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20 August 2020, Volume 42 Issue 4

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Research paper

STUDY ON PALEOEARTHQUAKES ALONG THE JINGHE SECTION OF BOLOKENU-AQIKEKUDUKE FAULT

HU Zong-kai, YANG Xiao-ping, YANG Hai-bo, WU Guo-dong, LI Jun, ZHOU Ben-gang

2020, 42(4):  773-790.  DOI: 10.3969/j.issn.0253-4967.2020.04.001

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The Bolokenu-Aqikekuduk fault zone(B-A Fault)is a 1 000km long right-lateral strike-slip active fault in the Tianshan Mountains. Its late Quaternary activity characteristics are helpful to understand the role of active strike-slip faults in regional compressional strain distribution and orogenic processes in the continental compression environment, as well as seismic hazard assessment. In this paper, research on the paleoearthquakes is carried out by remote sensing image interpretation, field investigation, trench excavation and Quaternary dating in the Jinghe section of B-A Fault. In this paper, two trenches were excavated on in the pluvial fans of Fan2b in the bulge and Fan3a in the fault scarp. The markers such as different strata, cracks and colluvial wedges in the trenches are identified and the age of sedimentation is determined by means of OSL dating for different strata. Four most recent paleoearthquakes on the B-A Fault are revealed in trench TC1 and three most recent paleoearthquakes are revealed in trench TC2. Only the latest event was constrained by the OSL age among the three events revealed in the trench TC2. Therefore, when establishing the recurrence of the paleoearthquakes, we mainly rely on the paleoearthquake events in trench TC1, which are labeled E1-E4 from oldest to youngest, and their dates are constrained to the following time ranges: E1(19.4±2.5)~(19.0±2.5)ka BP, E2(18.6±1.4)~(17.3±1.4)ka BP, E3(12.2±1.2)~(6.6±0.8)ka BP, and E4 6.9~6.2ka BP, respectively. The earthquake recurrence intervals are(1.2±0.5)ka, (8.7±3.0)ka and(2.8±3)ka, respectively. According to the sedimentation rate of the stratum, it can be judged that there is a sedimentary discontinuity between the paleoearthquakes E2 and E3, and the paleoearthquake events between E2 and E3 may not be recorded by the stratum. Ignoring the sedimentary discontinuous strata and the earthquakes occurring during the sedimentary discontinuity, the earthquake recurrence interval of the Jinghe section of B-A Fault is ~1~3ka. This is consistent with the earthquake recurrence interval(~2ka)calculated from the slip rate and the minimum displacement. The elapsed time of the latest paleoearthquake recorded in the trench is ~6.9~6.2ka BP. The magnitude of the latest event defined by the single event displacement on the fault is ~M_W7.4, and a longer earthquake elapsed time indicates the higher seismic risk of the B-A Fault.

QUATERNARY FOLDING OF THE XIHU ANTICLINE BELT ALONG FORELAND BASIN OF NORTH TIANSHAN

WANG Hao-ran, CHEN Jie, LI Tao, LI Yue-hua, ZHANG Bo-xuan

2020, 42(4):  791-805.  DOI: 10.3969/j.issn.0253-4967.2020.04.002

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Tianshan is one of the longest and most active intracontinental orogenic belts in the world. Due to the collision between Indian and Eurasian plates since Cenozoic, the Tianshan has been suffering from intense compression, shortening and uplifting. With the continuous extension of deformation to the foreland direction, a series of active reverse fault fold belts have been formed. The Xihu anticline is the fourth row of active fold reverse fault zone on the leading edge of the north Tianshan foreland basin. For the north Tianshan Mountains, predecessors have carried out a lot of research on the activity of the second and third rows of the active fold-reverse faults, and achieved fruitful results. But there is no systematic study on the Quaternary activities of the Xihu anticline zone. How is the structural belt distributed in space?What are the geometric and kinematic characteristics?What are the fold types and growth mechanism?How does the deformation amount and characteristics of anticline change?In view of these problems, we chose Xihu anticline as the research object. Through the analysis of surface geology, topography and geomorphology and the interpretation of seismic reflection profile across the anticline, we studied the geometry, kinematic characteristics, fold type and growth mechanism of the structural belt, and calculated the shortening, uplift and interlayer strain of the anticline by area depth strain analysis.
In this paper, by interpreting the five seismic reflection profiles across the anticline belt, and combining the characteristics of surface geology and geomorphology, we studied the types, growth mechanism, geometry and kinematics characteristics, and deformation amount of the fold. The deformation length of Xihu anticline is more than 47km from west to east, in which the hidden length is more than 14km. The maximum deformation width of the exposed area is 8.5km. The Xihu anticline is characterized by small surface deformation, simple structural style and symmetrical occurrence. The interpretation of seismic reflection profile shows that the deep structural style of the anticline is relatively complex. In addition to the continuous development of a series of secondary faults in the interior of Xihu anticline, an anticline with small deformation amplitude(Xihubei anticline)is continuously developed in the north of Xihu anticline. The terrain high point of Xihu anticline is located about 12km west of Kuitun River. The deformation amplitude decreases rapidly to the east and decreases slowly to the west, which is consistent with the interpretation results of seismic reflection profile and the calculation results of shortening. The Xihu anticline is a detachment fold with the growth type of limb rotation. The deformation of Xihu anticline is calculated by area depth strain analysis method. The shortening of five seismic reflection sections A, B, C, D and E is(650±70) m, (1 070±70) m, (780±50) m, (200±40) m and(130±30) m, respectively. The shortening amount is the largest near the seismic reflection profile B of the anticline, and decreases gradually along the strike to the east and west ends of the anticline, with a more rapidly decrease to the east, which indicates that the topographic high point is also a structural high point. The excess area caused by the inflow of external material or outflow of internal matter is between -0.34km² to 0.56km². The average shortening of the Xihubei anticline is between(60±10) m and(130±40) m, and the excess area caused by the inflow of external material is between 0.50km² and 0.74km². The initial locations of the growth strata at the east part is about 1.9~2.0km underground, and the initial location of the growth strata at the west part is about 3.7km underground. We can see the strata overlying the Xihu anticline at 3.3km under ground, the strata above are basically not deformed, indicating that this section of the anticline is no longer active.

DISCUSSION ON THE TIMING AND ITS TECTONIC SIGNIFICANCE OF ANGULAR UNCONFORMITY IN HETAO BASIN IN THE LATE QUATERNARY SEDIMENTS

BAI Luan-xi, XU Xi-wei, LUO Hao, LI Kang, LI Meng-ni, WEI Lei-ming, WANG Qi-xin, ZHAO Jun-xiang

2020, 42(4):  806-824.  DOI: 10.3969/j.issn.0253-4967.2020.04.003

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The interaction between the continental-continental collision of the Indian-Eurasian plate and the westward underthrusting of Pacific plate is generally considered to be the cause of the destruction of North China Craton. At present, there are still doubts in the researches worldwide about the dynamic mechanism of the formation and evolution of the Ordos peripheral fault-depression system and the contemporary tectonic stress field.
The Hetao Basin is a Cenozoic fault basin located between the Ordos block and the Yinshan Mountains. Due to the effect of uplift of the Tibet Plateau and the continuous subduction of the Pacific plate, graben faulting of different intensities occurred in different periods of Cenozoic around the Ordos block. Late Quaternary lacustrine facies sedimentary strata are widely developed in Hetao Basin. The Haolaigou profile, Bianqianhao profile and the Langshan profile in this study are all located in Hetao Basin. According to the lithology and structural analysis of the upper Pleistocene series in the three profiles, angular unconformities of phase 1-2 are recorded in the lacustrine facies sediments with a thickness of about 10m. The dating results of the Haolaigou profile, Bianqianhao profile and Langshan profile show that the formation time of both unconformities is 80ka BP.
Using the tectonic geology, Quaternary geology, stratigraphy, sedimentology and a variety of dating methods, we also carry out a comprehensive study and obtain the following results:
(1)The analysis of lithological and structural features of Haolaigou profile, Bianqianghao profile and Langshan profile in the Hetao Basin shows that multi-phase angular unconformities events are recorded in the lacustrine strata of a thickness of nearly 10m. These unconformities represent the tectonic movement in the late Pleistocene period since the 80ka BP and they may be widely distributed in the North China region. They are probably the direct products of the latest tectonic movement in the Quaternary period.
(2)The present tectonic movement initiates at about 80ka BP. It not only causes multiple angular unconformity events, but also leads to the disappearance of the Hetao ancient lake. The rapid regional epeirogenetic uplifting of the Ordos block since 76.4ka BP should also be the specific manifestation of this tectonic movement. Because of the influence of the accelerated uplifting and eastward spreading of the Qinghai-Tibet plateau in the late Quaternary, the NEE thrusting effect of the Ordos block is enhanced and affected.

THE CRUSTAL SHALLOW STRUCTURES AND FAULT ACTIVITY DETECTION IN XINYI SECTION OF TAN-LU FAULT ZONE

GU Qin-ping, XU Han-gang, YAN Yun-xiang, ZHAO Qi-guang, LI Li-mei, MENG Ke, YANG Hao, WANG Jin-yan, JIANG Xin, MA Dong-wei

2020, 42(4):  825-843.  DOI: 10.3969/j.issn.0253-4967.2020.04.004

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The Tan-Lu fault zone is the largest active tectonic zone in eastern China, with a complex history of formation and evolution, and it has a very important control effect on the regional structure, magmatic activity, the formation and distribution of mineral resources and modern seismic activity in eastern China. Xinyi City has a very important position as a segmental node in the Shandong and Suwan sections of the Tan-Lu fault zone. Predecessors have conducted research on the spatial distribution, occurrence and activity characteristics of the shallow crustal faults in the Suqian section of the Tan-Lu belt, and have obtained some new scientific understandings and results. However, due to different research objectives or limitations of research methods, previous researches have either focused on the deep crustal structure, or targeted on the Suqian section or other regions. However, the structural style and deep-shallow structural association characteristics of Xinyi section of Tan-Lu belt have not been well illustrated, nor its activity and spatial distribution have been systematically studied. In order to investigate the shallow crustal structure features, the fault activities, the spatial distribution and the relationship between deep and shallow structures of the Xinyi section of the Tan-Lu Fault, we used a method combining mid-deep/shallow seismic reflection exploration and first-break wave imaging. Firstly, a mid-deep seismic reflection profile with a length of 33km and a coverage number greater than 30 was completed in the south of Xinyi City. At the same time, using the first arrival wave on the common shot record, the tomographic study of the shallow crust structure was carried out. Secondly, three shallow seismic reflection profiles and one refraction tomography profile with high resolution across faults were presented. The results show that the Xinyi section of Tan-Lu fault zone is a fault zone composed of five concealed main faults, with a structural pattern of “two grabens sandwiched by a barrier”. The five main faults reveal more clearly the structural style of “one base between two cuts” of the Tan-Lu fault zone. From west to east, the distribution is as follows: on the west side, there are two high-angle faults, F₄ and F₃, with a slot-shaped fault block falling in the middle, forming the western graben. In the middle, F₃ and F₂, two normal faults with opposite dip directions, are bounded and the middle discontinuity disk rises relatively to form a barrier. On the east side, F₂ and F₁, two conjugate high-angle faults, constitute the eastern graben. The mid-deep and shallow seismic reflection profiles indicate that the main faults of the Xinyi section of Tan-Lu fault zone have a consistent upper-lower relationship and obvious Quaternary activities, which play a significant role in controlling the characteristics of graben-barrier structure and thickness of Cenozoic strata. The shape of the reflective interface of the stratum and the characteristics of the shallow part of the fault revealed by shallow seismic reflection profiles are clear. The Mohe-Lingcheng Fault, Xinyi-Xindian Fault, Malingshan-Chonggangshan Fault and Shanzuokou-Sihong Fault not only broke the top surface of the bedrock, but also are hidden active faults since Quaternary, especially the Malingshan-Chonggangshan Fault which shows strong activity characteristics of Holocene. The results of this paper provide a seismological basis for an in-depth understanding of the deep dynamics process of Xinyi City and its surrounding areas, and for studying the deep-shallow tectonic association and its activity in the the Xinyi section of the Tan-Lu Fault.

Research paper

JOINT INVERSION OF SURFACE WAVE DISPERSION AND RECEIVER FUNCTIONS FOR CRUSTAL AND UPPERMOST MANTLE STRUCTURE BENEATH CHINESE TIENSHAN AND ITS ADJACENT AREAS

KONG Xiang-yan, WU Jian-ping, FANG Li-hua, CAI Yan, FAN Li-ping, WANG Wei-lai

2020, 42(4):  844-865.  DOI: 10.3969/j.issn.0253-4967.2020.04.005

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The Tienshan orogenic belt is one of the most active intracontinental orogenic belts in the world. Studying the deep crust-mantle structure in this area is of great significance for understanding the deep dynamics of the Tienshan orogen. The distribution of fixed seismic stations in the Tianshan orogenic belt is sparse. The low resolution of the existing tomographic results in the Tienshan orogenic belt has affected the in-depth understanding of the deep dynamics of the Tienshan orogenic belt. In this paper, the observation data of 52 mobile seismic stations in the Xinjiang Seismic Network and the 11 new seismic stations in the Tienshan area for one-year observations are used. The seismic ambient noise tomography method is used to obtain the Rayleigh surface wave velocity distribution image in the range of 10~50s beneath the Chinese Tienshan and its adjacent areas (41°~48° N, 79°~91° E). The joint inversion of surface wave and receiver function reveals the S-wave velocity structure of the crust and uppermost mantle and the crustal thickness below the station beneath the Chinese Tienshan area(41°~46° N, 79°~91° E). The use of observation data from mobile stations and new fixed seismic stations has improved the resolution of surface wave phase velocity imaging and S-wave velocity structure models in the study area.
The results show that there are many obvious low-velocity layers in the crust near the basin-bearing zone in the northern Tienshan Mountains and the southern Tienshan Mountains. There are significant differences in the structural characteristics and distribution range of the low-velocity zone in the northern margin and the southern margin. Combining previous research results on artificial seismic profiles, receiver function profiles, teleseismic tomography, and continental subduction simulation experiments, it is speculated that the subduction of the Tarim Basin and the Junggar Basin to the Tienshan orogenic belt mainly occurs in the middle of the Chinese Tienshan orogenic belt, and the subduction of the southern margin of the Tienshan Mountains is larger than that of the northern margin, and the subduction of the eastern crust is not obvious or in the early subduction stage. There are many low-velocity layers in the inner crust of the Tienshan orogenic belt, and most of them correspond to the strong uplifting areas that are currently occurring. The thickness of the crust below the Tienshan orogenic belt is between 55km and 63km. The thickness of the crust(about 63km)is the largest near the BLT seismic station in the Bazhou region of Xinjiang. The average crustal thickness of the Tarim Basin is about 45km, and that of the Junggar Basin is 47km. The S-wave velocity structure obtained in this study can provide a new deep basis for the study of the segmentation of the Tienshan orogenic belt and the difference of the basin-mountain coupling type.

STUDY ON TANGSHAN-HEJIAN-CIXIAN EARTHQUAKE FAULT ZONE BY SHALLOW SEISMIC EXPLORATION METHOD

WANG Ji, GAO Zhan-wu, LIU Fang-xiao, WANG Wan-he, ZHAO Guo-cun, XU Wei

2020, 42(4):  866-880.  DOI: 10.3969/j.issn.0253-4967.2020.04.006

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The location of the buried faults, the fault broken layers and the depth of breakpoints in the Tangshan-Hejian-Cixian seismotectonic zone are not clear. We implemented 4 shallow seismic exploration profiles on the Daming Fault, Cangxi Fault, and Dachengdong Fault. Line DZ1 is located on the Daming Fault in the southeast of Daming County. Five breakpoints were dectectd, which are all normal faults, with depths of 95~125m and displacements about 6~12m, offsetting late Pleistocene but not the Holocene. Line DZ2 is located in the east of Xianxian County to dectect the Cangxi Fault. Three breakpoints were detected, all are normal faults, with depths of 170~190m and displacements about 7~10m. The upper breakpoints of the three faults cut the middle Pleistocene. The lines DZ3 and DZ4 are located in the west of Litan Town, Dacheng County. Four breakpoints were detected, with the upper breakpoint depth of 120~130m and displacements about 5~15m. They are all normal faults, and the upper breakpoints of the faults cut the Pleistocene strata.
The result of the exploration of Cixian-Daming Fault is not consistent with the buried depth 1 200m proposed by XU Hua-ming. It is proved that the activity of the fault is also consistent with the overall activity of the Cixian-Daming Fault, which is an active fault since late Pleistocene.
The Dachengdong Fault and Cangxi Fault offset the middle Pleistocene strata. Although the late Pleistocene active faults are generally defined as active faults in the practice of active tectonics research in China, strong earthquakes in eastern China have shorter recurrence period, and earthquakes of magnitude 6 or so may also occur in some middle Pleistocene active faults.
During the compilation of GB18306-2015 “Seismic ground motion parameter zonation map of China”, there were no late Pleistocene active faults in the M6~6.5 potential source areas in eastern China. Therefore, we believe that the Dachengdong and Cangxi faults still have the ability to generate earthquake of magnitude 6 or so, and the faults have some similarities with the seismogenic structures of Xingtai earthquake swarm. Under the action of the latest tectonic stress field, the “deep faults” tearing ruptured successively and expanded upwards, resulting in stress migration and loading between two neighbouring en-echolon concealed faults, so, the Dachengdong and Cangxi faults are the product of this three-dimensional rupture process. The Dachengdong Fault is a “newly-generated” fault resulting from the tearing rupturing and upward expanding of the pre-existing concealed “deept faults” in the middle and lower curst.

RELOCATION AND FOCAL MECHANISMS OF YUNNAN TONGHAI EARTHQUAKE SEQUENCE OF AUGUST 2018

LI Tong, GUO Zhi, GAO Xing

2020, 42(4):  881-892.  DOI: 10.3969/j.issn.0253-4967.2020.04.007

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The August 2018 Tonghai earthquake sequence and focal mechanisms of the two main shocks about M_S5.0 were investigated through relocation of catalog data and inversion of event waveform recordings downloaded from the China National Seismic Network. The epicenter of 2018 Tonghai earthquake locates in the southern edge of Chuandian block, where the Xiaojiang Fault separates the Chuandian block from South China block in the east, and the Red River Fault separates the Chuandian block from the Indo-China in the southwest. These two faults blocked and absorbed the continuous southward movement of Chuandian block, significant tectonic stress has been built up in the southern tip of Chuandian block. As a seismicity active zone, Tonghai has been struck by a M7.0 strong earthquake in 1970. The August 2018 Tonghai earthquake is the major earthquake occurring in Tonghai and surrounding areas since the 1970 strong earthquake. Therefore, detailed focal mechanism study for 2018 Tonghai earthquake sequence is crucial for the earthquake relief effect for the Tonghai and surrounding area. In this study, we first relocate the epicenter of earthquake sequence by using the double difference relocation method, then we inverted focal mechanism for two main earthquakes with magnitude of ~5.0 by using the W-phase method. The relocated epicentral locations of Tonghai earthquake sequence show a NE-SW trend narrow band, predominant depth range of 5~10km and near vertical seismogenic fault. The focal parameters for the August 13 earthquake are: strike of 298.2°, a dip of 45.2°, a rake of -172.9° and strike of 203.2°, a dip of 84.9°, a rake of -45.0°, respectively, and magnitude of M_W=5.07; Focal parameters for the August 14 earthquake are: strike of 297.0°, a dip of 63.6°, a rake of -161.5° and strike of 198.5°, a dip of 73.5°, a rake of -27.7°, respectively, and magnitude of M_W=4.89. Combined analysis of the relocated epicentral locations and obtained focal mechanisms suggest that the seismogenic fault of the August 2018 Tonghai earthquake sequences could be related to the NE-striking Mingxing-Erjie Fault, and the temporal and spatial distribution characteristics of the earthquake sequence is consistent with the regional seismotectonic background.

RESEARCH ON NEOGENE-QUATERNARY STRATIGRAPHIC STRUCTURE AND SHALLOW TECTONIC FEATURES IN THE NORTH SECTION OF DAXING FAULT ZONE BASED ON SHALLOW SEISMIC REFLECTION PROFILING

HE Fu-bing, XU Xi-wei, HE Zhen-jun, ZHANG Xiao-liang, LIU Li-yan, ZHANG Wei, WEI Bo, NI Jing-bo

2020, 42(4):  893-908.  DOI: 10.3969/j.issn.0253-4967.2020.04.008

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The Daxing Fault is an important buried fault in the Beijing sub-plain, which is also the boundary fault of the structural unit between Langgu sub-sag and Daxing sub-uplift. So far, there is a lack of data on the shallow tectonic features of the Daxing Fault, especially for the key structural part of its northern section where it joins with the Xiadian Fault. In this paper, the fine stratigraphic classifications and shallow tectonic features of the northern section in the main Daxing Fault are explored by using three NW-trending shallow seismic reflection profiles. These profiles pass through the Daxing earthquake(M6¾)area in 1057AD and the northern section of the main Daxing Fault. The results show that seven strong reflection layers(T₀₁—T₀₃, T_Q and T₁₁—T₁₃)are recognized in the strata of Neogene and Quaternary beneath the investigated area. The largest depth of strong reflection layer(T₁₃)is about 550~850ms, which is interpreted as an important surface of unconformity between Neogene and Paleogene or basement rock. The remaining reflection layers, such as T₀₁ and T_Q, are interpreted as internal interfaces in Neogene to Quaternary strata. There are different rupture surfaces and slip as well as obviously different structural features of the Daxing Fault revealed in three shallow seismic reflection profiles. The two profiles(2-7 and 2-8)show obvious rupture surfaces, which are the expression of Daxing Fault in shallow strata. Along the profile(2-6), which is located at the end of the Daxing fault structure, a triangle deformation zone or bending fracture can be identified, implying that the Daxing Fault is manifested as bending deformation instead of rupture surfaces at its end section. This unique structural feature can be explained by a shearing motion at the end of extensional normal fault. Therefore, the Daxing Fault exhibits obviously different tectonic features of deformation or displacement at different structural locations. The attitude and displacement of the fault at the shallow part are also different to some extent. From the southwest section to the northeast section of the fault, the dip angle gradually becomes gentler(80°~60°), the upper breakpoint becomes deeper(160~600m), and the fault displacement in Neogene to Quaternary strata decreases(80~0m). Three shallow seismic reflection profiles also reveal that the Daxing Fault is a normal fault during Neogene to early Quaternary, and the deformation or displacement caused by the activity of the fault reaches the reflection layer T₀₂. This depth is equivalent to the sedimentary strata of late Early-Pleistocene. Therefore, the geometry and morphology of the Daxing Fault also reveal that the early normal fault activity has continued into the Early Pleistocene, but the evidence of activity is not obvious since the late Pleistocene. The earthquakes occurring along the Daxing Fault, such as Daxing earthquake(M6¾)in 1057AD, may not have much relation with this extensional normal fault, but with another new strike-slip fault. A series of focal mechanism solutions of modern earthquakes reveal that the seismic activity is closely related to the strike-slip fault. The Daxing Fault extends also downwards into the lower crust, and may be cut by the steeply dipping new Xiadian Fault on deep seismic reflection profile. The northern section of the Daxing Fault strikes NNE, with a length of about 23km, arranged in a right step pattern with the Xiadian Fault. Transrotational basins have been developed in the junction between the northern Daxing Fault and the southern Xiadian Fault. Such combined tectonic features of the Daxing Fault and Xiadian Fault evolute independently under the extensional structure background and control the development of the Langgu sub-sag and Dachang sub-sag, respectively.

DIFFERENTIATION DIRECTION OF TELLURIC CURRENTS IN THE SOUTHERN SECTION OF THE TANLU FAULT ZONE

ZHANG Xin, DU Xue-bin

2020, 42(4):  909-922.  DOI: 10.3969/j.issn.0253-4967.2020.04.009

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The Tancheng-Lujiang Fault is an important tectonic boundary in eastern China. The southern part of the Tancheng-Lujiang Fault is located south of Baohai Bay, which is an area with a dense population and frequent economic activities. It is worth conducting an in-depth study on the southern section of the fault, especially in the aspect of geophysical exploration and seismicity analysis. Electrical structure detection is an important way to interpret the structural activity of the fault. It can also analyze and explore the influence of the fault on the physical properties of both sides of the fault based on the geoelectrical observation data. In the study area, there are densely distributed stations of geoelectrical observation, including 27 fixed stations distributed along the fault zone from the southern Baohai Bay to Nanjing, Jinagsu Province. The continuous observations and recording of these stations provide a favorable condition for studying the tectonic activity of Tancheng-Lujiang Fault.
In the long-term observation of geoelectric observation network, the geoelectric field measurements of long- and short-spacing measuring tracks in the same direction at the same station vary significantly because of the effect of long-term stability of the observation system and the environment near the electrodes. Also, the data curve changes complicatedly and seems to be in a mess. However, there are three basic facts of observation existing in the geoelectric field change: 1)The variation amplitude of the geoelectric field changes observed on the long- and short-spacing measuring tracks in the same direction at the same station(including tidal response changes and the rapid change events such as short periods or pulses)is the same or very close; 2)The E_x and E_y components at the same station always show the same variation in the same time period, or the opposite, which is related to the anisotropy of the medium under the station; 3)The rapid changes of the minute values of the geoelectric field observed at different stations are synchronous in a wide spatial area. In this study, in order to take full advantage of these basic facts, we only use the amplitude variation of geoelectric field with time.
Based on the data of 27 geoelectric field observation stations in the study area, we used the current density vector and streamline to characterize telluric current with its divergence and vorticity calculated in the southern Tancheng-Lujiang Fault in this paper. The results show that: 1)the telluric current shows the phenomenon of opposite directional differentiation in the southern part of the fault zone, the direction of the current vector is NE on the east side, while the direction is NW to SW on the west side; (2)The divergence and vorticity of telluric current also show the differentiation phenomenon along the fault, the positive/negative maximum of vorticity and divergence mainly occurs near the fault zone and the direction of alternating positive and negative gradient(or negative gradient)of vorticity or divergence is consistent with the strike of the fault zone. By analyzing the current superposition simulation results and comparing them with previous studies, an interpretation model of the above phenomenon is established in this paper. The results agree with previous studies on the electrical structure of this region. Besides, the results that telluric current differentiates along the fault zone may improve our understanding of the process of deep electrical and material migration.

Research paper

STUDY ON CHARACTERISTICS OF GRAVITY VARIATION BEFORE AND AFTER HUTUBI M_S6.2 EARTHQUAKE

WEI Shou-chun, ZHU Yi-qing, ZHAO Yun-feng, ZHANG Song, LIU Fang, LI Rui, GAO Duo-wen

2020, 42(4):  923-935.  DOI: 10.3969/j.issn.0253-4967.2020.04.010

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The Hutubi M_S6.2 earthquake of December 8, 2016 is a pure thrust event in the northern Tianshan thrust fold belt. The earthquake occurred between the Qigu Fault and the Junggar southern margin fault, which are both thrust faults. Based on mobile gravity measurements from 2013 to 2018 in the northern Tianshan, the gravity net adjustment was accomplished using Urumqi absolute gravity observation point as the datum, and the absolute gravity value of surface observation points were obtained. In order to eliminate the seasonal effect on gravity variation, the paper uses the observation data in May per annual as studying objects and obtains the temporal-spatial dynamic evolution images of gravity field differences in the northern Tianshan at different time scales as well as the time series of gravity variation of some points in the adjacent area of the epicenter. The characteristics of regional gravity variation before and after the Hutubi M_S6.2 earthquake on December 8, 2016 and their relations are analyzed systematically. The results show that: 1)The gravity variations in the study area are dramatic in generally, and the contours of gravity variation are consistent with the main faults basically. There was a four-quadrant distribution near the epicenter before the earthquake, and the Hutubi M_S6.2 earthquake occurred near the center of the four-quadrant distribution and at the turn of the gravity variation contour. The three years' cumulative gravity variation before the earthquake and the gravity variation after the earthquake are inversed, and the variation amplitudes are equivalent, suggesting that the M_S6.2 earthquake has released the stress and the energy accumulated before the earthquake. 2)This paper focuses on the analysis of gravity variation at the observation points on both sides of the Junggar southern margin fault near the epicenter. Regional gravity variation and gravity time series show that gravity variations at the same side of the Junggar southern margin fault are basically consistent, however, gravity variations at the different sides of the Junggar southern margin fault are different from each other obviously, indicating the difference of material migration laws in different structural regions. In addition, the strain energy accumulated in the epicenter is basically released after the earthquake, and the area nearby the epicenter tends to be stable. 3)The Hutubi M_S6.2 earthquake occurred near the center of the four-quadrant and at the turn of the high-gradient zone of gravity variation, reflecting the location of strong earthquake is related to the distribution of four-quadrant of regional gravity variation, the high-gradient zone of regional gravity variation and its turn. It has a unique advantage in determining the location of strong earthquake using gravity variation results. The regional spatial-temporal gravity variation before the earthquake is manifested as a systematic evolution process of “steady state→regional gravity anomaly→four-quadrant distribution→earthquake occurring in the reverse process”. Studying the temporal-spatial evolution characteristics of gravity field before and after Hutubi M_S6.2 earthquake has important practical significance for understanding the occurrence law of large earthquakes and capturing the precursory information of earthquakes.

RESTORATION OF THE ORIGINAL TOPOGRAPHY OF THE XIAONANHAI LANDSLIDE IN CHONGQING AND CALCULATION OF ITS VOLUME

ZHOU Xin, ZHOU Qing, GAO Shuai-po, LI Xiao-feng

2020, 42(4):  936-954.  DOI: 10.3969/j.issn.0253-4967.2020.04.011

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As documented in history, an M6¼ earthquake occurred between Qianjiang, Chongqing and Xianfeng, Hubei(also named the Daluba event)in 1856. This earthquake caused serious geological hazards, including a lot of landslides at Xiaonanhai, Wangdahai, Zhangshangjie and other places. Among them, the Xiaonanhai landslide is a gigantic one, which buried a village and blocked the river, creating a quake lake that has been preserved to this day. As the Xiaonanhai landslide is a historical earthquake-induced landslide, it is impossible to obtain the remote sensing image and DEM data before the earthquake, which brings certain difficulties to the estimation of landslide volume and the establishment of numerical simulation model. In this paper, the original topography before the earthquake is inferred by the methods of geomorphic analogy in adjacent areas and numerical simulation, and the volume of the Xiaonanhai landslide body is calculated. Firstly, the principle and application of UAV aerial photography are introduced. We employed an unmanned airplane to take pictures of the Xiaonanhai landslide and adjacent areas, yielding high-precision DOM images(digital orthophoto graph)and DEM data which permit generating terrain contours with a 25m interval. We also used the method of intensive manual depth measurement in waters to obtain the DEM data of bottom topography of Xiaonanhai quake lake. Based on field investigations, and combining terrain contours and DOM images, we described the sizes and forms of each slump mass in detail. Secondly, considering that the internal and external dynamic geological processes of shaping landforms in the same place are basically the same, the landforms such as ridges and valleys are also basically similar. Therefore, combining with the surrounding topography and landform of the Xiaonanhai area, we used MATLAB software to reconstruct two possible original landform models before the landslide. The original topography presented by model A is a relatively gentle slope, with a slope of 40°~50°, and the original topography presented by model B is a very high and steep slope, with a slope of 70°~80°. Thirdly, Geostudio software is used to conduct numerical simulation analysis on the slope stability. The safety factor of slope stability and the scale of landslide are analyzed under the conditions of static stability, seismic dynamic response and seismic dynamic response considering topographic amplification effect. The results show that large landslide is more likely to occur in model B, which is more consistent with the reality. In order to verify the credibility of recovered DEM data of valley bottom topography, we visited the government of Qianjiang District, collected the drilling data of 11 boreholes in two survey lines of Xiaonanhai weir dam. It is verified that the recovered valley bottom elevation is basically consistent with that revealed by the borehole data. Finally, according to the two kinds of topographic data before and after the landslide, the volume of the landslide is calculated by using the filling and excavation analysis function of ArcGIS software. There is a gap between the calculation results of filling and excavation, the filling data is 3×10⁶m³ larger than the excavation data. The reasons are mainly as follows: 1)Due to the disorderly accumulation of collapse blocks, the porosity of the accumulation body became larger, causing the volume of the fill to expand; 2)It has been more than 150a since the Xiaonanhai earthquake, and the landslide accumulation has been seriously reconstructed, therefore, there are some errors in the filling data; 3)The accumulation body in Xiaonanhai quake lake might be subject to erosion and siltation, this may affect the accuracy of the filling data. In conclusion, it is considered that the calculated results of the excavation are relatively reliable, with a volume of 4.3×10⁷m³.

MICROMETER SCALE MORPHOLOGY MEASUREMENT SYSTEM: A NEW TECHNIQUE FOR MICRO-TOPOGRAPHY MEASUREMENT ON FAULT PLANES

HAO Hai-jian, WEI Zhan-yu, HE Hong-lin, LIU Li-qiang, GUO Ling-li

2020, 42(4):  955-967.  DOI: 10.3969/j.issn.0253-4967.2020.04.012

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Earthquakes commonly occur in the sliding surface of the fault zone. The morphology of the sliding surface is the result of fault activities, and also it evolves with the activities. The irregular geometry of the fault plane affects the sliding resistance, the concentration and anisotropy of the stress distribution within the fault plane and the fault shear strength. So, the acquisition of high-precision morphological features is of great significance for studying the correlation between fault surface morphology and seismic nucleation, fracture propagation and termination. Due to the lack of reliable micron-scale morphological measurement apparatus, the study of the coherence of the fault surface morphology from large scale(unit: m-cm)to small scale(unit: μm)is subject to restrictions, as well as the study of the relationship between the micro-morphology of the experimental frictional surface and the rupture process. In order to improve the measurement accuracy of the fault plane and overcome the shortcomings of existing measurement methods, we have invented a morphology measurement system with independent intellectual property rights.
The measuring principle of this morphology measurement system is based on the laser rangefinder theory. The frame of this system consists of four parts: Braced Frame, Moving Scanner Unit, System-Controlling Unit and Data Collection Unit. Braced Frame is made up of high-adjustable frame, loading stage, dust-proof box and isolation platform, which is used to provide a vibration isolation, light proof and dust-proof measuring environment. Moving Scanner Unit contains a laser head and a two-dimensional translation stag, the laser head is used to measure vertical distance and a two-dimensional translation stage carrying a laser head moving in X-axis and Y-axis orientation to provide X, Y coordinate values. System-Controlling Unit includes two-dimensional translation stage controller, laser head controller and signal convertor. The function of this part is mainly to control operation of other parts. The Data Collection Unit is composed of computer system and software module. This part connects other parts for receiving and storing data. In order to improve the scan efficiency, we developed new software by which we can precisely control the measuring process and efficiently process the acquired data. The software is comprised of five modules: 1)Move Module, this module is used to control the original moving of the laser head relative to the two-dimension translation stage and display the 3-dimensional coordinate information in real time; 2)Set Parameters of Scan Area, the function of this module is to obtain the XY coordinate values of four corner points of the target area to scan; 3)Scan Method Module, though this part, we can control the point spacing in the X-axis orientation by inputting velocity of laser header, as well as the point spacing in X-axis orientation by inputting the Y-step parameter; 4)Pre-Scan Module, there are three functions in this module to inspect whether the z-value of the target area is beyond the range of the laser head or not, estimate consuming time for scanning the object area under the predefined parameters and to estimate the size of the result file; and 5)Scan Module, the function of this module is to store the scanning data.
We scanned the camera lens and the standard plate whose standard deviations are lower than 5μm to acquire the precision of the measurement system, and the results show that the precision of the plane positioning (X-axis and Y-axis direction)is better than 3.5μm; the vertical measurement precision is better than 4.5μm. The highest resolution of the measurement system is constrained by the performance of the laser head and two-dimension translation stage, and the horizontal resolution can reach 0.62μm, vertical resolution 0.25μm. When the needed resolution is lower than the highest, we can achieve it through adjusting the parameter of the velocity in the X-axis orientation and steps in the Y-axis orientation. To test the practical effect of the measurement system, we scanned an area of frictional surface of experimental rock using this system and obtained a high-resolution topography data. From the DEM interpolated from the cloud data, we can observe the striation on the fault plane and the variation of the roughness distribution. The roughness and slope distribution results show that the topography measurement system can meet our requirements for analyzing the microscopic morphology on the micrometer scale.
Compared with traditional measurement devices, the morphology measurement system has the following advantages: 1)The measurement system can obtain the data even in a valley region with a large dip angle on the surface because the vertically emitted beam by the laser head is practically perpendicular to the surface. So compared with other means, it can avoid producing a blank area of measurements and get a complete area; 2)the measurement system has a larger measurement range of 30cm×30cm. When the high-resolution measurement is performed on a large scale, the error caused by the registration of multiple measurement results can also be avoided.

A PRELIMINARY STUDY ON THE METHOD OF SEISMIC INTENSITY ASSESSMENT BASED ON RESIDENTIAL BUILDING DATA AND HIGH RESOLUTION REMOTE SENSING IMAGES

GUO Jian-xing, ZHANG Yu-xiang, JI Jian-zhong, YUAN Xiao-xiang, XIAO Ben-fu

2020, 42(4):  968-980.  DOI: 10.3969/j.issn.0253-4967.2020.04.013

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After destructive earthquakes, the assessment result of seismic intensity is an important decision-making basis for emergency rescue, recovery and reconstruction. This job requires higher timeliness by government and society. Because remote sensing technology is not affected by the terrible traffic conditions on the ground after the earthquake, large-scale seismic damage information in the earthquake area can be collected in a short time by the remote sensing image. The remote sensing technique plays a more and more important role in rapid acquisition of seismic damage information, emergency rescue decision-making, seismic intensity assessment and other work. On the basis of previous studies, this paper proposes a new method to assess seismic intensity by using remote sensing image, i.e. to interpret the building collapse rate of a residential quarter after an earthquake by high-resolution remote sensing images. If there already are detailed building data and building structure vulnerability matrix data of a residential area, we can calculate the building collapse rate under any intensity values in this residential area by using the theory of earthquake damage prediction. Assuming that the building collapse rate interpreted by remote sensing is equal to the building collapse rate predicted by using the existing data, it will be easy to calculate the actual seismic intensity of the residential area in this earthquake event. Based on this idea, according to the relevant standard specifications issued by China Earthquake Administration, this paper puts forward some functional models, such as the calculation model of building collapse rate based on remote sensing, the data matrix model of residential building structure, the prediction function matrix model of residential building collapse rate and the prediction model of residential building collapse rate. A formula for calculating seismic intensity by using remote sensing interpretation of collapse rate is also proposed. To test and verify the proposed method, this paper takes two neighboring blocks of Jiegu Town after the Yushu M7.1 earthquake in Qinghai Province as an example. The building structure matrix of the study block was constructed by using pre-earthquake 0.6m resolution satellite remote sensing image(QuickBird, acquired on November 6, 2004), post-earthquake 0.2m aerial remote sensing image(acquired by National Bureau of Surveying and Mapping, April 15, 2010) and some field investigation data. The building collapse rate in the two blocks was calculated by using the interpretation results of seismic damage from the Remote Sensing Technology Coordinating Group of China Seismological Bureau. The seismic damage matrix of building structures in Yushu area is constructed by using the abundant scientific data of the scientific investigation team of the project “Comprehensive Scientific Investigation of the Yushu M7.1 Earthquake in Qinghai Province” of China Seismological Bureau. On this basis, the collapse rate prediction function of different structures in Yushu area is constructed. According to the prediction function of collapse rate and the building structure matrix of the two blocks, the building collapse rate under different intensity values is predicted, and the curve of intensity-collapse rate function is drawn. By comparing the building collapse rate interpreted by remote sensing and the intensity-collapse rate function curve of this two blocks, the seismic intensity of both blocks are calculated to be the same value: Ⅸ degree, which is consistent with the results of the field scientific investigation of the earthquake. The validation shows that the method proposed in this paper can effectively avoid the influence caused by the difference of seismic performance of buildings and accurately evaluate seismic intensity when using remote sensing technique. The method has certain application value for earthquake emergency work.

RESIDUAL POST-IR IRSL SIGNALS OF POTASSIUM FELDSPAR FROM MODERN SAG POND DEPOSITS OF CENTRAL ALTYN TAGH FAULT: IMPLICATION FOR DATING YOUNG PALEOSEISMIC EVENTS

QIN Jin-tang, CHEN Jie, LI Tao

2020, 42(4):  981-992.  DOI: 10.3969/j.issn.0253-4967.2020.04.014

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The Altyn Tagh Fault(ATF)is one of the most prominent active strike-slip faults in the India-Eurasia collision. Fresh features of surface ruptures, which are attributed to seismic events taking place in the last millennium, are identified at several sites along the Che'erchen River to Qingshui River section on the central part of ATF. Accurate chronology of these earthquake events would help understand the spatial-temporal relationship of the recent earthquakes. However, great difficulties are encountered. The central ATF is located in the arid area, and the vegetation cover is so limited that rare organic materials appropriate for radiocarbon dating can be found in the sediments. Luminescence dating technique may serve as an alternative to directly determine the burial ages of the earthquake related sediments. The optically stimulated luminescence(OSL)signal of quartz, which has been widely employed for luminescence dating, displays unwanted charateristics for accurate dating. Firstly, the quartz OSL signal is not sensitive to irradiation, which leads to low signal-to-noise ratio or even no measurable quartz OSL signal. Secondly, the targeted samples of the last millennium are very young, and the radiation dose received during the burial is expected to be less than 3~4Gy, which futher deteriorates the signal-to-noise ratio of the quartz OSL signal. Therefore, quartz OSL signal is not appropriate for dating the sediments relevant to the recent earthquakes on ATF.
The infrared stimulated luminescence(IRSL)signal of potassium feldspar is an alternative, and it is in usual an order of maginitude more sensitive to raidation than the quartz OSL signal. The enhanced signal-to-noise ratio makes it applicable to young samples. The post-IR IRSL signal has been successfully applied to date the sediments beyond the Holocene, however, the relatively slow bleaching of the post-IR IRSL signal poses challenges on applying it to young sediments, especially for the sediments deposited during the last millennium. In this study, we investigated the feasibility of using post-IR IRSL signal from potassium feldspar to date the earthquake events of the last millennium by employing modern sag pond deposits with different sorting and expected equivalent dose(D_e)of 0Gy. Choosing an appropriate measurement procedure and identifying the well bleached pottassium feldspar grains are essential for post-IR IRSL dating of young sediments. The non-fading characteristic of the post-IR IRSL₁₇₀ signal measured at 170℃ following a prior IR stimulation at 110℃ was verified by employing the D_e plateau test with respect to the signal integration interval and IR stimulation temperature together. Reducing the amount of potassium feldspar grains mounted on an aliquot would help reveal the among grains variation of bleaching level of post-IR IRSL₁₇₀ signal before depostion and identify the most sufficiently bleached grains. Therefore, the post-IR IRSL₁₇₀ D_e values of 2mm aliquots were measured for three samples with different sedimentary textures. The median of D_e distribution of well sorted and stratified sag pond deposits is consistent with the minimum D_e value inferred from the minimum age model(MAM-3) and finite mixture model(FMM), while for the poorly sorted deposits, the median is significantly overestimated compared with the minimum D_e values from the MAM-3 and the FMM. The minimum D_e values of 0.6~0.8Gy of all three samples are consistent with the unbleachable residual dose previously reported for post-IR IRSL signals measured at similar temperature for well bleached samples. It implies that by combined use of small aliquot and statistical age models, the well-bleached potassium feldspar grains could be identified. Such an intrinsic unbleachable component needs to be properly corrected when earthquake events of last millennium are to be dated in this area. Otherwise, the post-IR IRSL₁₇₀ age would be overestimated by 200~300a.
The post-IR IRSL₁₇₀ procedure investigated in this study is not only applicable for dating the paleoearthquake events along the Altyn Tagh Fault, but also with great potential to be applied to other tectonically active area. With consideration of the potential variability in post-IR IRSL signal characteristics of potassium feldspar grains from different origins, the signal stability needs to be routinely inspected. The modern analog sample would also be informative for justifying the measurement procedure and analytical method employed.

UNIFIED EARTHQUAKE CATALOG FOR CHINA’S SEAS AND ADJACENT REGIONS AND ITS COMPLETENESS ANALYSIS

XIE Zhuo-juan, LI Shan-you, LÜ Yue-jun, XU Wei-jin, ZHANG Yu-ling, LIU Wen-xin

2020, 42(4):  993-1019.  DOI: 10.3969/j.issn.0253-4967.2020.04.015

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China’s seas and adjacent regions are affected by interactions among the Eurasian plate, the western Pacific plate, and the Philippine Sea plate. Both intraplate and plate-edge earthquakes have occurred in these regions and the seismic activities are frequent. The coastal areas of China are economically developed and densely populated. With the development and utilization of marine energy and resources along with the development of national economy, the types and quantity of construction projects in the marine and coastal areas have increased, once an earthquake happens, it will cause huge damage and loss to these areas, therefore, the earthquake-related research for these sea areas cannot be ignored and the need for study on these areas is increasingly urgent. One type of essential basic data for marine seismic research is a complete, unified earthquake catalog, which is an important database for seismotectonics, seismic zoning, earthquake prediction, earthquake prevention, and disaster reduction. Completeness and reliability analysis of an earthquake catalog is one of the fundamental research topics in seismology.
At present, four editions of earthquake catalogs have been officially published in China, as well as the earthquake catalogue compiled in the national fifth-generation earthquake parameter zoning map, these catalogs are based on historical data, seismic survey investigations, and various instrumental observations. However, these catalogs have earlier data deadlines and contain the earthquake records for only the offshore regions of China, which are extensions of coastal land. Distant sea regions, subduction zones, and adjacent sea regions have not been included in these catalogs. Secondly, there were no cross-border areas involved in the compilation of earthquake catalogs in the past. It was not required to use magnitudes measured by other countries’ seismic networks and observation agencies to develop an earthquake catalog with a uniform magnitude scale, moreover, there was no formula suitable for the conversion of magnitude scale in China’s seas areas and adjacent regions. Little research has been conducted to compile and analyze the completeness of a unified earthquake catalog for China’s seas and adjacent regions. Therefore, in this study, we compiled earthquake data from the seismic networks of China and other countries for China’s seas and adjacent regions. The earthquake-monitoring capabilities of different sea areas at different time periods were evaluated, and the temporal and spatial distribution characteristics of epicentral location accuracy for China’s seas and adjacent regions were analyzed. We used the orthogonal regression method to obtain conversion relationships between the surface wave magnitude, body wave magnitude, and moment magnitude for China’s seas and adjacent regions, and established magnitude conversion formulae between the China Seismic Network and the M_L magnitude of the Taiwan Seismic Network and the M_S magnitude of the Philippine Seismic Network. Finally, we developed an earthquake catalog with uniform magnitude scales for China’s seas and adjacent regions.
On the basis of the frequency-magnitude distribution obtained from the magnitude-cumulative frequency relationship (N-T) and the Gutenberg-Richter(GR)law, we conducted a completeness analysis of the unified earthquake catalog for China’s seas and adjacent regions, Then, we identified the beginning years of each magnitude interval at different focal depth ranges and different seismic zones in the earthquake catalog.
This study marks the first time that a unified earthquake catalog has been compiled for China’s seas and adjacent regions, based on the characteristics of seismicity in the surrounding sea regions, which fills the gap in the compilation of the earthquake catalogue of China’s seas and adjacent areas. The resulting earthquake catalog provides a basis for seismotectonics, seismicity study, and seismic hazard analysis for China’s seas and adjacent regions. The catalog also provides technical support for the preparation of seismic zoning maps as well as for earthquake prevention and disaster reduction in project planning and engineering construction in the sea regions. In addition, by evaluating the earthquake-monitoring capability of the seismic networks in China’s seas and adjacent regions and analyzing the completeness of the compiled unified earthquake catalog, this study provides a scientific reference to improve the earthquake-monitoring capability and optimizing the distribution of the seismic networks in these regions.