autoclast generated by lava flowage and quenching. This classification not only emphasizes the difference in the forming process of different volcanic products, but also helps to distinguish the different mechanism in volcanological research and hazard estimation. Different types of pyroclastic rocks are formed with different fragment mechanisms and diagenetic ways, and some specific pyroclastic rocks represent various special types and scales of volcanic hazards. Although they are usually classified as primary clastics, the hazard caused by autoclastic breccia is significantly different. Cryptoexplosive breccia, although we have employed a rock name from pyroclastic rocks, is actually more concerned with its resource economics. 3)When we study the genetic types of pyroclastic rocks, the most important basis for identification is the forming mode of the materials, that is, the type of fragmentation, which include primary volcanism and secondary volcanism. Primary clasts are divided into pyroclast, which is formed by the direct action of volcanic eruption, and autoclast, which is produced by the flow process of lava flows, While secondary(exogenous)volcanism includes various kinds of exogenous clasts(epiclast)formed by volcanic surface processes. According to the proportion of magma and water content at eruptive environment, explosive eruption can be divided into three types: magmatic eruption, phreatomagmatic eruption and phreatic eruption, which represent the most basic process of explosive eruption, and are also the problems of genetic classification and identification often faced in the study of pyroclastic rocks.

Research paper

STUDY ON THE MAGMATIC PROCESSES OF POST-COLLISIONAL POTASSIC VOLCANIC ROCKS FROM WEST KUNLUN: TAKING THE PULU AND KANGXIWA VOLCANIC ROCKS AS EXAMPLES

DING Ran, LUAN Peng, YU Hong-mei, WEI Fei-xiang, ZHAO Bo, YANG Wen-jian, XU Jian-dong

2024, 46(2):  312-335.  DOI: 10.3969/j.issn.0253-4967.2024.02.005

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The West Kunlun region is located in the northwest margin of the Qinghai-Xizang Plateau. Due to the subduction and collision of the Indian plate, this region has many post-collisional potassic volcanic areas of different sizes. Scholars have conducted many volcanic geology and petro-geochemistry work in the West Kunlun volcanic area, mainly focusing on the origin of deep magmas and plate dynamics. However, the magmatic processes of these potassic volcanic rocks are still unclear. To reveal the magmatic processes in the West Kunlun region and understand the mechanism of volcanic eruption, we analyzed the whole-rock major elements, structure and compositions of the phenocrysts, the crystal size distribution(CSD), and magma crystallization temperature and pressure conditions of the Pulu and Kangxiwa volcanic rocks. The results show that the magma sources of the two volcanic regions are close and their trace element characteristics are similar. Still, their rock types and mineral compositions are significantly different. The Pulu volcanic rocks are mainly trachyandesite and basaltic trachyandesite. The phenocrysts are composed of plagioclase, olivine, clinopyroxene and a small amount of orthopyroxene. The Kangxiwa volcanic rocks are mainly phonotephrite, consisting of clinopyroxene, biotite, and a small amount of olivine and plagioclase. The erosion and zonation of plagioclase, olivine and clinopyroxene were observed under a microscope. There are Nb-Ta and Ti negative anomalies in the two regions, with relative enrichment of large ion lithophile elements(LILF)and light rare earth elements(LREE), indicating that the magma source area has the characteristics of island arc magma, which is related to plate subduction. Based on the analysis of previous Sr-Nd datas, we suggest that the magmas from these two volcanic areas originated from enriched sources.

According to the erosion characteristics, zonal composition data, and the concave CSD pattern, we suggest that the magma in Pulu mixed with acidic magma, whereas the magma in Kangxiwa may only mix with the internal magma, resulting in a large amount of melt erosion of phenocrysts. In Pulu volcanic rocks, the retention time of the smaller size(<5mm)crystal is 190-332a, and that of crystallographic size(>5mm)is 339-860a. The CSD curves of clinopyroxene phenocrysts in Kangxiwa phonotephrites kink at the size of 1.5mm. In Kangxiwa volcanic rocks, the residence time of smaller crystallographic size(<1.5mm)is 5.8-6.4a, and that of larger crystallographic size(>1.5mm)is 9.6-21.2a. The CSD curves of the volcanic rocks from Pulu and Kangxiwa volcanic regions are concave upward, indicating that magma mixing may have occurred both in the two volcanic regions. The An values of the core and the rim of the normal zoning plagioclases and the For value of the normal zoning olivines in the Pulu volcanic rocks vary widely, and the feldspars with low An values at the rim are out of balance with the melt. This indicates that the magma of the Pulu volcanic group mixed with the acidic magma. The Mg# of the normal zoning clinopyroxenes in the Kangxiwa volcanic rocks has a narrow range, and they are all in balance with the melt. The crystallization pressure at the rim was low, and the decompression caused a large number of resorbed phenocrysts to melt. This indicates that the mixing of phenocrysts with different degrees of melting and erosion may result in upward concave CSD curves of clinopyroxenes in the Kangxiwa volcanic rocks, so the Kangxiwa volcanic rocks may only have internal magma mixing.

The mineral-melt equilibrium thermometers show that the equilibrium temperature and pressure of the Pulu volcanic rocks are 1 035-1 218℃, 5.1-9.9kbar, respectively, and the corresponding depth is 19.4~37.3km. The equilibrium temperature of Kangxiwa volcanic rocks is 1 154-1 282℃, the equilibrium pressure is 1.2-11.6kbar, and the corresponding depth is 4.3~43.7km. The variation range of equilibrium pressure in the Kangxiwa region is large, which may be related to the deep fault zone. In this study, by quantitatively studying the magmatic processes of post-collisional potassic volcanic rocks in the West Kunlun region, we provide CSD calculations of the volcanic rocks, reveal the migration and evolution processes of magmas in the crustal magma reservoir, and provide important information for the volcanic activities in the northwest margin of the Qinghai-Xizang Plateau and its surrounding regions.

PRECISE RELOCATION OF SMALL-TO-MODERATE-SIZED EARTHQUAKES IN THE DATONG VOLCANIC GROUP AND SURROUNDING AREAS

XU Yong-qiang, LEI Jian-she

2024, 46(2):  336-356.  DOI: 10.3969/j.issn.0253-4967.2024.02.006

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In the present study we collect a large amount of arrival times from 3 218 earthquakes in the Datong volcanic group and surrounding areas from January 2008 to January 2023 through the China Seismic Network Center and relocated these earthquakes using double-difference location algorithm, finally obtain 2 447 relocate earthquakes. Our result shows that most earthquakes occurred above a depth of 16km, and earthquakes in the basin occurred at depths of 5-16km. There are fewer earthquakes occur near the surface at depths of 0-2km, while 6km and 11km are the dominant depths for earthquakes. The overall strike trending of these earthquake sequences is NE-SW, which is consistent with the regional active faults and controlled grabens and semi-graben-type faulting basins. In addition, these earthquakes are more concentrated near the Kouquan fault zone and in the Datong-Yanggao earthquake zone in the eastern part of the volcanic group. The average location errors of these earthquakes in the east-west, north-south, and vertical directions are about 0.21km, 0.22km, and 0.30km, respectively, with an average travel time residual of 0.14s.

The earthquakes near the Kouquan fault zone changed from deeper and more concentrated in the south to shallower and more scattered in the north. The earthquake sequences in the northern part of the southern section and the southern part of the middle section of the Kouquan fault zone are deeper along the NE-SW direction, roughly vertically distributed on the Kouquan fault. The earthquake sequences in the northern part of the middle section of the Kouquan fault zone did not occur on the Kouquan fault, and the distribution of earthquakes is relatively scattered, and earthquakes with larger magnitude are mostly concentrated at shallow depth, which may be related to the thick sedimentary coal-bearing strata and mining activities in the area. The strike trending of these earthquakes in the northern section of the Kouquan fault zone is, along the NE-SW direction, roughly distributed on the Kouquan fault. However, there are also earthquakes in the northern part of the Kouquan fault zone, which may suggest that the activity of the Kouquan fault zone has extended there.

The focal depth in the source areas of the Datong-Yanggao earthquake is mostly concentrated at depths of 3-16km on the hidden fault parallel to the NE-SW trending Dawangcun fault to the east. The hidden fault has a large dip angle and dips towards NW, which intersects with the Tubao fault and the Liulengshan piedmont fault, likely related to the aftershock activity of the Datong-Yanggao earthquake.

Earthquakes occur frequently in the middle section of the Huairen fault, followed by the southern section, and there are few earthquakes in the northern section. The seismic activity of the Shuiyu fault, the east fault of the Cailiangshan mountains, and the Yanggao-Tianzhen fault is relatively weaker. There are some seismic activities in the central part of the northern margin fault of the Tianzhen-Yanggao Basin. Earthquakes in volcanic areas occurred at the boundaries of volcanic clusters, while the seismicities inside the volcanic group area were not very strong, which suggests that the boundary of volcanic clusters is more prone to stress accumulation and earthquake generation than the interior of volcanic clusters.

Based on the new seismic results of ambient noise tomography in the area, it is found that earthquakes are not only related to faults, but more importantly, most earthquakes occur near the high-to-low-velocity anomaly boundaries. Furthermore, there are obvious low-velocity anomalies visible beneath most earthquake source areas, which may suggest that the occurrence of these earthquakes is closely related to fluids carried by the upwelling of thermal materials rising to the crust from the mantle and reducing the effective normal stress on the fault planes.

SIMULATION OF THE ROCK SURFACE LUMINESCENCE SIGNALS ON BEDROCK FAULT SCARPS BY STICK-SLIP AND CREEP MOVEMENTS

LUO Ming, CHEN Jie, QIN Jin-tang, YIN Jin-hui, YANG Hui-li, LIU Jin-feng, GONG Zhi-jun

2024, 46(2):  357-370.  DOI: 10.3969/j.issn.0253-4967.2024.02.007

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The reconstruct of the stick-slip and creep histories is essential for understanding fault activities and seismic hazard assessment. Distinguishing stick-slip and creep using geodetic technology has become a hot research area in recent years, but distinguishing and estimating seismic slip and creep on geological timescales(e.g., over hundreds of years)is challenging due to the lack of historical, geodetic and remote sensing data extending back more than a few hundred years. This study uses a newly developed dating technique(rock surface optically-stimulated-luminescence(OSL)dating)combined with the OSL decay parameters of granite samples from the Langshan fault in Inner Mongolia to simulate optically stimulated OSL-depth curves and depths of half saturation of luminescence signal under various scenarios such as fault seismic slipping, creeping, and erosion of colluvial wedge. The study compares these OSL-depth profiles, especially the depths of the half saturation, under different slipping modes, and summarizes their features.

During fault seismic slip, samples at different heights along the fault scarp display a “step-like” distribution pattern at their depths of half saturation. While during creep, however, they exhibit a “slope-like” pattern. Such differences may lie in that the slope during accelerating creeping is steeper than the slope during constant-speed creeping. Correspondingly, the resolution of residual luminescence-depth profile and depth of half saturation is also higher during accelerating creeping. During intra-earthquake creep events between seismic slip occurrences on the bedrock fault scarp, the distribution of half-saturation depth in the samples includes segments resembling both “steps” and “slopes”, which indicate the seismic slip and creep activities of the fault respectively. If the samples at the base of the colluvial wedge have had a sufficiently long last exposure time, the luminescence-depth profile and half-saturation depth distribution due to the erosion of the colluvial wedge would be approximately the same as in the three-phase seismic slip scenario. This indicates that samples previously buried by the colluvial wedge may be considered within the seismic displacement. Conversely, if the last exposure time of the base samples at the base of the colluvial wedge is short, the bleaching depth of the luminescence signal of these base samples will be noticeably shallower than that of the other samples within the seismic displacement, indicating the observed erosion of the colluvial wedge in this case. Furthermore, the seismic displacement ideally should include the buried location of the colluvial wedge. Therefore, when the luminescence curves and half-saturation depth distributions fail to identify the presence of the colluvial wedge, it is acceptable to include the buried location of the colluvial wedge in the seismic displacement calculation. Conversely, the luminescence-depth curves and half-saturation depth distributions document the erosion caused by the colluvial wedge. The simulation results demonstrate that this method can effectively distinguish between fault slipping and creeping, obtain corresponding displacements, and potentially record the erosion of colluvial wedge.

This study also analyzes the temporal resolution of the method for distinguishing fault activity times and the spatial resolution for quantifying displacements. The specific situation is as follows. When exposure age of the bedrock fault scarp is within a thousand years, the rock surface OSL dating method can easily distinguish types of active slips and seismic displacements for the earthquakes with a recurrence interval of hundreds of years. When exposure age of the bedrock fault scarp is in the range of 10⁰-10¹ka, the method can easily distinguish types of active slips and seismic displacements for the earthquakes with a recurrence interval exceeding a thousand years. When exposure age of the bedrock fault scarp is over ten-thousand years, the resolution of this method may be significantly reduced. The spatial resolution of seismic displacements using this method depends on interval between sampling and testing samples, typically in 10~30cm.

CHARACTERISTICS OF FOCAL MECHANISM AND STRESS FIELD IN THE EASTERN BOUNDARY OF THE SICHUAN-YUNNAN BLOCK

GUO Xiang-yun, FANG Li-hua, HAN Li-bo, LI Zhen-yue, LI Chun-lai, SU Shan

2024, 46(2):  371-396.  DOI: 10.3969/j.issn.0253-4967.2024.02.008

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It is important to study the characteristics of the tectonic stress field studies which could provide a deeper understanding of the internal stress environment of the crust. It can provide useful assistance for exploring the relationship between the tectonic stress field and earthquake development. At the same time, it plays an important role in understanding block interactions, fault movement, tectonic deformation, and revealing the dynamic mechanical processes of the continent. The focal mechanism solutions contain abundant information reflecting the stress field.

In this paper, using the broadband records from 128 permanent and temporary regional stations from the Chinese National Seismic Network(CNSN)deployed in the Sichuan-Yunnan Province and its adjacent, we determined the focal mechanisms of 3 951 earthquakes by the cut-and-paste(CAP)method and the HASH method. The friction coefficient and stress properties of the main active fault and characteristics of the tectonic stress field in this area are analyzed by using two different methods which are the damped inversion method(STASI)and iterative joint inversion method from focal mechanisms.

The results of the focal mechanisms show that: there are 2 512 strike-slip earthquakes in the study area, accounting for 63.58% of all earthquakes; there are 818 normal fault type and normal strike-slip type earthquakes, accounting for 20.70% of all earthquakes; there are 621 reverse strike slip and reverse thrust earthquakes, accounting for 15.72% of all earthquakes. The most of earthquakes in the study area are distributed in active fault zones, the strike of the fault plane is consistent with the orientation of active fault zones. It revealed predominantly strike-slip faulting characteristics of earthquakes in the Eastern Boundary of the Sichuan-Yunnan Block, while the reverse thrust of earthquakes is mainly concentrated in the Longmenshan fault zone, as well as the NW trending Mabian-Yanjin Fault and the NE trending of Ludian-Zhaotong and Lianfeng faults which lied on the eastern boundary of the Sichuan-Yunnan block. Overall, the characteristics of the source mechanism are consistent with the regional tectonic background.

Results of the stress field inversion confirmed main active fault in the Eastern Boundary of the Sichuan-Yunnan Block is under a strike-slip stress regime, maximum and minimum compressional stress axes are nearly horizontal. The maximum compressional axes are primarily oriented in NW-SE and NWW-SEE direction, and they experience a clockwise rotation from north to south. Against the strike-slip background, normal faulting stress regimes and reverse faulting stress can be seen in the regional areas. The most prominent is the Daliangshan fault zone, which has obvious differences from the overall characteristics of the stress field with the eastern boundary of the Sichuan Yunnan Block. The maximum horizontal principal stress in the northern section shows a nearly EW direction, with a strike-slip type stress property, and the NW-SE direction in the southern section, with a thrust type stress property. The distribution characteristics of the stress field are consistent with the fault type of sinistral strike-slip and thrust on the eastern boundary of the Sichuan Yunnan block

The shape ratio R-value varies significantly, the R-value in the Sanchakou area is relatively high, with obvious extrusion characteristics, the R-values of the Xianshuihe fault zone, Anninghe fault zone and Xiaojiang fault zone are all between 0.25-0.5, showing NE-SW compression and NW-SE tension, and the tensile stress may be much less than the compressive stress(strike-slip type). The R values of the northern segment of the Daliangshan fault zone, the southern segment of the Anninghe fault zone, and Zemuhe fault zone are all between 0.5-1, showing NW-SE compression and NE-SW tension, and the compressive stress is greater than the tensile stress. To sum up, the current stress characteristics of the eastern boundary of the Sichuan Yunnan rhombic block are shear strain and local compression or tension.

There are different friction coefficients of the main faults in the study area: The Anninghe fault zone is 0.60, the Xianshuihe and Zemuhe fault zones are 0.80, the Xiaojiang fault zone is 0.75 and northern and southern sections of the Daliangshan fault zone are 0.65 and 0.85. The friction coefficients of the Xianshuihe Fault, the southern section of the Daliangshan Fault, and the Zemuhe Fault are above 0.75. The high friction coefficients of these fault zones may be because they are strike-slip faults, and the friction coefficients themselves are relatively high. The southern section of the Xiaojiang fault zone may be related to the development of fault gouges in the fault zone.

JOINT INVERSION OF THE RUPTURE PROCESS OF 2018 M_L5.7 XINGWEN EARTHQUAKE BASED ON SEISMIC AND INSAR OBSERVATIONS

MIAO Si-yu, ZHANG Hai-jiang, GU Ning, LI Jun-lun, TAN Yu-yang, HUA Si-bo, ZHANG Yong

2024, 46(2):  397-413.  DOI: 10.3969/j.issn.0253-4967.2024.02.009

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The M_L5.7 Xingwen earthquake on December 16, 2018 is very likely induced by shale gas hydraulic fracturing, which caused not only massive landslides and rock collapse, but also some casualties in the surrounding area, with the direct economic loss of about 50 million CNY. It is of great significance to determine the source rupturing process of such an induced earthquake with large magnitude.

Finite fault inversion is one of the commonly adopted methods to determine coseismic slip displacement distribution. For finite fault inversion, various data have different sensitivities to various aspects of the rupture process. The seismic data can provide the historical information about the earthquake rupture process because it contains the Doppler effect of the space-time rupture behavior on the fault. In comparison, the near-field geodetic data(such as InSAR and GPS)can constrain the fault parameters and the static slip distribution well because they contain the surface motion information. Therefore, the reliability of the inversion for the complex seismic rupture process can greatly be improved by combined use of seismicdata and InSAR data.

In this study, strong-motion seismic data recorded at 8 near-field stations are chosen and filtered by a band-pass of 0.15-0.60Hz. The same InSAR data used in Wang et al.(2022)is adopted in this joint study. For inversion, a sufficiently large potential fault plane of 15km long and 10km wide is chosen and divided into 15×10 subfaults. Finally, the rupture process is obtained by joint inversion of strong-motion seismic data and InSAR data. The results show that the earthquake is characterzied by a typical unilateral rupture with the rupturing direction nearly towards the north. The duration of the rupture process was 6s, and the energy release was mainly concentrated in the first 5s. The rupture process is segmented and can be divided into two stages. The first stage is distributed from 1-3s and is located in the range of 0~5km from the source; and the 2nd stage is distributed from 3-5s and is located between 6 and 8km from the source. The coseismic slip is mainly concentrated in areas shallower than 5km, with a peak slip of approximately 0.27m. This can be used to explain why the Xingwen earthquake with a magnitude of M_L5.7 caused relatively serious damages.

Combined with the distribution of foreshocks and aftershocks, it can be seen that the foreshocks were mainly concentrated to the eastern edge of the major coseismicslip zone, which are close to some hydraulic fracturing wells. This suggests that these foreshocks occuring at the edge of the main rupture zone has a certain correlation with fluids, and the presence of fluids further leads to the fault weakening of the mainshock due to the increase of pore pressure and the decrease of effective compressive stress, which plays a triggering role in the occurrence of the Xingwen earthquake. The aftershocks are mainly distributed around the main slip zone, which are caused by after slips after the mainshock. The results from seismic inversion, InSAR inversion and joint inversion of the two data types reveal that the Xingwen earthquake is a northward unilateral rupture. The rupture propagation direction and coseismic slip distribution may be related to the physical property changes along the fault plane.

Compared with the two single inversion results, the joint inversion overcomes the influence of uneven distribution of seismic stations, improves the resolution of slip distribution, and produces results that are more consistent with the real physical process. The slip model obtained by joint inversion in this study can be helpful for further understanding the mechanisms of induced earthquake, the correlation between induced earthquake and geological structure, earthquake disaster assessment and post-earthquake disaster prevention and hazard mitigation.

STUDY ON THE SEISMOGENIC STRUCTURE OF THE 2022 GUJIAO M_L4.1 EARTHQUAKE IN SHANXI PROVINCE BASED ON FOCAL MECHANISM AND SEISMIC LOCATION

DONG Chun-li, ZHANG Guang-wei, LI Xin-wei, WANG Yue-jie, DING Da-ye, GONG Zhuo-hong

2024, 46(2):  414-432.  DOI: 10.3969/j.issn.0253-4967.2024.02.010

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Understanding the mechanism of earthquake sequence in the mining area is important for the time-dependent hazard assessment. An earthquake of M_L4.1 occurred in Gujiao, Taiyuan, Shanxi on February 20th, 2022, which caused strong ground motion in Gujiao and surrounding counties. The epicenter of this earthquake is located in the area of Lvliang uplift, where historical earthquakes are relatively rare. In addition, the coal resources are well developed in the earthquake source area which has attracted much attention from society and local governments.

To investigate the mechanism and the seismogenic fault of Gujiao M_L4.1 earthquake, we first apply the double-difference location method to retrieve highly accurate hypocenter locations. The results show that the earthquakes mainly occur at a depth range of 3~5km, and display a dominant distribution direction nearly EW-trending, which differs significantly from the NE-trending fault distribution pattern in this region. We further collect the broad-band seismic waveforms from the regional network of Shanxi province to perform focal mechanism inversion. The inversion results show that the Gujiao earthquake is a left-slip seismic event with a moment magnitude of M_W3.96. The optimal double-couple solution is characterized by a strike of 90°, dip of 80°, and a rake angle of -21° for fault plane Ⅰ, while for the fault plane Ⅱ, the strike is 184°, dip is 69°, and rake angle is -169°. The best centroid depth is estimated to be at 3km. This earthquake shows an extremely shallow focal depth. Moreover, By using cluster analysis method, we obtained the central solution for the seismogenic fault plane of the GuJiao earthquake, with a fault strike of 91°and a dip angle of 70°. The focal solutions show that the earthquake exhibit a strike-slip type, and the orientations of earthquake sequence coincide well with the focal mechanisms.

In addition, to discuss the effect of Gujiao M_L4.1 earthquake on regional stress, we calculate the stress drop of this seismic sequence. The results show that the stress drop is significantly smaller than that of the regional earthquakes, exhibiting at least one order of magnitude lower than that of the background earthquakes in the same region. This phenomenon reflects that the stress level in the focal area of the GuJiao earthquake is not high, suggesting that the background stress enhancement in the focal area is not obvious.

Based on regional geological structure, we found that the known faults in the region are all high-angle normal faults, and the strike of these faults are inconsistent with the focal mechanism solution of Gujiao earthquake sequence, which suggests that the existing faults are not the seismogenic fault. Taking the regional mining activities into account, we speculated that mining may cause strong disturbance to the stress field, and lead to stress redistribution within the rock mass. Such coal mining activity may generate a high stress disturbance on the hidden fault plane, and then the fault become the carrier of stress transfer. So we conclude that the seismogenic mechanism of the Gujiao-seismic sequence may be related to coal mining activities near the focal area, which leads to local stress changes, thus resulting in the activation of preexisting hidden faults and triggering the occurrence of the Gujiao earthquake.

GEOCHEMICAL CHARACTERISTICS AND GENESIS OF SOIL GAS IN THE PINGYUAN M5.5 EARTHQUAKE

SU Shu-juan, CHEN Qi-feng, SUN Hao, LIU Jun, FENG Liang-le, XU Ji-long, YANG Yan-ming, LUO Kun-li

2024, 46(2):  433-448.  DOI: 10.3969/j.issn.0253-4967.2024.02.011

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At 2:33 am on August 6, 2023, a M5.5 earthquake occurred in Pingyuan county, Dezhou city, Shandong Province. The faults within the epicenter and adjacent areas are deeply buried by the thick Quaternary sediment cover on which human activity is intensive, which makes it difficult to determine the location of the buried active faults from the surface based on geological and geomorphological evidences. It is necessary to detect the location of the buried active faults around earthquake areas and estimate their seismic risk.

In this study, based on the epicenter distribution direction of major earthquake and aftershocks, seismic and geological data of earthquake areas, and damage degree of local buildings, 4 survey lines with a length of 30km were arranged across the epicenters and adjacent areas, and the concentrations of Rn, CO₂ and Hg in soil gas were measured on site, and the results are as follows:

(1)There are obvious spatial differences in the concentrations of soil gas near the epicenter and its vicinities within the distance of 30km. Gas concentrations are relatively high near the epicenter areas and the east and west ends of 4 arranged survey lines, in contrast to those which are relatively low in other non-structural control regions. The spatial distribution pattern of Rn concentration in soil gas is basically consistent with that of CO₂, which may be due to CO₂ used as a carrier gas of Rn to migrate to the surface. At the southern end of the Lingxian-Guanxian Fault(F₁), the spatial concentration patterns of Rn and CO₂ gases exhibit multiple peaks or wide anomalous zones. It is speculated that the deformation zone of the fault rupture at this location is relatively wide, and there may be secondary permeable fracture zones in the west of the F₁. The escape form of Rn and CO₂ gas indicates that there may indeed be multiple small fault branches near the F₁, and the fault structure is relatively complex.

(2)The spatial concentration distributions of Hg, Rn and CO₂ in the epicenter areas are similar to that in its eastern region. However, in the western region of the epicenter areas, the spatial concentration distributions of Hg, Rn and CO₂ vary greatly, and the Rn and CO₂ concentrations near the Jiucheng Fault(F₃) in the west of the epicenter regions are higher than those near epicenters. It is speculated that this phenomenon may be related to the high-concentration gas migration caused by strong seismic tectonic activities and the special nasal geological structure controlled by F₃.

(3)The concentrations of Rn, CO₂ and Hg in the soil show high-value anomaly zones near the F₁ and F₃, and the concentrations of Rn and CO₂ in the west of F₃ exceed those in the epicenter area. After further earthquake relocation analysis, the spatial distribution of aftershocks exhibit a trend from F₁ to F₃. Combined with geochemical and geophysical research results, it is inferred that Pingyuan M5.5 earthquake should be related to the deep tectonic activities of F₁ and F₃.

Above research results show that the soil gas geochemical method can be applied to define the location and distribution direction of the buried faults with thick overburden, which provides an important criterion for earthquake trend tracking analysis. This study is of greatly scientific significance in determining the dynamic source and genetic mechanism of Pingyuan M5.5 earthquake, identifying potential strong earthquake hazard areas, and assessing the risk of future earthquakes in the study area.

STUDY ON THE RELATIONSHIP BETWEEN LITHOSPHERIC MAGNETIC FIELD AND GEOLOGICAL STRUCTURE AND SEISMIC ACTIVITY: TAKING THE 2021 M_S6.4 YANGBI EARTHQUAKE AS AN EXAMPLE

CHEN Zheng-yu, NI Zhe, ZHOU Si-yuan, JIN Yun-hua, YANG Xin-jun

2024, 46(2):  449-461.  DOI: 10.3969/j.issn.0253-4967.2024.02.012

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The lithospheric magnetic field is an important part of the earth’s magnetic field, which is affected by many factors, such as rock’s magnetization environment, underground geological structure, rock mineralogical composition, thermal and pressure state, and the deep tectonic evolution process. Most earthquakes occur in the crust and uppermost mantle, known as the lithosphere. The preparation and occurrence of earthquakes are usually accompanied by long-term accumulation and sudden release of energy, which will lead to changes in the thermal and pressure state of rocks, causing magnetic susceptibility variation in the lithosphere. Previous studies show that seismic activities can cause abnormal changes in the geomagnetic field, and there is an obvious correlation between the lithospheric magnetic field and seismic activities. The M_S6.4 Yangbi earthquake on May 21, 2021, provided a unique opportunity to study the dynamic evolution of seismo-magnetic anomaly.

Five-term repeat station vector geomagnetic data observed in Yangbi and surrounding areas from 2018-2021 were used in this paper, the first four terms were observed before the earthquake, and the fifth term was observed after the earthquake. After data processing and model calculation, the lithospheric magnetic fields before and after the earthquake are obtained, lithospheric magnetic field models are established using the Surface Spline method, and annual variations are calculated. Based on the analysis of lithospheric magnetic field combined with the regional geological structure, the Northwest Sichuan Subblock shows negative magnetic anomalies, which is consistent with the geological structural characteristics in the study area, altitude and crustal thickness increase sharply from Central Yunnan Subblock to Western Sichuan Plateau. Small areas of positive and negative magnetic anomalies are alternatively distributed in the Central Yunnan Subblock, which reflects the heterogeneity of deep lithosphere structure. The negative magnetic anomaly in the western boundary of the study area is also consistent with the geological characteristics of the Qingzang Plateau. There is also a correspondence between lithospheric magnetic field anomalies and faults, especially the total intensity. Negative magnetic anomaly strips are distributed along the strike of the Honghe Fault and Lijiang-Xiaojinhe Fault, while Weixi-Qiaohou-Weishan Fault appears at the junction of positive and negative magnetic anomalies. The statistical analysis of the M_S6.0 and above seismic events from 1970 to 2021 shows that there is a correlation between lithospheric magnetic anomalies and seismic activities. Most earthquakes occur in the weak magnetic anomaly area, especially near zero contour. The earthquakes tend to be distributed in anomaly gradient belts, and the number of earthquakes in the negative anomaly area is higher than that in the positive anomaly area. Analyzing the characteristics of the pre- and post-seismic changes of declination and total intensity near the epicenter of Yangbi M_S6.4, it is found that the epicenter of the Yangbi earthquake is located near the zero-contour-line of declination. During the preparation of the Yangbi earthquake, the total intensity gradually changed from a balanced distribution of positive and negative anomalies to the overall negative changes, and the magnetic anomalies recovered the trend of the balanced distribution of positive and negative changes after the earthquake.

GENESIS OF SONGYUAN EARTHQUAKES BASED ON 3D RESIDUAL DENSITY STRUCTURE

LIU Wen-yu, CHENG Zheng-pu, NIAN Xiu-qing, CHEN Yan, HU Yu-ling, QIN Zu-jian, SHAO Ming-zheng

2024, 46(2):  462-476.  DOI: 10.3969/j.issn.0253-4967.2024.02.013

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Since 2006, more than 10 earthquakes with the magnitude of 4 or higher have occurred continuously in Ningjiang district and Qian’an county, Songyuan city of Jilin province, with tens of thousands of aftershocks. The largest earthquake is the magnitude 5.8 earthquake on November 23, 2013 in Qiangorlos Mongolian autonomous county, Songyuan city, Jilin province. Frequent earthquake activities not only cause a large number of damaged buildings, but also trigger geological disasters such as sand liquefaction and slope instability, which have attracted widespread attention from the public and government departments. In order to comprehensively understand the causes of earthquakes in the Songyuan area, much studies on earthquake anomalies, seismogenic structures, and source depths have benn conducted recently. However, there are still some difference opinions in understanding the causes of earthquakes in the Songyuan area, for example, the major different viewpoints include: 1)it is caused by the regional stress release caused by Pacific subduction, mainly based on the consistency between the stress field in the source area and the background stress field of the Pacific plate subducting towards the edge of the east Asian continent; 2)it is related to deep melt or fluid migration, mainly based on the existence of low-speed and low resistance anomalous bodies in the deep part of the seismic area; 3)it is related to long-term oil and gas extraction, the main basis is that the seismic source spectrum exhibits characteristics of early and fast attenuation compared to typical structural earthquakes, and the time-domain and frequency-domain characteristic parameters such as waveform complexity and spectral ratio are also significantly different from typical structural earthquakes. In addition, the seismic source mechanism contains a large number of nondual force source components and shallow seismic source depth. Moreover, there is also significant controversy over the seismogenic structure, with the focus on whether the Qian’an and Ningjiang earthquake regions are controlled by the same fault zones. Some scholars infer that the seismogenic structures in both areas are the NE-trending Fuyu-Zhaodong Fault based on the source mechanism solution. Another group of scholars believe that the Qian’an earthquake area is controlled by NW-trending hidden faults, while the seismic structure in the Ningjiang earthquake area is the second NW-trending Songhuajiang Fault. The reason for the above controversy lies in the lack of necessary constraints on the deep structure of the seismic area.

To solve above problems, in this article we studied the three-dimensional residual density structure of the Songyuan earthquake area by performing nonlinear conjugate gradient focusing inversion on regional gravity data. Combined with oil drilling and reflection seismic data, some new insights were obtained as follows: 1)The residual density anomaly in the study area shows a high-low alternating strip distribution, with the southern anomaly trending NNW and the northern anomaly transitioning to NNE, This feature reflects the different deep earthquake environments in the Qian’an earthquake area and the Ningjiang earthquake area. The former’s source is located in the high-density anomaly body and its edge of Chaganhua, while the latter’s source is located in the middle of the low-density anomaly zone in Songyuan, indicating different rock types in the two locations. The basement of the Qian’an earthquake area is composed of limestone and metamorphic volcanic rocks, while the basement of the Ningjiang earthquake area is composed of fractured granite magmatic rocks. The stability of the basement structures in both areas is poor; 2)The seismic structures of the two are different. The former is controlled by the Chaganhua Fault and Qian’an Fault, while the latter is controlled by the Songyuan Fault and the Fuyubei Fault; 3)The formation of earthquakes is related to factors such as regional stress, basement structure, deep gas/fluid migration, and long-term oil and gas extraction. Long term water injection and oil recovery have damaged geological structures and stress environments, which may be important triggering factors. 4)In terms of the seismic source mechanism, we concluded that under the sustained action of regional stress in the near EW direction in the Songyuan area, the basement fault structures and fractures in the Qian’an and Ningjiang earthquake areas are extremely developed. Deep gas/fluid migrates upwards along the basement fractures, further reducing the stability of the basement structure. Long term water injection, oil and gas recovery, as well as fracturing, greatly damage the structural integrity of the cap rock and the distribution of deep stress, which results in the compressive and torsional strike slip movements in the relevant structures.

THE CHARACTERISTICS AND MECHANISM OF FLUID ANOMALIES IN THE DAZHAI OBSERVATION WELL OF PU’ER, YUNNAN PROVINCE BEFORE THE M5.9 MOJIANG EARTHQUAKE ON SEPTEMBER 8, 2018

HU Xiao-jing, FU Hong, ZHANG Xiang, LI Li-bo, HUANG Jiang-pei, LI Qiong, GAO Wen-fei

2024, 46(2):  477-491.  DOI: 10.3969/j.issn.0253-4967.2024.02.014

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The precursors before earthquakes are very useful to earthquake prediction, and fluid anomalies before earthquakes are very important to precursory observations. This paper reviews the characteristics of hydrochemical ions and well-aquifer permeability anomalies of the Dazhai observation Well in Pu’er, which is in the Yunnan-Southwestern region of China, for all M≥5.5 earthquakes since 2004. We find that both the chemical ions and physical parameters before the Mojiang M5.9 earthquake exhibited the largest magnitude of changes since observation, and the abnormal state was much stronger than that of previous historical earthquakes, but the magnitude of the earthquake was below 6. About 1.5-2a before the M5.9 Mojiang earthquake, the composition of hydrogen and oxygen isotopes of the water samples in the Dazhai observation Well showed a significant deviation, accompanied by a continuously increasing concentration of fluoride ions from sources at deeper depths. This might suggest that the deep material in the earthquake source area began to be active. At the same time, starting one year before the earthquake, the phase lag of the water level in the wellhole changed from negative to positive, indicating that the source and pathway of well water recharge have been changed. In addition, around half a year before the earthquake, the continuously observed water chemical ions at shallow depths in the wellhole began to show a dramatic change. Moreover, macroscopic anomalies of hot spring water volume increased sharply before the earthquake, showing a remarkable evolution process from deep to shallow, from background to short-term, and from micro anomalies to macro anomalies before the earthquake. To investigate the causes and mechanisms of this phenomenon, we attempt to discuss the abnormal evolution process before the M5.9 Mojiang earthquake from the aspects of regional deep material activity and regional stress level. The abnormal concentration of the hydrochemical ions and the change of aquifer permeability observed continuously at the Dazhai observation well before the M5.9 Mojiang earthquake were caused by the continuous increase in shear stress in the region, which caused the aquifer to be compressed, resulting in a vertical fluid recharge and ultimately the alternation and mixing of different aquifer water bodies. In addition to being controlled by the continuous increase in regional vertical shear stress, the abnormal formation process was also accompanied by the intense activity of deep-sourced chemical elements such as helium isotope and fluoride ion. The abnormal evolution process showed a remarkably coupled process of migration from deep to shallow, which may be the reason why the shallow ion anomaly before the M5.9 Mojiang earthquake was the most significant among all the observed cases. Therefore, the evolution process of fluid activity starting from the deep and continuously transmitting to the surface with the accumulation of regional stress is essential to the abnormal evolution of the hydrological phenomenon before the M5.9 Mojiang earthquake. The regional stress and the process of deep material activity are the biggest differences between the M5.9 Mojiang earthquake and other historical earthquake cases in the study area, which will be the two main factors to be considered when similar ion changes occur again in the future. Our study provides insight into a comprehensive understanding of the predictive significance of underground fluid anomalies in the Dazhai well and the coupled evolution process of deep-shallow fluid anomalies before the earthquake.

Application of new technique

CONSTRUCTION METHOD OF SEISMOTHERMAL INFRARED BACKGROUND FIELD BASED ON GPR-LSTM

SONG Dong-mei, ZHANG Man-yu, SHAN Xin-jian, CUI Jian-yong, WANG Bin

2024, 46(2):  492-511.  DOI: 10.3969/j.issn.0253-4967.2024.02.015

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Seismic monitoring is a very important and challenging task. The continuous development of remote sensing technology has strengthened our ability to monitor the Earth’s surface on a macro scale. Research shows that an abnormal rise in surface temperature usually occurs before an earthquake, so a variety of anomaly extraction algorithms have been applied to the study of seismic thermal anomalies. Among them, the extraction method based on background field is widely used because of its strong mechanism interpretation. However, the previous anomaly methods based on the background field mainly limit the background field to a certain fixed threshold value, and ignore the small range of normal LST fluctuations caused by external factors(non-seismic). Therefore, a method of constructing a seismothermal infrared background field based on GPR-LSTM is proposed in this paper. The main idea of the method is that the LST background field is obtained by adding the established annual variable reference field and the fluctuation range of normal LST. First, the LST exhibits a range of fluctuations due to non-seismic factors such as solar radiation, weather, and human activities. Therefore, it is not reasonable to take a fixed value of the LST background field but it should have a certain fluctuation range. Therefore, the dynamic fluctuation characteristics of the background field should be reflected in the construction process of the background field in this study. Secondly, the reason why this method uses the LSTM model to predict the annual variable reference field of the earthquake period based on the annual variable reference field of the non-earthquake period is that on the one hand, the LSTM model can predict the law of long time series data, so it can better learn the annual variation law of LST in the non-earthquake year. At the same time, the LST trend of increasing or decreasing year by year caused by climate change can be predicted, which is conducive to more truly describing the LST trend of the real background field in the year of the earthquake period.

The method includes two parts: the establishment of the annual variable reference field, the calculation of the residual fluctuation range of the actual LST and the construction of the background field. Based on the MODIS surface temperature product, the precursory thermal anomaly information of the 2008 Wenchuan earthquake in Sichuan Province and the Yutian earthquake in Xinjiang Province was extracted and analyzed by using the proposed method. Based on the MODIS surface temperature data of the 2008 Wenchuan earthquake in Sichuan Province and the Yutian earthquake in Xinjiang Province, the proposed method is used to detect and analyze the thermal anomalies before the earthquake. The following conclusions are drawn:

(1)The established LST background field on the Qinghai-Xizang Plateau is consistent with the actual variation law of LST and is relatively stable on the whole. In the time dimension, the background field of LST shows the annual variation of high summer and low winter. In terms of spatial dimension, the established LST background field is generally high in the south, low in the middle and low in the north, and the temperature value of the background field is the lowest at the highest altitude of the Qinghai-Xizang Plateau.

(2)The thermal anomalies obtained based on the new algorithm are usually distributed along the fault zone of the Qinghai-Xizang Plateau, and the anomaly evolution law is obvious. For the Yutian earthquake in Xinjiang, the trend of thermal anomalies is consistent with that of the fault zone in most cases, and the thermal anomalies are mainly distributed in the northern margin of the fault zone. For the Wenchuan earthquake in Sichuan Province, with the onset time approaching, the thermal anomalies gradually moved southward along the Longmenshan fault zone from the north margin, filled the entire fault zone before the earthquake, and disappeared after the earthquake. This proves the validity of the proposed background field construction method.

(3)Compared with non-seismic years, the spatial characteristics of thermal anomalies along faults are more obvious in seismic years, and the duration and amplitude of the anomalies are longer. This indicates that the tectonic activity in seismic years is more active than that in non-seismic years, resulting in more significant abnormal warming of surface temperature.

(4)The evolution law of thermal anomalies of earthquakes with magnitude 7 or above on the Qinghai-Tibet Plateau can be summarized as incubation—disappearance—accumulation—disappearance—earthquake occurrence, and the strength of thermal anomalies appears in the way of repeated changes. This indicates that the thermal anomaly is not continuously enhanced or weakened before the earthquake but shows the characteristics of repeated fluctuations between strong and weak until the earthquake eruption thermal anomaly disappears.

In conclusion, the proposed method can not only ensure that the established background field is not affected by tectonic factors but also fully consider the small range of normal fluctuation of surface temperature caused by non-tectonic factors, which makes the extraction of thermal infrared anomaly information more accurate. This provides new technical means and ideas for the extraction of earthquake anomaly information and is helpful for the further development of earthquake monitoring.