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20 August 2022, Volume 44 Issue 4

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Research paper

NEW UNDERSTANDING OF THE MAGMA EVOLUTION OF CHANGBAISHAN-TIANCHI VOLCANO BASED ON MELTS SIMULATION

ZHOU Bing-rui, PAN Bo, YUN Sung-hyo, CHANG Cheol-woo, YAN Li-li

2022, 44(4):  831-844.  DOI: 10.3969/j.issn.0253-4967.2022.04.001

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Changbaishan-Tianchi volcano(CBS-TC), located in Jilin Province on the border between China and North Korea, is the largest composite volcano around China, which is still active. The eruption stages of this large Quaternary composite volcano can be roughly divided into 2.0~1.48Ma shield forming stage, 1.48~0.05Ma cone forming stage and the explosive eruption stage since 50000 years ago. Its great eruption activities(the Millennium Eruption)from 946AD to 947AD and magmatic disturbances from 2002 to 2005 have attracted great attention of the government and scholars.

Predecessors have done a lot of researches on Tianchi volcano, including its eruption periods, distribution of eruptive products, disaster assessment and so on. Geophysical data show that there are anomalies in the lower part, indicating the existence of magma chambers or conduits, but the accurate boundary and depth of magma chambers need to be further explored. The study of petro-geochemistry shows that the products of shield forming stage of Tianchi are mainly potassic trachy-basalts. The MgO^# of these basic magma is lower than that of the primary magma in Northeast China, indicating that they are the evolved magma undergoing the process of fractional crystallization. In the past, the cone forming stage was considered to have the characteristic of “bimodal” eruptions, that is, the cone forming eruptions of high SiO₂ trachytic/comenditic magma was accompanied by the low SiO₂ basaltic magma, which formed small cinder cones on the edifice. In recent years, some drilling data show that there are thick basaltic trachy-andesite and trachy-andesite strata under the cone, indicating that the products of the cone forming stage of Tianchi include early basaltic trachy-andesite, medium trachy-andesite and late trachyte. Their SiO₂ and Na₂O+K₂O contents are increasing with the degree of evolution. Since the late Pleistocene, Tianchi volcano has entered the stage of explosive eruptions with strong caldera forming effect. The eruptive products are mainly comenditic/trachytic airborne pumice, ignimbrite and so on. However, there are still many disputes about the magmatic evolution of CBS-TC, especially the evolution process from basalt to trachy-andesite, trachyte and comendite. In this study, we did abundant field geological investigation and collected rock samples of each eruptive stage of CBS-TC, and carried out whole-rock geochemical analysis. The results show that major elements of these samples have continuous linear trends with increasing of SiO₂ content in magma, and the distribution of rare earth elements and trace elements is also consistent, which indicates a continuous evolution process. Meanwhile, compared with intermediate-basic magma, the trachyte and comendite magma in Tianchi has a characteristic of high Th/La and ⁸⁷Sr/⁸⁶Sr values, indicating that the magma has also experienced assimilated contamination by crustal materials. In order to verify this fractional crystallization with assimilation(AFC)process of Tianchi magma, the author uses petro-thermodynamic simulation(MELTS model)to calculate the magma evolution. The condition parameters used in the simulation include temperature, pressure, oxygen fugacity, water content, etc. Those parameters are considered as close as possible to the real situation in the magma system. The conditions of pressure and water content are still controversial, which are limited by this simulation. It is found that the evolution of Tianchi magma tends to have occured under the conditions of low pressure(2kbar)and high water content(≥0.5wt%), and about 10% granitic assimilates were mixed in the late stage of evolution, which is consistent with the previous research on the location of magma chambers and melt inclusions. The simulation results are consistent with the trends of tested major elements of Tianchi volcano. To sum up, we found that besides fractional crystallization, assimilation and contamination of shallow crustal granite also play an important role in the evolution of basalt to comendite.

In this paper, the magmatic evolution of Tianchi volcano has been studied systematically, during which the method of petro-thermodynamic simulation combined with geochemical analysis is used. A series of new understandings have been obtained, including the eruption sequence, magmatic evolution, and contamination processes of Tianchi volcanic rocks. This analysis procedure provides a certain reference for the future study. The conclusions help to better understand this largest active volcano in China, and provide new ideas for interpretation of volcanic monitoring data, which helps prevent volcanic disasters. The study also provides references for the regional construction planning of the government.

THE CHARACTERISTICS OF DEEP ELECTRICAL STRUCTURE IN LONGGANG VOLCANIC AREA, JILIN PROVINCE

ZHAO Ling-qiang, HU Ya-xuan, WANG Qing-liang, ZHU Yi-qing, CAO Cong, LI Zhong-wei, QI Wei, WEN Yu-long

2022, 44(4):  845-858.  DOI: 10.3969/j.issn.0253-4967.2022.04.002

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The Longgang volcano group, located about 150km west of the Tianchi volcano in Changbaishan, is one of the typical monogenic volcanoes formed in China since the Quaternary. The volcano group has the characteristics of high-density distribution and multi-center explosive eruption. At present, more than 160 low-level craters, volcanic cones and caldera lakes have been discovered. The eruption of Longgang volcano group is characterized by multi-cycle, multi-period and multi-stage eruption. In recent years, a large number of studies have shown that Jinlongdingzi volcano in the northwest of Longgang volcanic group underwent a large-scale eruption about 1600 years ago, and this volcanic group now has potential eruption risk. By exploring the electrical structure of the crust and upper mantle in the volcanic area, the structure of the underground magma system can be imaged, which provides key data for volcanic eruptive hazard modeling and further enriches our understanding of the formation mechanism of continental monogenetic volcano in Northeast China. In this paper, the data of a magnetotelluric profile with broadband dense measuring points with a length of more than 160km from Meihekou city in the west to the Changbaishan in the east, passing through the core area of Longgang volcano and Jinlongdingzi volcano, are used for phase tensor decomposition and two-dimensional inversion to obtain the deep electrical structure characteristics along the profile. Whether there are high-level magma chambers in the crust in Longgang volcanic area is discussed. The analysis shows that high-resistivity structures are distributed at different depths in the crust beneath the Longgang volcanic group and its adjacent area, and the high-resistivity structures are deeper under the early volcanic group, which are speculated to be related to the consolidation of magma. There are some obvious large-scale low-resistivity structures under the high-resistivity structures. These low-resistivity structures correspond to the distribution depth of high-resistivity structures in the upper crust of the region and have various depths from west to east. On the whole, these low-resistivity structures may be interconnected at the lower crust and mantle scales and show a trend of continuing to extend to the east and west sides of the study area. It is supposed that these low-resistivity structures are the magmatic system of the middle and lower crust, and the crustal uplift and seismic activity in the study area may be related to the magmatic activity. There may be a magma channel beneath the newly erupted Jinlongdingzi volcano(below 10km), connecting the magma system of the middle and lower crust, and the magma above 10km may have been consolidated. C3 area with a wide range of magma occurrence at a depth of about 30km is located in the east of Longgang volcanic area, which relatively corresponds to the depth and location of magma occurrence obtained from the inversion of previous deformation data. The deformation data reveal that the crustal uplift rate above the region is large, and the seismic data reveal that the region is seismically active, which is a region worthy of keeping an eye on the magmatic activity. The low-resistivity structures of the middle and lower crust found in the eastern part of the section show that they continue to extend to the eastern Changbaishan Tianchi volcanic area. Combined with previous magnetotelluric and seismological research results, it is speculated that the Longgang volcanic group and the Changbaishan volcano may share one magmatic system in the middle and deep parts. The results obtained can provide geophysical basis for volcanic eruption risk prediction and disaster evaluation in the Longgang volcano group.

STUDY ON VOLCANIC GEOLOGY AND HISTORY OF ERUPTION IN EMAN AREA, NORTHERN HAINAN ISLAND

YANG Wen-jian, ZHAO Bo, YU Hong-mei, XU Jian-dong, PAN Bo, WANG Xi-jiao

2022, 44(4):  859-875.  DOI: 10.3969/j.issn.0253-4967.2022.04.003

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As one of the largest Quaternary volcanic clusters in China, the volcanic activities of Qiongbei are characterized by multi-stage and multi-cycle. However, the eruption era of Eman volcanic rocks located in the northwest of Qiongbei volcanic cluster is still controversial. In this paper, we present a comprehensive study of volcanic geology and geomorphology, whole-rock major elements, K-Ar geochronology of volcanic rocks and ¹⁴C geochronology of conch, in order to reveal the epoch of volcanic activity and eruption characteristics of Eman volcanic field. According to the field geological survey, it is found that Eman volcanic field has many craters, such as Bijialing, Chunliling, Bingmajiao, Longmenjilang, Longmen Pharos, and Zhangwu. The main types of eruptions are effusive eruption, phreatomagmatic explosive eruption and weakly magmatic explosive eruption. Lava flows almost cover the entire volcanic field, with an area of about 26.3km², which are mainly formed by the eruptions of Bijialing and Chunliling volcanoes in the central-south of the volcanic field. Among them, Bijialing volcano consists of five volcanoes, with steep-slope cones and grayish-black block lavas. Chunliling volcano is located in the east of Bijialing, with gentle slope cone, few lava outcrops and spherical weathering. However, the distribution of base-surge deposits, spattering deposits and scoria is relatively small, and limited to the vicinity of Longmenjilang to Wucaiwan and Zhangwu Village. They were formed by phreatomagmatic explosive eruptions of Bingmajiao and Zhangwu volcano, as well as weakly magmatic explosive eruptions of Longmenjilang and Longmen Pharos volcanoes. Moreover, compared with the Holocene Shishan and Late Pleistocene Daotang basalts in Haikou, Eman volcanic rocks have a wider range of silicon(SiO₂=51.39%~55.00%)and alkali(K₂O+Na₂O=3.51%~8.48%)content. Nevertheless, they are general intermediates, mainly composed of basaltic andesite, basaltic trachyandesite and trachyandesite, and experienced fractional crystallization of olivine and clinopyroxene in the process of magmatic evolution. Combining the weathering degree of volcanic rocks(spherical weathering, red clay layer), volcanic geology and geomorphology(cone morphology, slope), petrology and petrogeochemistry(difference of major elements, olivine phenocryst alteration), K-Ar age of volcanic rocks(0.12~0.44Ma)and ¹⁴C age of conch((43.27±0.67)kaBP), we conclude that the eruption era in Eman volcanic field belongs to the Middle and Late Pleistocene.

A STUDY ON LONG-TERM GRAVITY VARIATION AND ITS MECHANISM IN THE WESTERN YUNNAN EARTHQUAKE PREDICTION STUDY AREA

HAO Hong-tao, WANG Qing-hua, ZHANG Xin-lin, WEI Jin, WU Gui-ju, HU Min-zhang

2022, 44(4):  876-894.  DOI: 10.3969/j.issn.0253-4967.2022.04.004

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The long-term variation of gravity field can provide an important reference for studying the regional dynamic background. In 1980s, the Western Yunnan Earthquake Prediction Study Area was established by the State Seismological Bureau for the purpose of monitoring and predicting earthquakes. Since 1984, gravity monitoring in the Western Yunnan Earthquake Prediction Study Area has been carried out continuously by Yunnan Earthquake Agency and Institute of Seismology of China Earthquake Administration. In this research, long-term gravity variations in the Western Yunnan Earthquake Prediction Study Area are obtained by using 62 campaigns relative gravity data and absolute gravity data of Xiaguan station, Lijiang station and Eryuan station from 1986 to 2014 in this area. On this basis, we first analyze the relationship between gravity variation and tectonic activity background in combination with fault distribution and historical seismicity. Then, the mechanism of gravity variation is discussed combined with the vertical crustal deformation, crustal structure and dynamic background. The main results are as follows.

1)After a fine processing of the gravity data, gravity variation rates of 87 gravity stations are obtained, among which 77 stations show negative change. This indicates that the long-term gravity variation background in the study area is dominated by negative changes. The annual average rate of 87 gravity stations is about -1.24×10^-8m/s², which is consistent with the mean gravity variation rate in the Qinghai-Tibet Plateau and its adjacent areas by absolute gravity observation from the existing research result.

2)In terms of spatial distribution, the intensity of gravity variation is closely related to the distribution of fault zones and historical strong earthquakes. In the area along the north section of Honghe Fault and Longpan-Qiaohou Fault, gravity variation shows strongly negative anomalies, and the frequency of historical strong earthquake activity is the highest. This indicates that the gravity variation clearly reflects the strong activity background of the fault zone on the west boundary of the central Yunnan secondary block. In the east area of Chenghai Fault, the spatial distribution of gravity variation is symmetrically positive and negative in a four-quadrant pattern. In 2003, the Dayao M_S6.2 and M_S6.1 earthquakes occurred in the central area of the four-quadrant, and their coseismic gravity variations caused by strike-slip dislocation are consistent with observed four-quadrant characteristics. Therefore, the observed gravity variations reflect the shear stress background in this region. While in the southwest of Yunnan, which is located in the west of the Honghe Fault, the magnitude of gravity variations and frequency of seismic activity decreased significantly in comparison with that of the Honghe fault zone and its eastern region. This indicates that the Honghe fault zone, which is the boundary of Sichuan and Yunnan rhombus block, has an obvious boundary control effect on the gravity field variations and tectonic activity in the study area.

3)Vertical displacement velocities of GNSS stations in the study area are collected, and then the vertical displacement gravity effect and observed gravity variations of 13 GNSS stations are analyzed. The result shows that ground surface vertical movement is dominated by uplift, and the gravity effect of surface vertical displacement is basically consistent with the observed gravity variation in the direction. This indicates that observed gravity variation reflects the uplift background of crustal vertical movement in this area. The magnitude of the average observed gravity variation is about 0.784×10^-8m·s^-2/a after removing the denudation-caused gravity change rate, while the average gravity effect of vertical displacement is about -0.252×10^-8m·s^-2/a, accounting for just about 30% of the observed gravity variation. Therefore, observed gravity variation cannot be explained completely by vertical displacement, and mass loss in the study area is also an important factor causing gravity variation.

4)The Western Yunnan Earthquake Prediction Study Area is located in the middle and lower crustal flow channel in the Qinghai-Tibet Plateau. The overall negative trend of gravity variation may be caused by Moho surface subsidence and surface uplift induced by the crustal flow. The spatial distribution details of gravity variation have a good correlation with the density distribution of the middle and upper crust. Therefore, we speculated that spatial distribution details of gravity variation are caused by activity of specific fault and local material distribution changes.

CHARACTERISTICS OF SEISMIC GRAVITY ANOMALY AND GROUND SUBSIDENCE GRAVITY ANOMALY

LIAO Gui-jin, YE Dong-hua, DENG Zhi-hui, LI Chong, TANG Guo-ying, HU Wei-ming

2022, 44(4):  895-908.  DOI: 10.3969/j.issn.0253-4967.2022.04.005

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Gravity anomalies occurred at Lufeng, Bomei, Nantang, Jiazi, Xixi and other measuring points in the Guangdong Shantou mobile gravity measurement network before and after the Lufeng M_L4.3 earthquake on September 24^th, 2015, and the area was named as Lufeng gravity anomaly area, where the accumulated change of gravity in two years was greater than three times of the RMS errors of the observations and medium-term decline and reverse turning rise appeared. Meanwhile, gravity anomalies also appeared in the measurement points of Chendian and Heping, so the area was named as Chendian gravity anomaly area, where the gravity showed continuous monotonic increase. The two gravity anomaly areas were adjacent to each other, but the nature of the gravity anomalies might be different, there might be seismic gravity anomalies and ground subsidence gravity anomalies. In order to analyze the development trend of later earthquakes, it is necessary to determine the nature of gravity anomaly. The method of data analysis and field verification was used to distinguish the nature of gravity changes in each gravity anomaly area. The results mainly show that: 1)the Lufeng gravity anomaly area is of seismic gravity anomaly, while the Chendian gravity anomaly area is of ground subsidence gravity anomaly. Through the characteristic analysis of seismic gravity anomalies and ground subsidence gravity anomalies, we had a better understanding of the correlation between seismic gravity field evolution and seismic development, which helped to extract the precursor information of seismic gravity changes and predict earthquake. 2)The gravity changes at the observation points of Lufeng, Bomei, Nantang, Jiazi and Xixi in Lufengthe gravity anomalies area began to decline synchronously in August 2014 and rose synchronously in August 2015, forming a positive gravity anomaly area with regional synchronous decline and reverse turning rise. Through anomaly investigation and verification, no interference source to the observation environment was found, and the Lufeng gravity anomaly area was of seismic gravity anomalies. It was the manifestation of the evolution process of the seismic gravity field during the preparation of the Lufeng M_L4.3 earthquake on September 24, 2015. 3)The gravity changes at Heping and Chendian observation points in Chendian gravity anomalies showed a continuous monotonic increase resulting from the pumping of a large amount of groundwater in Chendian and Heping, which led to ground subsidence, house and ground cracking, so the gravity anomaly was related to land subsidence. It is determined that the nature of the anomaly was land subsidence gravity anomaly. The gravity change caused by ground subsidence at Chendian from March 1995 to July 2016 was 292μgal, and the gravity change caused by ground subsidence at Heping from March 2006 to July 2016 was 137μgal. In the analysis of seismic gravity anomalies, the gravity changes at Heping and Chendian should deduct the gravity changes caused by ground subsidence. 4)For obvious gravity anomalies, the background conditions of the anomalies should be understood in detail, such as geological conditions, the use of domestic water and industrial water, ground subsidence, ground fissures, house fissures, etc., and the source of the anomaly should be found. It is necessary to collect water level observation data and hydrological observation data in gravity anomaly area for trend analysis. 5)According to the comparative analysis of the characteristics of time series gravity variation curve of the gravity anomaly points in the gravity anomaly area, the gravity anomalies of the observation points in Chendian gravity anomaly area showed a long-term continuous monotonic increase, while that in the Lufeng gravity anomaly area showed a medium-term decline and reverse turning rise. By analyzing the gravity variation of the adjacent observation points in the abnormal area, and analyzing the difference of the geological conditions and the field survey data, we can basically judge the nature of the gravity anomalies.

STUDY ON THE LATE QUATERNARY ACTIVITY OF THE WEST XIADIAN FAULT IN BEIJING PLAIN

SHEN Jun, DAI Xun-ye, XIAO Chun, JIAO Xuan-kai, BAI Qilegeer, DENG Mei, LIU Ze-zhong, XIA Fang-hua, LIU Yu, LIU Ming

2022, 44(4):  909-924.  DOI: 10.3969/j.issn.0253-4967.2022.04.006

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Beijing plain is a strong earthquake tectonic area in China, where the Sanhe-Pinggu earthquake with M8 occurred in 1679.The seismogenic fault of this earthquake is the Xiadian Fault. An about 10km-long earthquake surface fault is developed, striking northeast. Deep seismic exploration reveals that this surface fault is a direct exposure of a deep fault cutting through the whole crust, and it is concealed in the Quaternary layers to both ends. Previous studies have not yet revealed how the deep fault with M8 earthquake extended to the southwest and northeast. In the study of Xiadian Fault, it is found that there is another fault with similar strike and opposite dip in the west of Xiadian Fault, which is called the West Xiadian Fault in this paper. In this study, six shallow seismic profiles data are used to determine the location of this fault in Sanhe city, and the late Quaternary activity of the fault is studied by using the method of combined drilling, magnetic susceptibility logging and luminescence dating.

The results of shallow seismic exploration profiles show that the fault is zigzag with a general strike of NE and dip NW. In vertical profile, it is generally of normal fault. It shows the flower structure in one profile, which indicates that the fault may have a certain strike-slip property. On two long seismic reflection profiles, it can be seen that the northwest side of the fault is a half graben structure. This half graben-like depression, which has not been introduced by predecessors, is called Yanjiao fault depression in this paper. The maximum Quaternary thickness of the graben is 300m. The West Xiadian Fault is the main controlling fault in the southern margin of the sag.

The Xiadian Fault, which is opposite to the West Xiadian Fault in dips, controls the Dachang depression, which is a large-scale depression with a Quaternary thickness of more than 600m. The West Xiadian Fault is opposite to the Xiadian Fault, and there is a horst between the West Xiadian Fault and the Xiadian Fault. The width of the horst varies greatly, and the narrowest part is less than 1km. The West Xiadian Fault may form an echelon structure with Xiadian Fault in plane, and they are closely related in depth.

According to the core histogram and logging curves of ten boreholes and eight effective dating data, the buried depth of the upper breakpoint of the concealed fault is about 12m, which dislocates the late Pleistocene strata. The effective dating result of this set of strata is(36.52±5.39)ka. There is no evidence of Holocene activity of the fault, but it is certain that the fault is an active fault in the late Pleistocene in Sanhe region. The vertical slip rate is about 0.075mm/a since late Pleistocene, and about 0.03mm/a since the late period of late Pleistocene. These slip rates are less than those of the Xiadian Fault in the same period. According to our study, the vertical slip rate of Xiadian Fault since late Pleistocene is about 0.25mm/a.

Although the latest active age, the total movement amplitude since Quaternary and the sliding rate since late Pleistocene of West Xiadian Fault are less than those of Xiadian Fault, its movement characteristics is very similar to that of Xiadian Fault, and the two faults are close to each other in space, and closely related in deep structure. It can be inferred that the fault is probably a part of the seismogenic structure of the 1679 Sanhe-Pinggu M8 earthquake. In a broad sense, the Xiadian fault zone is likely to extend to the southwest along the West Xiadian Fault.

PRELIMINARY STUDY ON FAULTED LANDFORMS AND AGES OF RECENT STRONG EARTHQUAKE ACTIVITY ON THE KARAKORUM FAULT IN NGARI, TIBET

XU Wei, LIU Zhi-cheng, WANG Ji, GAO Zhan-wu, YIN Jin-hui

2022, 44(4):  925-943.  DOI: 10.3969/j.issn.0253-4967.2022.04.007

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The Karakoram Fault is located in the west of the Qinghai-Tibet Plateau and crosses Kashmir, Xinjiang and Tibet in China. It is a large normal dextral strike-slip fault in the middle of the Asian continent. As a boundary fault dividing the Qinghai-Tibet Plateau and the Pamir Plateau-Karakoram Mountains, the Karakoram Fault plays a role in accommodating the collision deformation between the Indian plate and the Eurasian plate and in the tectonic evolution of the western Qinghai-Tibet Plateau. The fault trace in Ngari area is clear and the faulted landforms are obvious, which show strong activity characteristics in late Quaternary. As a large active fault, only one earthquake of magnitude 7 has been recorded on the Karakoram Fault since the recorded history, namely, the Tashkurgan earthquake of 1895 at its north end. There are no records of strong earthquakes of magnitude≥7 along the rest of the fault, and no paleo-seismic research has been carried out. Ages of recent strong earthquake activity and earthquake recurrence intervals are not clear, which greatly limit the accuracy of seismic risk assessment. In this study, we investigated the fault geometry and faulted landforms in Ngari area, collected OSL samples of the faulted landforms and sag ponds in Zhaxigang, Menshi and Baga towns and preliminarily discussed the ages of recent strong earthquake activity.

Study shows that the fault can be divided into three sections by Zhaxigang town and Suoduo village, and the structure and properties of each section are significantly different. In west Zhaxigang town section, the fault is dominated by dextral strike-slip with certain vertical movement, it is almost straight on the surface, with river terraces, alluvial-proluvial fans and water system faulted ranging from tens to hundreds of meters. In Zhaxigang town to Suoduo village section, the normal faulting is remarkable, the main fault constitutes the boundary fault between Ayilari Mountain and Gar Basin; fault facets and fault scarps are common along the fault line, there are also secondary faults with the same or opposite dip as the main fault developed near the piedmont basin. In east Suoduo village section, the main part of the fault is located at the south foot of Gangdise Mountain, and in addition to the piedmont fault, several approximately parallel faults are also developed on the southern alluvial-proluvial fans and moraine fans which are mainly dextrally faulted with certain vertical component.

According to the analysis of the faulted landforms and dating of the OSL samples collected from the sag ponds and faulted landforms in the west of Zhaxigang town, the east of Menshi town and the east of Baga town, the ages of recent strong earthquake activity on the fault are analyzed as follows. In the west of Zhaxigang town, the age of recent strong earthquake activity of the fault is constrained to be close to 2.34kaBP according to the average OSL dating results of KKF-3 and KKF-4. In the east of Menshi town, the recent earthquake activity age of fault f₂ is 4.67~3.01kaBP, but closer to 3.01kaBP according to the OSL dating results of KKF-11 of the youngest faulted geomorphic surface and average OSL dating results of KKF-6 and KKF-13 collected from sag ponds. In the area near Angwang village, Baga town, it is inferred that the recent strong earthquake activity age of the fault is close to 2.54kaBP according to the OSL dating results of KKF-2 collected from sag pond. If the faults of above three places are active at the same time, the age of recent strong earthquake activity of the fault is close to 2.63kaBP. The Karakorum Fault in Ngari area has obvious segment boundaries, and the activity of each segment and in its internal branch faults is most likely to be independent.

The earthquake recurrence interval on the fault is estimated to be 2.8ka according to the slip rate and the amount of displacement. From the above analysis, it can be seen the time since the last strong earthquake activity of Karakorum Fault may have been very close to the interval of earthquake recurrence. If the fault is characterized by a quasi-periodic in-situ recurrence, the energy accumulation in the fault may have reached a very high degree and the risk of recurrence of strong earthquake events of the fault may be very high, so more attention should be paid and more detailed research on the paleo-earthquake events and recurrence intervals should be carried out as quickly as possible.

PROVENANCE TRACING OF PB ISOTOPES OF FLUVIAL DETRITAL K-feldspar FROM THE YELLOW RIVER BASIN

LIN Xu, LIU Hai-jin, LIU-ZENG Jing, WU Zhong-hai, LI Zhao-ning, CHEN Ji-xin, LI Ling-ling, HU Cheng-wei

2022, 44(4):  944-960.  DOI: 10.3969/j.issn.0253-4967.2022.04.008

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The collision of the Indian plate with Eurasia in the early Cenozoic era drove the emergence of the Tibetan plateau. At the same time, the subduction of the western Pacific plate towards Eurasia resulted in the stretching and thinning of the lithosphere in eastern Asia, leading to a series of faulted basins and marginal seas. The macro-geomorphic pattern of East Asia was finally established under the control of these two tectonic domains. In this case, the Yellow River, which originated from the Tibetan plateau and flowed through the Loess Plateau and the North China Plain, carried a huge amount of detrital material into the Bohai Sea, which played an important role in the regional geochemical cycle, environmental change, sedimentary flux and the diffusion of detrital material in the shelf sea. Therefore, tracing sediment sources in the Yellow River Basin is of great importance for understanding the coupling relationship between uplift and denudation in the northeastern Tibetan plateau, East Asian monsoon evolution, and detrital material accumulation. However, the Yellow River Basin spans multiple climatic and tectonic zones with different provenance areas, so it is particularly critical to select appropriate provenance tracing methods.

Although K-feldspar is more vulnerable to chemical weathering than zircon, it is a widely distributed rock forming mineral and can best represent the provenance characteristics of a certain area. The non-clay minerals in the Yellow River Basin are mainly composed of quartz and feldspar. At the same time, the Pb isotope ratios(²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb)of K-feldspar in different blocks are much different from those of Nd and Sr isotope systems and are often used to construct regional Pb isotope geochemistry province, continental crust evolution, and reconstruct paleocurrent direction, etc. In recent years, detrital K-feldspar Pb isotopic composition has been successfully used to trace the provenance of the Indus, Yangtze and Mississippi Rivers. But this method has not been carried out in the Yellow River Basin. Therefore, we systematically analyzed the detailed K-feldspar Pb isotopic compositions from the Yellow River Basin, and compared the results with the potential source areas to determine the specific source areas. It also can provide basic comparative data for future studies on the formation age of the Yellow River and material source areas of the Loess Plateau and deserts in the northwestern China.

We analyzed 15 samples from the Yellow River Basin and obtained 967 in-situ Pb isotopic results of K-feldspar grains by laser erosion inductively coupled plasma mass spectrometer(LA-MC-ICP-MS). K-feldspar grains in the samples from the Yellow River are angular, subangular and subcircular, with diameters ranging from 20μm to 300μm. The ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of K-feldspar grains from the source of the Yellow River to Lanzhou city range from 20 to 16 and 42 to 36. However, some ratios of ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb of K-feldspar grains from the Lanzhou city range from 23 to 19 and 40 to 37, respectively. The ²⁰⁶Pb/²⁰⁴Pb ratio of most K-feldspar samples in Bayannur city is greater than 19, and the maximum value is 24.79, while this ratio from Hequ and Hancheng cities located in the middle reaches of the Yellow River is less than 18.5. The ²⁰⁶Pb/²⁰⁴Pb ratios of the Mesozoic sandstone near the Hequ city range from 16 to 15. The ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of K-feldspar grains from the Weihe River, which is the largest tributary of the Yellow River, range from 19 to 17 and 40 to 37. The ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of K-feldspar grains in the Fenhe River, Yiluohe River, Kaifeng and Lijin cities range from 21 to 14 and 42 to 33. The comparison results of ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios show that the Pb isotopic compositions of K-feldspar grains in the upper Yellow River, Daxiahe River and Huangshui River are significantly different from those in the Lanzhou city. The Pb isotopic composition of K-feldspar grains from the Yellow River from the Lanzhou city is consistent with that in the Bayannur city, which is influenced by similar eolian provenance. K-feldspar grains from the Yellow River and Fen River in the Jinshan Gorge are mainly from the Loess Plateau. By contrast, the K-feldspar grains in the Weihe River are mainly derived from the Qinling Mountains. The Pb isotopic compositions of K-feldspar grains in the Kaifeng and Lijin cities of the lower Yellow River are different to those in the upper Yellow River and the North China Plate, but similar to those in the middle reaches of the Yellow River. The Loess Plateau plays a leading role in the source of K-feldspar gains in the middle and lower reaches of the Yellow River.

THE RESPONSE OF FLUVIAL LANDFORM TO THE EVOLU-TION OF FAULT STRUCTURE IN THE NORTHERN ZHONGTIAO MOUNTAINS FAULT

LU Ben-tian, LI Zhi-gang, LIANG Hao, YANG Jing-jun, ZHENG Wen-jun

2022, 44(4):  961-975.  DOI: 10.3969/j.issn.0253-4967.2022.04.009

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As an important part of the land geomorphic unit, river is one of the main geological forces to shape the surface morphology. The fluvial geomorphic development characteristics are extremely sensitive to tectonic activities and record rich tectonic deformation information in geological history. Therefore, through the information extraction and quantitative analysis of bedrock river, we can reverse the relevant information about the tectonic evolution history. By extracting topographic information, comprehensively analyzing the spatial differences of fluvial geomorphological parameters, sieving the influencing factors such as tectonic, climatic and lithological characteristics, and quantifying the intensity of tectonic activity have become an important research tool for the segmental differences of active faults.

The Northern Zhongtiao Mountains Fault is an active fault that controls the uplift of the Zhongtiao Mountains and subsidence of the Yuncheng Basin, and can be divided into the Hanyang, Yongji, Yanhu and Xiaxian sections from south to north. The activity of each section of the fault is closely related to the shaping of the present-day topography of the Zhongtiao Mountains, and it is a typical area for applying quantitative analysis of fluvial landform to the study of the segmentation differences along the fault. So we can effectively study the distribution characteristics of tectonic activity in the fault zone through the river geomorphological features of Zhongtiao Mountains. In this paper, by extracting information on the river topography of the bedrock mountain watershed system on the northern slopes of the Zhongtiao Mountains, parameters such as the normalized steepness index k_sn, slope S, geometric features of the stream longitudinal profile of the drainage system, the location of the knickpoints and the amount of variant incision between upstream and downstream of the knickpoints are obtained. The results show that the bedrock channels on the northern slopes of the Zhongtiao Mountains has experienced accelerated incision in the longitudinal direction, and that the spatial variation of geomorphological parameters such as the normalized steepness index k_sn, slope S and fluvial incision in the lateral direction is dominated by tectonic uplift, with high values in the Hangyang-Yongji section and decreasing in a segmental manner towards the west, which is consistent with the topographic relief of the Zhongtiao Mountains, but contradicts the high slip rate area and the Cenozoic subsidence centre(the Salt Lake).

The geomorphic response to the slip rate is inconsistent with the topographic relief of the Zhongtiao Mountains, which is high in the west and low in the east. The high value area of geomorphic parameters reveals that the present active tectonic area of the Northern Zhongtiao Mountains Fault is located in the Hanyang-Yongji segment in the south, rather than the salt lake segment with high activity rate. The reason may be related to the migration of part of the activity of Huashan piedmont fault along the NE-trending hidden fault of Huayin Shouyang to the Hanyang Yongji segment of Zhongtiao Mountains. It suggests that the tectonic activity center of the Northern Zhongtiao Mountains Fault moves westward. Compared with the structural deformation caused by the change of sedimentary center, the time scale of river geomorphology response to structural deformation is shorter, and the landform is transformed most rapidly, which leads to the inconsistency between the geomorphological parameters and structural activities of the fault at the Northern Zhongtiao Mountains Fault.

A DISCUSSION ON THE RELATIONSHIP BETWEEN THE SUR-FACE RUPTURE ZONE IN FRONT OF THE AMUNIKESHAN MOUNTAIN AND THE 1962 M6.8 EARTHQUAKE

YAO Sheng-hai, GAI Hai-long, YIN Xiang, LIU Wei, ZHANG Jia-qing, YUAN Jian-xin

2022, 44(4):  976-991.  DOI: 10.3969/j.issn.0253-4967.2022.04.010

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The investigation of seismogenic structure of historical strong earthquakes and the research on the genetic link between earthquakes and active faults are a basic seismogeologic work. In particular, the investigation of seismic surface rupture zones and the study of seismogenic structures are extremely important for understanding the characteristics of their tectonic activities. The determination of the macro-epicenter provides important evidence for the site selection for post-disaster reconstruction and avoidance. Due to the diversity of the rupture process in the focal area, the macro-epicenter and the micro-epicenter may not be identical. As the magnitude increases, the larger the focal area of an earthquake is, the more significant the gap between the macro-epicenter and the micro-epicenter will be.

The northern margin of the Qaidam Basin is an area with frequent earthquakes, where many earthquakes with magnitude above 6.0 occurred in the history. In the early and late 1990s, small earthquake swarms with long duration and high frequency occurred in this area, which caused considerable losses to the local industry. Since the Delingha earthquake of magnitude 6.6 in 2003, two earthquakes with magnitude 6.3 and 6.4 occurred in the northern margin of the Qaidam Basin in 2008 and 2009, which aroused great attention of researchers. A new research focus has emerged on this area, and many scholars conducted in-depth research on the faults of the northern margin of the Qaidam Basin.

The author conducted a preliminary remote sensing interpretation of the Amunikeshan Mountain segment of the northern margin of the Qaidam Basin and found that there is a very straight linear feature in the image of the Amunikeshan mountain front. On the basis of remote sensing interpretation, a related study was carried out on the Amunikeshan segment of the northern margin fault of the Qaidam Basin, which was considered to be a Holocene active fault. Since the late Holocene, the horizontal movement rate of the fault is 2.50~2.75mm/a, and the vertical movement rate is(0.43±0.02)mm/a. A 30km-long earthquake surface rupture zone was found in front of Mount Amunikeshan. It is preliminarily believed that the rupture might be caused by a strong historical earthquake. According to the catalogue of historical strong earthquakes and local chronicles, there were earthquakes of magnitude 6.8 and 6.3 occurring in this area on May 21, 1962 and January 19, 1977, respectively. There has been no detailed research report on these two earthquakes.

Through on-the-spot geological investigation, it is found that there are fault scarps, fault grooves, seismic bulges and ridges, twisted water system and other landforms developed along the line, forming a surface rupture zone with a strike of N30°-40°W, a coseismic displacement of 2.3m, and a length of about 22km. Through trenching and excavation, the trench section reveals several faults, indicating the characteristic of multi-stage activity. In the section, the faults ruptured to the surface, and the late Quaternary activity is obvious. Combining surface relics, geological dating, and micro-geomorphic measurements, it is determined that the nature of the fault is mainly strike-slip with thrust. The investigation has found many seismic geological disasters, such as landslides, rockfalls and ground fissures along the fault, which are judged to be generated in recent decades or centuries.

Based on the empirical statistical relationship between magnitude and surface rupture, and the empirical relationship between strike-slip fault and rupture length, the average magnitude required for producing a 22km-long earthquake surface rupture is 6.79, and the average magnitude for producing a 2.3m coseismic displacement is 7.03. In combination with the surface rupture, trench profile, geological dating, seismic geological disasters, empirical formula calculation, historical earthquake catalogue, local chronicles and other documents, it is considered that the rupture zone is most likely produced by the North Huobuxun Lake M6.8 earthquake on May 21, 1962, and its seismogenic fault is the Amunikeshan Mountain segment of the northern margin fault of the Qaidam Basin.

Since the study area has no permanent residents or buildings(structures), which are taken as the basis for inquiring and investigating the earthquake intensity, we are unable to draw the earthquake intensity map.

THE FOCAL MECHANISM AND TECTONIC SIGNIFICANCE OF THE M_S5.6 EARTHQUAKE ON JULY 24, 2009 IN NIMA, TIBET

LI Zong-xu, HE Ri-zheng, JI Zhan-bo, LI Yu-lan, NIU Xiao

2022, 44(4):  992-1010.  DOI: 10.3969/j.issn.0253-4967.2022.04.011

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The paper collects the seismic waveforms of the M_S5.6 earthquake that occurred in southern Nima, central Tibe on July 24, 2009 recorded by Tibet seismic network and the mobile seismic networks of the orresponding period, i.e. Western Tibet/Y2 and TITAN. The seismic waveform data were preprocessed by rglitches, rmean, rtrend, taper, transfer and filtering. Then we hand-picked the arrival times of the P-and S-waves(0.05~2Hz for P wave, and 0.05~0.5Hz for S wave). The Hypo2000 method was applied to accurately relocate the earthquake.

Because the earthquake occurred in the hinterland of Tibetan plateau, there are few local seismic stations available. Since the seismic stations and seismic phase information used in processing by different institutions are different, the epicenter location and focal mechanism determined by various institutions are different. Compared with the result(31.30°N, 86.10°E)relocated by Tibet seismic network, our result(31.08°N, 86.05°E)is more reliable due to the uniform distribution of stations used in our study, which is roughtly identical to the GCMT result(31.05°N, 86.10°E)inverted by the moment tensor method.

Based on the relocated result, we apply the Cut-and-Paste(CAP)inversion method to invert the focal mechanism and focal depth. The waveform is decomposed into Pn1 and surface wave to perform cross-correlation fitting of theoretical waveform and actual waveform, respectively. To suppress the noise and influence of the source region medium, the bandpass filter is selected as 0.05~0.15Hz for body wave and 0.05~0.1Hz for surface wave. We set the earthquake source time function as 5s and search for the best focal depth at the depth of 1~30km, and the search step is 1km concerning the magnitude of the earthquake. The result shows that the earthquake has a best-fitting focal depth of 19.3km from the mean sea level and is of strike-slip faulting(the nodal plane Ⅰ: 220°/82°/-17° and nodal plane Ⅱ: 314°/73°/-171°).

The shear stress and normal stress of the two nodal planes of the earthquake are calculated according to the stress field characteristics of the earthquake area. The generation of the earthquake is consistent with the stress field characteristics of NS compression and EW extension in the region. Referring to the near-EW strike-slip fault zone constrained by the EW-trending Wozang Fault and the NWW-trending Zhala Fault in the 1︰250000 regional geological survey map near the epicenter area, it is inferred that the earthquake is of EW-trending dextral strike-slip faulting.

Most of the earthquakes that occurred along the 31°N belt near this earthquake area are EW-trending strike-slip ones, even in the interior of the Tangra-Yumco Rift. Considering the physical properties beneath Tibetan plateau, the low-velocity and high-conductivity layers are widely distributed in the depth range of 20km to 30km in the thick crust. According to surface geology and deep structures revealed by regional geophysics(receiver function, magnetotellurics, and tomography)of the region, the earthquake occurred on the top of the brittle-ductile transition zone with a low seismic velocity between the middle and upper crust beneath the south boundary faults of the Seng-ge Kambab-Lhaguo Tso-Yongzhu-Jiali ophiolite mélange zone(SYMZ), 30km away from the Tangra-Yumco Rift to the west. The occurrence of the earthquake indicates that SYMZ, which formed in the Late Jurassic, was reactivated in an EW-trending strike-slip manner during the quick uplift of the plateau. This cognition is of great significance to understand the geodynamic mechanisms of the EW-trending extension within the Tibetan plateau.

THREE-DIMENSIONAL S-WAVE VELOCITY DISTRIBUTION BASED ON AMBIENT NOISE ANALYSIS IN SOUTH CHINA BLOCK AND ITS ADJACENT AREAS

GONG Meng, LÜ Jian, ZHENG Yong, XIE Zu-jun, SHENG Shu-zhong, ZHANG Xing-mian

2022, 44(4):  1011-1028.  DOI: 10.3969/j.issn.0253-4967.2022.04.012

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The South China block, located in the east of the Eurasian plate, mainly consists of the Yangtze block and the Cathaysia block. The South China block is bounded by the eastern margin of the Qinghai-Tibet Plateau in the west, the Qinling-Dabie orogenic belt in the north, and its eastern boundary extends from the southeast coast to the north, through the Taiwan Strait, and then along the Ryukyu Island arc to the west direction. The neotectonic movement of the South China block is intense. It is not only the continental margin with the most active crustal growth and continental accretion, but also the tectonic belt with the most intense core-mantle mass transfer and the coupling zone of the inner layers of the Earth. Therefore, the crust-mantle velocity structure of the South China block and its formation and evolution have always been a hot topic in earth science research.

In this paper, we collected continuous vertical component broadband seismic data between January 1, 2010 and December 31, 2012 from the regional networks of 609 stations and used ambient noise tomography method to inverse the three-dimensional S-wave velocity structure of South China block and its adjacent area. Firstly, the seismograms are cut into daily segments and decimated at a sampling rate of 1Hz. After the removal of the mean, trend, and instrument response, a 3~150s band-pass filter is applied. In order to reduce the effect of earthquakes and instrumental irregularities on cross-correlations, we normalized the seismograms with a time-frequency normalization method. Then, we computed daily cross-correlations for each station pairs and stacked all of them by using normalized linear stacking method to obtain cross-correlation functions. Next, the phase velocity dispersion curves of Rayleigh surface wave were extracted by frequency-time analysis method. Finally, the three-dimensional S-wave velocity structure of the study area was obtained by using nonlinear Bayesian Monte Carlo inversion method.

The results show that the S-wave velocity distribution has a good correlation with surface geological and tectonic features, and could clearly reveal the lateral velocity variation in the crustal. The shallow S-wave velocity in basin and graben area presents low velocity anomaly due to the influence of sedimentary layer. The high velocity anomaly exists in the middle and lower crust of Jianghan Basin and Sichuan Basin, indicating that the middle and lower crust of these basins are cold and hard. Due to the phenomenon of arching existing in the upper mantle of Sichuan Basin, the S-wave velocity of the crust and mantle is relatively high in the upper mantle, meanwhile, the S-wave velocity in the center of the basin is higher than that in the edge. Although both the Yangtze block and Cathaysia block are located in the South China block, their upper mantle S-wave velocity structures are quite different due to their different evolutionary processes. The high S-wave velocity of the Yangtze block indicates the internal structure of the block is relatively stable, while the low S-wave velocity of the Cathaysia block indicates the strong magmatic activity during its evolution. The crust-mantle S-wave velocities in the west of the southwest boundary of the South China block show low velocity anomalies, which may indicate the existence of asthenosphere in the middle and lower crust of the eastern margin of the Qinghai-Tibet Plateau. The S-wave velocity structures of the eastern and western parts of the Qinling-Dabie orogenic belt are quite different, and the crustal thickness transition zone is the boundary of the S-wave velocity structure, which is high in the east and low in the west. The crust-mantle S-wave velocity of Ordos block is relatively high, indicating that the inner structure of ordos block is relatively stable. However, the S-wave low velocity anomaly in the upper mantle at the southwest corner of the Ordos Basin may indicate that the heat flow of the upper mantle of the North China Craton has begun to “invade” the Ordos lithosphere.

DEEP STRUCTURES OF THE MIDDLE-SOUTHERN SEGMENT OF LANLIAO FAULT ZONE REVEALED BY DEEP SEISMIC REFLECTION PROFILE

LI Qian, SONG Qian-jin, FENG Shao-ying, JI Ji-fa, DUAN Yong-hong, HE Yin-juan, QIN Jing-jing

2022, 44(4):  1029-1045.  DOI: 10.3969/j.issn.0253-4967.2022.04.013

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The research area involved in this paper is the middle-southern segment of Liaocheng-Lankao fault zone(Lanliao fault zone)and its adjacent area. In order to study the fine crustal structure image and the tectonic features of the faults in this tectonic zone, we conducted a 70km-long deep seismic reflection profile along EW direction in Puyang City, Henan Province and got clearer lithospheric structure image along the profile.

As regards data acquisition, we applied the geometry with 30m group interval, 1 160 recording channels and more than 90 folds. Seismic wave exploding applies the 30kg shots of dynamite source with the hole depth of 40~50m. In addition, in order to ensure the signal-to-noise ratio of the deep reflector, explosive quantity of dynamite source is increased to 96kg every 1 000m interval. In data processing, the most important thing is to improve the signal-to-noise ratio. Data processing methods mainly include one-dimensional time-varying filtering combined with two-dimensional filtering, tomographic static correction, residual static correction, deconvolution, normal moveout correction(NMO), dip moveout correction, common mid-point(CMP)stack and post-stack denoising, post-stack migration, etc.

The section with high signal-to-noise ratio has been obtained. There are obvious characteristics of reflection wave groups in the crust, which reflects abundant information about geological structure. On this section, according to this study, the characteristics of deep and shallow structure and crustal reflection structures on both sides of the Lanliao fault zone are obviously different. The crust in this area is composed of brittle upper crust and ductile lower crust. There are rich reflective layers and clear tectonic framework in the upper crust. In the western area of Lanliao fault zone, there is a set of dense reflectors with strong energy, which reflects the sedimentary interface of different times since Mesozoic in the basin. The basement slope with gentle dip to the east is the bottom boundary of the “dustpan-shaped” sedimentary depression. The reflected wave of the crystalline basement presents a group of strong reflection wave groups with good continuity in the eastern area of Lanliao fault zone, which are parallel unconformities on the Ordovician strata of Paleozoic or older strata. There are some secondary faults in the hanging wall of Lanliao Fault, which together with the Lanliao fault zone control the tectonic framework of “dustpan-shaped” sedimentary depression, the Dongpu sag. The reflection structure of the lower crust is relatively simple. On the whole, it is mainly arc reflection with strong energy and short duration.

The depth of Moho surface beneath the central-southern Lanliao fault zone in this area is 31.7~34.8km, where the fault is characterized by a strong reflection band with piecewise continuous distribution in horizontal direction and a duration of about 0.3~0.8s in vertical direction. And it is a transition zone with a certain thickness after geological deformation, rather than a sharp first-order discontinuity, which is consistent with the research results of Li Songlin et al.(2011). This profile reveals 2 deep faults(F_D1 and F_D2)that offset the Moho surface, extend down to the top of the upper mantle and create conditions for the upwelling of hot materials from asthenosphere and the energy exchange in this area. It may also be the cause of arc reflection in the lower crust.

The deep seismic reflection profile shows that faults in the upper crust are well developed. Lanliao Fault is the largest boundary fault in this area, which controls the formation and evolution of the “dustpan-shaped” sedimentary depression and plays an important role in the filling of Paleogene strata in the sag. Pucheng Fault F_P1 and Weixi Fault F_P3 are developed in the hanging wall of Lanliao Fault, which are basement normal faults in the same direction as Lanliao Fault and control the structural framework of the depression. Pucheng Fault, Weixi Fault and Lanliao Fault constitute a domino fault system, which makes the basement of the depression incline to the SEE direction. In addition, a reverse secondary normal fault(Changyuan Fault F_P2)is developed in the hanging wall of Lanliao Fault, which intersects with Weixi Fault F_P3 at TWT 3.0s. These faults and Lanliao faults jointly control the basic structural pattern of the sedimentary sag.

The deep and shallow tectonic framework in this area is controlled by the shallow faults in the upper crust and the deep faults in the lower crust. Deep faults(F_D1 and F_D2)create conditions for the upwelling of hot materials from asthenosphere, while shallow faults play an important role in the formation and evolution of basin structures.

RELATIONSHIP BETWEEN DECREASING AMPLITUDE OF b-VALUE AND THE SEISMOGENIC ZONE OF THE WENCHUAN M_S8.0 EARTHQUAKE

CHEN Li-juan, CHEN Xue-zhong, LI Yan-e, GONG Li-wen

2022, 44(4):  1046-1058.  DOI: 10.3969/j.issn.0253-4967.2022.04.014

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The b-value in the G-R relation proposed by Gutenberg-Richter is often used to evaluate the regional stress level. A lot of studies on the b-value before the M_S8.0 earthquake occurring in Wenchuan, Sichuan on May 12, 2008 have been carried out, and the conclusions are different. There was a big difference in the distribution of low b-value area before the Wenchuan earthquake through space scanning. Most studies on temporal variations believe that there was a significant drop in b-value before the Wenchuan earthquake, but the decrease patterns are different, some results show that the b-value decline was not obvious before the earthquake.

The selection of study area is important in analyzing the temporal variations of b-value. The existing researches are mainly based on the temporal variations of b-value in the surrounding area of the seismogenic zone or rupture area of Wenchuan earthquake. The earthquake samples in the selected area may contain the ones irrelevant with the seismogenic process of the main shock, but these samples will affect the statistical characteristic, leading to the real seismogenic information to be ignored. The low b-value area obtained by spatial scanning is the absolute b-value during the study period, while this low b-value may be caused by the difference in the medium strength or geological structure, and has nothing to do with the dynamic change process of stress. Earthquakes may occur in area with low b-value, and there is no obvious low-value abnormality in absolute b-value before some big earthquakes, while there is a significant decline process before these big earthquakes. The decrease of b-value indicates that the stress level is rising, which reflects the dynamic evolution process of seismogenicity. Therefore, in this study, attention is paid to the decrease process of b-value and the spatial distribution of the decrease range. Based on the theoretical basis that the b-value decreases with the increase of stress, this paper attempts to determine the seismogenic zone before the Wenchuan earthquake by the range of decrease in b-value(Δb). Taking the epicenter of the Wenchuan earthquake as the center, the earthquake catalog from January 2000 to April 2008 in the range of 10°×10° is selected, and the abnormal grid points with a significant decrease in b-value before the Wenchuan earthquake are searched by grid point search method, and the temporal variations of b-value in the region where the abnormal grid points locate are analyzed.

The results show that the abnormal grid points are distributed around the Longmenshan fault zone, Anninghe fault zone and Ma’erkang, when Δb≥5%, and the largest anomalous area is located in the Longmenshan fault zone. The anomaly range decreased gradually with the increase of Δb, which converged to the Longmenshan fault zone and the Ma’erkang area when Δb≥20%, and concentrated in the Dujiangyan-Beichuan section of the Longmenshan fault zone when Δb≥30%, which is located in the southern section of the main rupture zone of the Wenchuan earthquake. The initial rupture point of the Wenchuan earthquake is located in the southeast margin of the anomaly zone. The Longmenshan fault zone is not the only area that experienced the decline in b-value before the Wenchuan earthquake in the study region, but it experienced the largest decline in b-value. The main shock finally occurred in the area with the largest Δb, indicating that the loading of stress before a large earthquake was not limited to the focal area and the stress increased in a large range. The Longmenshan fault zone was subjected to the maximum stress loading before the Wenchuan earthquake, which led to its final rupture. The maximum slip, main rupture range and seismogenic zone of the Wenchuan earthquake obtained by previous studies are similar to the area with the largest Δb obtained in this paper, indicating that the abnormal area with a large Δb can reflect the spatial range of preparation of strong earthquakes. This provides a way for judging the location of strong earthquakes, that is, the region with the largest Δb is more likely to generate strong earthquakes in the future.

We selected an area greater than the distribution of grid points with Δb≥25% as the anomaly area. Earthquakes in this area were calculated. The b-value was calculated with 500 earthquake samples as a time window and the temporal variations of b-value were obtained by sliding with 10 earthquakes. The temporal variations of b-value in the abnormal area are shown in Fig.5b. It can be seen that before the Wenchuan M_S8.0 earthquake, the temporal variations of b-value in the anomaly area experienced three stages of change. In the first stage, the b-value declined continuously from 1.5 to 1.1 from May 2002 to the end of 2005, with a decrease rate of about 27%in three and a half years. In the second stage, the b-value was relatively stable for 2 years, remaining at about 1.1 from 2006 to the end of 2007. In the third stage, the b-value decreased rapidly. In early 2008, the b-value decreased from 1.15 to 1.05 just before the Wenchuan earthquake, reaching a decrease rate of about 9% in just 4 months. Then, the Wenchuan M_S8.0 earthquake occurred. The temporal variations of b-value are similar to the acoustic emission test results of rock, which may reflect the three stages of the development of rock crack underground. The b-value continues to decline while new cracks are constantly formed under the compression of tectonic stress in the first stage. The b-value basically remains stable in the second stage. The relative relationship among the rupture events of different sizes basically remains unchanged, the tectonic stress continues to increase, and the rock internal crack presents a progressive steady extension. The b-value rapidly decreases in the third stage just before the main shock. The rapid decline of b-value before the Wenchuan earthquake may provide a reference for short-term prediction.

The grid point search method has the advantage of searching for the area with Δb in a wide range before strong earthquake, and then, the area with the strongest stress accumulation can be identified according to the amplitude of Δb, thus the seismogenic zone of the future strong earthquakes can be determined. In the actual prediction research, the combination of b-value and other disciplines can provide a useful reference for judging the seismogenic zone.

PRELIMINARY ANALYSIS FOR RUPTURE PROCESS OF THE MAY 22TH, 2021, MADOI(QINGHAI) M_S7.4 EARTHQUAKE

DENG Wen-ze, LIU Jie, YANG Zhi-gao, SUN Li, ZHANG Xue-mei

2022, 44(4):  1059-1070.  DOI: 10.3969/j.issn.0253-4967.2022.04.015

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At 02:04a.m. on May 22th, 2021, a M_S7.4 earthquake struck Madoi County, Qinghai Province, China. The depth of this earthquake is 17km. The epicenter locates at 34.59° north latitude and 98.34° east longitude. It is another major earthquake occurring on a secondary fault within the Bayan Har block in the northern central Tibet Plateau during the past 30 years. Fast finite fault inversion and detailed focal mechanism inversion of the Madoi earthquake can help us better understand the seismogenic environment and its relationship with the faults and thus provide the scientific basis for post-earthquake emergency management and disaster assessment.

In this study, firstly, we use W-phase method to determine focal mechanism of the main shock within 30 minutes after the origin time. The W-phase solution indicates that the main shock is a high-dip strike-slip event and the estimated centriod depth is 11km, the strike/dip/rake of two nodal planes of the optimum double couple model are 102°/81°/11° and 194°/79°/171°. Secondly, as of June 10^th, the China Earthquake Networks Center has reported 57 aftershocks with magnitude larger than 3.0, the distribution of aftershocks indicates a mainly NWW direction. We obtained focal mechanisms of moderate aftershocks with M_S≥4.0 inverted from regional stations in Qinghai, Tibet, Sichuan and Gansu Provinces with the method of full waveform fitting, 12 out of 15 aftershocks are of strike-slip which is consistent with the background tectonics, and the existence of two thrust and one normal type events probably indicates that the rupture process of the main shock was affected by structure in the crust. Finally, combined with the geological background and solution of focal mechanism, we select the nodal plane with strike 102°/dip 81°/ rake 11° as the real fault plane. We use finite fault inversion method to invert the rupture process of Madoi earthquake with teleseismic waveform data. The source time function shows that the total scalar moment M₀ is 1.73×10²⁰N·m(or moment magnitude M_W7.45 ), which is consistent with the result of GCMT. The rupture process has lasted 45 seconds, the energy releasing was slow in the primary 5 seconds, the majority energy released during 10~30s after the main shock, then, the rupture was weakening and the fault was healing gradually. The slip and aftershock distribution of Madoi earthquake indicate an asymmetry bilateral rupture mode. The average rake is~3°, indicating a mainly left-lateral slip. The rupture area is estimated as 140km in length and 15km in depth, the slip distribution on SE and NW of epicenter shows obvious segmentation characteristics. The peak coseismic slip is estimated to be 400cm at 0~20km along strike in SE direction at shallow depth. The rupture of the earthquake did break through the ground surface which possibly causes seismic disaster. On the SE side of the main shock, the slip distribution shows a development into deep crust, while on the NW side, the slip distribution shows a more complicate mode. Over all, our results suggest that the Madoi main shock ruptured on a left-lateral strike-slip fault with high-dip along NWW direction in the Bayan Har block. The rupture length along strike is approximately 140km, slightly less than the length of aftershock distribution and field investigation due to the clear bifurcation geometry at both ends. Focal mechanism result of aftershocks shows that most of them are strike-slip but with variety in strike and dip, indicating the complex seismogenic environment in the fault zone. The slip distribution along strike and depth is highly heterogeneous, indicating that the rupture model has more complicated geometry in the lower crust than the shallow crust which controls the variability of slip distribution.

TYPICAL CASE ANALYSIS ON SETBACK DISTANCE FOR URBAN BURIED ACTIVE FAULT: AN EXAMPLE SITE ALONG THE TANLU FAULT ZONE IN XINYI CITY

CAO Jun, LI Yan-bao, RAN Yong-kang, XU Xi-wei, MA Dong-wei, ZHANG Zhi-qiang

2022, 44(4):  1071-1085.  DOI: 10.3969/j.issn.0253-4967.2022.04.016

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With the acceleration of urbanization process, solving the earthquake and its associated disasters caused by buried active fault in urban areas has been a difficult issue in the construction of urban public security system. It is difficult to deal with the anti-seismic issues of cross-fault buildings using the existing techniques, therefore, reasonable setback distance for buried active fault in urban area is the only method for the planning and construction at the beginning. At present, theoretical research about setback for active fault is becoming more and more mature, and the mandatory national standard “Setback distance for active fault” will be enacted soon. As a result, how to work on the basis of these theories and national standards is in urgent. In recent years, the exploration of urban active faults was successively completed. However, there are no typical cases of how to make full use of the achievements of urban active fault projects in the follow-up work, and how to guide urban construction based on the project conclusions, so as to ensure urban safety and rational development of urban economy.

In this paper, taking a site along the Anqiu-Juxian Fault in the Tanlu fault zone in Xinyi city as an example, based on the results of 1︰10 000 active fault distribution map, and referring to the stipulation of national standard “Setback distance for active fault”, 12 shallow seismic survey lines with a spacing of less than 50m were laid out firstly, and the results of shallow seismic exploration show the existence of two high-dip faults in the site. Secondly, considering the shallow seismic survey results and the geologic site conditions, five rows of borehole joint profiles were selected along five of the shallow seismic survey lines. Based on the location of the faults and stratigraphy in the site revealed by the borehole joint profiles, and considering the latest research results of Quaternary stratigraphy and the conclusion of urban active faults detection, the west branch fault is constrained to be a Holocene active fault and the east branch fault is an early Quaternary fault. As a result, we precisely mapped the trace, dip and upper breakpoint of the fault in the site based on the shallow seismic exploration and joint borehole profile. The accurate positioning of the plane position of the active fault differs by about 200m from the 1:1000 strip distribution map.

According to the relevant national standards and scientific research results, active faults in the site shall be avoided. Based on the surface traces of active faults revealed by the accurate detection in the site, the active fault deformation zone was delineated, and the range of setback distance for active fault was defined outside the deformation zone. The detection results accurately determined the plane distribution of the active fault in the site, which meets the accuracy of the development and utilization of the site. Based on the accurately located active fault trace, and complying with the forthcoming national standard “Setback distance from active fault”, this study not only scientifically determines the setback distance for active fault in the site, but also releases the scarce land resources in the city. This result achieves the goal of scientifically avoiding potential dangerous urban hidden active fault and making full use of land.

The case detection process confirms that the results of urban active fault detection are still difficult to meet the fault positioning accuracy required for specific site development, and the range of active fault deformation zone within the site must be determined based on the precise positioning method for hidden active faults as stipulated in the national standard “Setback distance for active fault”. The national standard “Code for seismic design of buildings” only specifies the setback distance for active faults under different seismic intensity, but does not provide any clear definition of the accuracy of active fault positioning, so it is difficult to define the required active fault positioning degree and boundary range of the deformation zone of active fault in practice. The national standard “Setback distance for active fault” clearly defines various types of active fault detection and positioning methods, determines the scope of active fault deformation zone and the accurate setback distance for active fault in different cases. The specific case proves that before developing and utilizing specific sites along urban concealed active faults, relevant work shall be carried out according to the national standard “Setback distance for active fault” to effectively resolve the issue about the relations between urban development and urban safety, so the promulgation and implementation of national standard should speed up.