Table of Content

20 December 2020, Volume 42 Issue 6

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Research paper

RATE DEPENDENCE OF FRICTION OF HORNBLENDE AND IMPLICATIONS FOR UNSTABLE SLIPS

LIU Yang, HE Chang-rong

2020, 42(6):  1267-1281.  DOI: 10.3969/j.issn.0253-4967.2020.06.001

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Since the 21st century, the occurrence of tremor and slow-slip events in the subduction zones has increasingly attracted researchers' attention. It seems that minerals in the subduction zone which may be related to tremor and slow-slip events, have become a topic of concern. Hydrous minerals are generally lower in strength than anhydrous minerals, so the hydrous minerals in the subduction zone(e.g., hornblende, serpentine, talc, etc.)may control the frictional sliding behaviors in the subduction zones.
Although many geophysical data indicate that serpentinization may exist in the depth range around the mantle wedge, ocean drilling results indicate that hornblende is a common hydrous mineral in the mantle. In order to understand the frictional behaviors of hornblende as a common hydrous mineral in the subduction zones under hydrothermal conditions, we used pure hornblende as the material for simulating fault gouge samples. In a series of experiments with a certain confining pressure, the axial loading rate is between 0.04μm/s and 1.0μm/s. In these experiments, the velocity stepping tests were carried out under the confining pressure of 136MPa, and the pore pressure was 30MPa. The frictional sliding velocity is switched between 1.22μm/s, 0.244μm/s, and 0.048 8μm/s to obtain data on the response to the velocity change. The experimental results are as follows:
(1)In these experiments(C_P=136MPa, P_P=30MPa), except for the quasi-static oscillations at 505℃ and 607℃, at most experimental temperatures, hornblende fault gouge samples all show stable sliding behaviors. The steady-state friction coefficient ranges from about 0.70(607℃)to about 0.72(403℃), with an average value of about 0.71, and does not show systematic changes with temperature.
(2)In these experiments of hornblende fault gouge(C_P=136MPa, P_P=30MPa), velocity strengthening behaviors were observed at temperatures of 101 and 203℃. It changes to velocity weakening at 303℃. It is in transition state at 403℃, and changes to velocity weakening at 505℃, and the velocity weakening continues until the highest temperature in our experiment, 607℃. The absolute value of velocity weakening(b-a)is between 0.000 51 and 0.001 4, which is a weak velocity weakening.
We numerically fitted the experimental data and obtained the values of the friction constitutive parameters a, b, and D_c at each temperature.
Our results of data fitting show that the slowness law adequately reproduces both the non-oscillatory rate steps and the periodical slow slips. As a result, a and b-values for the series exhibit a similar trend up to 203℃. The maximum of a averaged over steps (~0.009) occurred at 101℃, associated with a step-averaged b of 0.001 3. As temperature increased to 203℃, the step-averaged a decreased rapidly to a level of 0.006 8, with the corresponding b-value of 0.005 3~0.005 5. The temperature at 303℃ is a turning point for the a-value from the decreasing trend to a monotonic increasing trend up till 607℃. In contrast to the a-value, the average b in the series shows a growing trend in the whole temperature range.
The average D_c was found to range from 12~23.5μm for non-oscillatory cases, with no systematic changes as related to temperature. Much smaller D_c of 2μm was inverted for the oscillatory slow slips at 505℃ and 607℃ in the series, indicating that it was the cause of heightened critical stiffness that approached the vicinity of the loading stiffness.
Hornblende has a weak velocity weakening, with (b-a)-value below 0.001 5, which is less than the (b-a)-value used in the numerical simulation of the slow-slip in the Cascadia subduction zone, indicating that the velocity weakening is very weak, and this weak velocity weakening is conducive to the generation of slow slip. Therefore, the degree of velocity weakening of hornblende is in line with the appropriate conditions for the occurrence of slow slip in the subduction zone. The slow slip event may occur in a wider range of effective normal stress.

ELECTRIC STRUCTRUE MODEL OF TIANCHI VOLCANO IN CHANGBAI MOUNTAINS BASED ON THREE-DIMENSIONAL AR-QN MAGNETOTELLURIC INVERSION

RUAN Shuai, TANG Ji, DONG Ze-yi, WANG Li-feng, DENG Yan, HAN Bing

2020, 42(6):  1282-1300.  DOI: 10.3969/j.issn.0253-4967.2020.06.002

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The Tianchi volcano in Changbai Mountains is a giant active volcano with potential risk of eruption, so more detailed researches of deep magma chamber as well as its closely related three-dimensional electric structure model are being concerned by many scholars. Based on adaptive regularized three-dimensional quasi-Newton inversion program developed by the author, this paper carries out three-dimensional magnetotelluric inversion including topography using the existing data observed decades ago. Our three-dimensional magnetotelluric adaptive regularization quasi-Newton inversion method improves the conventional quasi-Newton method by approximating Hessian matrix of the data misfit using LBFGS formula instead of total Hessian matrix which is approximated in conventional quasi-Newton inversion. This not only guarantees the precise regularization-term Hessian matrix, but also allows the regularization parameter to be adaptively updated on every inversion iteration without destroying the descent of total objective function. Thus, the regularization parameter's adaptive updating strategy on every iteration can be established based on L2-norm ratio of the data misfit function and regularization function, and our AR-QN inversion algorithm was implemented. The model synthetic data inversion results indicate that AR-QN inversion has very stable iteration flow, and is weakly dependent on initial model selection, which tends to be more suitable for the three-dimensional inversion task when MT survey sites are sparsely distributed on the surface.
Besides the impedances data, the tipper data which is more sensitive to horizontal conductive discontinuity was fitted in this three-dimensional inversion as well. Through comparison of qualitative analysis to measured data and anomalous-body-erasing forward modeling test, our new three-dimensional electric structure was proved to be a reliable model under the constraints of current magnetotelluric data. The three-dimensional electric structure is more accurate and rational compared with the electric structure obtained by previous researches based on two-dimensional inversion, and clearly characterizes the distribution of crustal magma chamber and magma channels with high resolution, even inverted by the limited spare-sites-distributed magnetotelluric dataset. Our result locates the crustal magma chamber on the northeast of Tianchi within the depths of 10~30km, and the general magma chamber along with magma channels system is represented as “V” shape. The magma channel of Tianchi volcano can be clearly shown on the EW profile 10km away from the north of Tianchi volcano. The magma chamber and channels system can be divided into three layers from shallow to deep: Magma channel with a depth of less than 5km and connecting to the Tianchi crater; alkaline fluid magma layer with a depth of 5~10km; and trachytic magma layer with a depth of 10~30km. As the coverage of the existing MT survey sites is still incomplete on the whole Tianchi volcano area, adding more MT survey sites in the northwest, southwest directions and especially in North Korea will help us get more reliable, higher-resolution three-dimensional electrical structure model for Changbaishan Tianchi volcano area.

ANALYSIS OF COSEISMIC VARIATIONS IN MAGNETIC FIELD OF THE LUSHAN M_S7.0 EARTHQUAKE IN 2013

SONG Cheng-ke, ZHANG Hai-yang

2020, 42(6):  1301-1315.  DOI: 10.3969/j.issn.0253-4967.2020.06.003

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Stress perturbation related to earthquakes could cause the anomalous changes of magnetic field. Numerous observation data from high-precision magnetometers before and after earthquakes have revealed obvious coseismic changes in the geomagnetic field during earthquakes. In order to assess the ability of observing geomagnetic variations related to earthquakes with present geomagnetic stations, the coseismic geomagnetic variations corresponding to the 2013 M_S7.0 Lushan earthquake are presented. Precise measurements of geomagnetic fields have been obtained at a continuous geomagnetic station, Chengdu(CDP), which is approximately 99km from the epicenter of the M_S7.0 earthquake. The 12^th Generation International Geomagnetic Reference Field is used to reduce the secular variation of geomagnetic field. The geomagnetic station, Wuhan(WHN)and Lanzhou(LZH), which are approximately 1 100km and 650km from the epicenter of M_S7.0 earthquake respectively, are regarded as reference stations to reduce the diurnal variation of geomagnetic field. A corrected method based on observed data from CDP, WHN and LZH is also used to correct the effect of short-period geomagnetic variations of external origin. Minute average values of CDP from April 16, 2013 to April 23, 2013 is used to analyze the coseismic geomagnetic changes because the ΣKp indices in this period are less than 16 which indicates that there are no strong magnetic disturbances, solar flares, or solar storms. The result reveals that coseismic geomagnetic variation at CDP is little, which is about +0.09nT and +0.04nT, with WHN and LZH as reference stations. Meanwhile, the coseismic geomagnetic variation is +0.02nT using the geomagnetic values sampled per second observed in 20s before and after earthquake from CDP.
The observed coseismic geomagnetic variations are generally interpreted in terms of the piezomagnetic effect in rocks, which are most probably generated by sudden stress changes resulting from earthquake rupturing. According to slip model of 2013 M_S7.0 Lushan earthquake which is constrained by GPS data, the coseismic stress changes are calculated. Then, piezomagnetic magnetic field in the epicenter and surrounding area of the 2013 M_S7.0 Lushan earthquake is calculated based on linear model of magnetization and stress. In the model, we assume that the initial magnetization is consistent with the present magnetic field, which has an inclination and declination of 47.2° and -1.7°(westward). The in-situ Curie depth is chosen as 30km because the magnetized rocks beneath 30km contribute negligibly to the ground geomagnetic field. The calculated piezomagnetic effect shows that the vertical component is downward and horizontal component is mainly southward resulting from stress release in footwall of faults. The vertical component is upward and the horizontal component is mainly northward in hanging wall of faults. When the stress sensitivity is 1×10^-3MPa^-1 and magnetization is 1A/m,the calculated piezomagnetic field is about 5nT, 1nT and +0.007nT in the epicenter, 15km away from epicenter and at CDP, respectively. According to coseismic geomagnetic variations acquired from observed data and calculated data, it is obvious that stations (sites) located close enough to the epicenter could record coseismic signals above several nano Tesla. This implies that it would be difficult to observe geomagnetic signals associated with moderate earthquakes with present geomagnetic stations in this area and we need more improved observations. The piezomagnetic model tended to underestimate the observed coseismic geomagnetic variations with stress sensitivity of 1×10^-3MPa^-1 and magnetization of 1A/m. In this study,a uniform initial magnetization of 1A/m is expected to be reasonable according to aeromagnetic data and ground-based geomagnetic data, which indicates a relatively small magnetic anomaly. In this case, the stress sensitivity of 2×10^-3~3×10^-3MPa^-1 is suitable for explaining the observed coseismic geomagnetic variations. This result provides new data for coseismic geomagnetic variations. We can evaluate the initial magnetization, the stress sensitivity and slip model of earthquake if more observed data in near-field are available.

ANALYSIS OF IN-SITU STRESS PARAMETERS OF YISHU FAULT ZONE BASED ON ORIENTATION OF DITF IN BHTV IMAGE

WANG Pu, WANG Cheng-hu, WANG Hong, CHEN Nian, ZHOU Hao, WEI Xue-yong

2020, 42(6):  1316-1334.  DOI: 10.3969/j.issn.0253-4967.2020.06.004

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The parameters of regional in-situ stress field are very important for the analysis of regional crustal stability, extraction of deep energy resource materials, and deep infrastructure construction. Moreover, the principal stress orientation is one of the important characteristic indicators of regional in-situ stress field. At present, there are many methods to determine the stress field, mainly including the method based on borehole(in-situ measurement)or focal mechanism solution(inversion). Comparing the way of in-situ measurement and inversion measurement, the in-situ measurement not only can measure the orientation of the in-situ stress, but also can obtain the magnitude of the in-situ stress. In the existing in-situ stress measurement and estimation methods based on borehole, the stress orientation is mainly determined by borehole induced failure structures when drilling or coring. Due to the concentration of tectonic stress in the borehole wall after drilling, the rock mass in the borehole wall area may be damaged, and borehole induced failure structures including borehole breakout(BO)and borehole-induced tensile fracture(DITF)will occur. Usually, borehole breakout occurs near the minimum principal stress orientation, and tensile cracks occur near the maximum principal stress orientation. In these two types of borehole induced failure structures, the DITF is more widely used to determine stress in-situ orientation, because DITF is the direct response of the orientation of the maximum horizontal stress in the stress field. The Yishu fault zone is the main active fault in the southern middle segment of Tan-Lu fault zone and the M_S8.5 Tancheng earthquake in 1668 occurred in this area, so determining the characteristics of the stress field in this area has important reference value for the judgment of Yishu fault stability. In order to accurately determine the orientation of the modern stress field of the Yishu fault zone, we drilled three boreholes with a depth of about 400m in the Yishu fault zone from 2012 to 2014, the connection direction of the three borehole positions is perpendicular to the strike of fault zone and covered the main faults and its branch faults. In the three deep boreholes, we use high-precision ultrasonic drilling TV system to implement a three-year repeat scan logging and hydraulic fracturing in-situ stress measurement with a view to analyzing the characteristics of the regional in-situ stress field. Through logging images, a total of 199 vertical fractures were accurately identified based on the characteristics of the damaged structure, including 99 drilling-induced fractures, 43 hydraulically induced fractures, and 57 growth fractures in the later stages(natural growth), simultaneously, we use the cyclic statistical method to determine the dominant horizontal principal stress orientation of the Yishu fault zone. As the result, the principal stress orientation is about 92.28° which is basically no change compared with previous research results, and the natural growth of vertical fractures was discovered for the first time near the dominant orientation. In order to verify this phenomenon, we compare the rock tensile strength of the borehole core with the stress profile obtained from the measured in-situ stress value in this research. Through Brazil Splitting Strip Test, Brazil Splitting Pyramid Test, Hollow Rock Pillar Test(oil injection and water injection)and the direct tensile test, we obtained the rock tensile strength of about 6~11MPa. Theoretical analysis results show that the maximum circumferential stress in the borehole is larger than the tensile strength of the rock, which satisfies the basic conditions for causing natural tensile failure, and it is theoretically possible to form the natural tension fracture in the current stress environment. This new phenomenon can further verify the accuracy of ultrasonic drilling TV in determining in-situ stress orientation, and acoustic borehole televiewer with high resolution can be widely used to in-situ stress measurement in the future.

JOINT INVERSION OF SITE VELOCITY STRUCTURE BY MICRO-TREMOR ARRAY RECORD: A CASE STUDY OF THE OBSER-VATION SITE 3^# OF XIANGTANG IN TANGSHAN

WANG Ji-xin, RONG Mian-shui, FU Li-yun, FU Lei

2020, 42(6):  1335-1353.  DOI: 10.3969/j.issn.0253-4967.2020.06.005

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The research on the exploration method of velocity structure of the site soil layer that is efficient, economic and easy for promotion and application is of great significance considering the importance of shear wave velocity structure in shallow underground for prediction and prevention of geological hazards. With no dependence on special hypocenter, no need for destructive drilling and a wide range of detectable depths, microtremor array applies to densely populated cities and plain areas, and has become one of the new research focuses in the field of geophysical exploration at home and abroad in recent years.
In the study of inversion record of wave velocity profile on shallow soil layer by using array observation records, surface wave dispersion(DC)or microtremor horizontal-to-vertical spectral ratio(MHVSR)inversion is generally carried out separately at present, but the velocity structure of inversion is often of obvious multi-solution. The dispersion curve mainly constrains the shear wave velocity of the loose sedimentary layer while the predominant frequency estimated from the peak value of MHVSR mainly constrains the thickness of the overburden. In addition, various results of the Rayleigh-wave dispersion curve indicate that the calculated frequency range of the phase velocity is higher than the predominant frequency. In view of this, a joint inversion method of DC and MHVSR is developed, and a new inversion strategy is proposed in this paper. Different from the existing inversion methods, in this paper, firstly the Rayleigh-wave dispersion curve is obtained from the data of microtremor array by Modified Space Autocorrelation Method(MSPAC)and Frequency-wavenumber analysis method(F-K), the results are compared with the theoretical fundamental order and one high order Rayleigh wave dispersion curve calculated from the borehole data, and the measured dispersion curve is fitted. Secondly, the dominant frequency and its corresponding amplification coefficient of the site are analyzed based on the Microtremor Horizontal-to-Vertical Spectral Ratio(MHVSR)recorded from a single station. As a result of the correlation between the dominant frequency and the thickness of the overburden, the depth of the site bedrock is determined, and then the initial velocity structure of the site is obtained by the improved half-wavelength method. Finally, the best velocity structure of the site is determined by the joint inversion of DC and MHVSR, and the S-wave transfer functions caused by the vertical incidence of the inversion model and the measured borehole model obtained by different inversion methods are compared.
The advantages of the inversion method in this paper lie in two aspects. On the one hand, in the extraction of surface wave dispersion curve, the comprehensive application of Modified Space Autocorrelation Method(MSPAC)and Frequency-wavenumber analysis method (F-K) widens the frequency range of extracting dispersion curve by a single method. On the other hand, in the determination of initial velocity structure, the problem of relying on certain prior information of the current other inversion methods is better solved with the improved half-wavelength method.
In this paper, the effectiveness and stability of the new inversion strategy are verified by a theoretical example and an array observation example. It is observed that the MHVSR of the single DC inversion model is different from that of the theoretical example model after the peak frequency(especially in the high frequency segment)under the initial model, and the DC of the single MHVSR inversion model is different from that of the theoretical example model in the lower frequency segment. However, the joint inversion of the two makes up for the high frequency difference of the MHVSR of the DC inversion model and the low frequency of the DC of the MHVSR inversion model, thus greatly reducing the multi-solution of the inversion model and reflecting well the site characteristics(amplification effect and predominant frequency)when the inversion model approaches the real site. Compared with the traditional seismic and electromagnetic exploration methods, the joint inversion method based on the microtremor array records in this paper is of more practical value in acquiring the velocity structure of shallow site.

THE RESEARCH ON FAULT PLANE SOLUTION AND GEOMETRIC MEANING OF THE LAOHUSHAN FAULT IN THE NORTHEASTERN TIBETAN PLATEAU

LIU Bai-yun, YIN Zhi-wen, YUAN Dao-yang, LI Liang, WANG Wei-huan

2020, 42(6):  1354-1369.  DOI: 10.3969/j.issn.0253-4967.2020.06.006

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The Laohushan fault zone is located in the northeast margin of the uplift area of the Tibetan plateau. It belongs to the eastern segment of the Laohushan-Maomaoshan-Jinqianghe Fault in the eastern segment of the North Qilian fault system. It was manifested as compressive thrust in early stage, but its mechanical properties changed into left-lateral strike-slip movement after middle Pleistocene. There occurred the Jingtai M_S6.8 earthquake in 1888, Tianzhu M_S6.2 earthquake in 1990 and Jingtai M_S5.9 earthquake in 2000 along the fault in history.
With the construction of the national important projects in earthquake industry-“Digital seismic network project of the 10th Five Year Plan” and “Chinese seismic background field detection project”, a number of modern seismological stations were built in Gansu Province and its adjacent areas. Contrast with seismographic network, the mobile broadband seismic array has the advantages of relatively dense stations, small spacing, uniform distribution, and high data integrity rate. Combining the two observational methods has gradually become the main development direction at home and abroad.
Based on the data of small earthquakes in the Laohushan zone recorded by 20 stations of the digital seismic network in Gansu and its adjacent seismic network during the years of 2008 to 2019, and 18 portable seismographic stations from the 2^nd-phase project of China Seismic Detection Array during the years of 2014—2015, we relocated the dense earthquakes by double-difference method and obtained the source parameters for 700 earthquakes. The accurately located small earthquakes distribute along both sides of the Laohushan Fault, which is NW-trending obviously. Most earthquakes distribute at the depth range of 0~10km of the earth's surface after the relocation, and the result shows that the focal depths are more concentrated.
Generally, the earthquakes are closely related to active tectonics, large earthquakes usually occur on fault zones with obvious activity, but the distribution of small earthquakes is related to the complex stress state underground and the complex structures of fault zones. We can inverse the shapes and positions of the fault planes using spatial distribution of hypocenters of small earthquakes according to the principle that clustered earthquakes occur near the faults. We obtained the parameters of the Laohushan Fault, which has a strike of 103°and a dip angle of 89°, by using the simulated annealing algorithm and the Gauss-Newton algorithm. On this condition, rake angle of the fault plane is further inferred from regional tectonic stress parameters. These inversion results of the fault parameters indicate that it's a left-lateral slip fault with a high dip angle and a length of 38km. It extends from Xijishui county town of Jingtai in the southeast to Songshan of Tianzh county town in the northwest. Comparing the inversion fault plane parameters and the focal mechanism solutions of the 1990 Tianzhu M_S6.2 and 2000 Jingtai M_S5.9 earthquakes, both of the results are identical. Besides, the spatial distribution of inverted fault plane and the location of Laohushan Fault by the previous studies are basically parallel.
In the past, the studies of active faults mainly focused on the qualitative researches of macroscopic survey. With the technological development of earthquake location and inversion method in recent years, many quantitative researches have gradually been carried out on the determination of active fault parameters. The inversion results of Laohushan fault plane and the previous studies on the geometric characteristics of the fault are verified each other. It is proved by facts that it's an important research means of active faults. It can provide more evidences for determining fault parameters by inversion.

LATEST PROGRESS ON ACTIVITY OF HESHAN-MODAOMEN SEGMENT, XIJIANG FAULT

LU Bang-hua, WANG Ping, WANG Hui-ying, LAI Zhong-ping, DENG Zhi-hui, BI Li-si, WAN Wan-he

2020, 42(6):  1370-1384.  DOI: 10.3969/j.issn.0253-4967.2020.06.007

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The Xijiang Fault is an important NW-trending fault with a length of~200km, located in the western part of the Pearl River Delta. A M4 3/4 earthquake occurred at the northern end of the fault(Sihui)in 1445 and a magnitude 5 earthquake occurred at the southern end of the fault(Modaomen Waters)in 1905. Heshan is the boundary between the southern and the northern segments of this fault. The southern segment which is called Heshan-Modaomen segment is mainly hidden faults. The activity of Heshan-Modaomen segment remains controversial due to the lack of systematic studies for the deep and shallow exploration, which affects the assessment and prevention of earthquake disaster risk. In this paper, we concentrate particularly on the distribution and activity of Heshan-Modaomen segment using seismic geological surveys, shallow seismic exploration, joint borehole profile detection, and Quaternary geochronology.
Field geological surveys show that the fault zone is prominently normal sinistral strike-slip faults, striking about N310°~330°W, with a width of 10~20m. Most of them dip northeast at angles of 60°~80°. Observations on typical outcrop show that cataclasite, breccias and siliceous rocks are developed on the faults. Fault planes often have smooth and polished surfaces and no fault geomorphology has been developed along the fault zone. The overlying eluvial weathered soil materials have not been disturbed or cut. We carried out shallow cross-fault sounding of 7 profiles in the hidden section of the fault zone using longitudinal wave reflection method of multifold coverage observation system. As a result, we obtained the reflection time sections of the target stratum and the main structure. A total of 13 breaking points to be investigated were explained. We also performed cross-fault drilling at the location of the seismic data interpretation profile and catalogued drilling cores. ¹⁴C and OSL samples were collected systematically. The ¹⁴C dating was performed by the BETA Laboratory in the United States and 16 valid age data were obtained. OSL dating was performed by the OSL Laboratory of China University of Geosciences(Wuhan)and 6 age data were obtained.
This paper presents the study results of two representative cross-fault profiles. The shallow exploration survey line XJ1 and the row drill profile P1 are located in the southern section of the fault where six boreholes are arranged. We find the existence of bedrock faults on the joint borehole profile. The grooves developed thereupon are filled with the late Pleistocene paleochannel deposits with no obvious faults observed. The overlying Holocene strata are horizontal and continuous, without cutting and disturbance. Combined with the stratigraphic age, we infer that the fault has been inactive for at least about 11 000 years. The shallow exploration survey line XJ2 and row drill profile P3 are located in the northern section of the fault, where a total of seven boreholes are arranged. The borehole sections reveal the existence of fault crushed zone in the underlying bedrock(Cambrian hornstone). The tectonites are mainly fault breccias and cataclastic rocks with chlorite alteration. Groove landforms are formed along the fault zone with strong erosion at the later stage, and filling and accumulation occurred since the Holocene transgression with no fracture cutting or stratum disturbance. According to the landform, the occurrence of faults and the development of transverse active faults, the Heshan-Modaomen segment of Xijiang Fault can be further divided into two segments with the boundary of Zhupai Island. Both of them have been inactive since the Holocene.

TECTONIC GEOMORPHOLOGY AND QUATERNARY SLIP RATE OF THE XITIESHAN SECTION OF THE NORTHERN MARGIN FAULT OF QAIDAM BASIN

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

2020, 42(6):  1385-1400.  DOI: 10.3969/j.issn.0253-4967.2020.06.008

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The northern margin fault of Qaidam Basin(NMFQB)dominates the deformation of the northeastern part of the Qaidam Basin. The study on the Quaternary slip characteristics of NMFQB is of great significance to understand the regional strain-partitioning pattern for the south Qilian orogenic belt, and the extrusion process in the Qaidam Basin. In this paper, Quaternary activities of the fault are discussed based on the remote sensing interpretation, geological survey, trench excavation, GPS topographic profile measurement and OSL dating. The results show that the NMFQB has obvious linear characteristics from the remote sensing image of Xitieshan section. A series of geomorphic traces, such as fault scarps, fault facets, water system displacement, show that the Xitieshan section of the NMFQB is a Holocene active strike-slip fault with minor thrust. Four-stage alluvial fans were identified in the Xitieshan area. The DEM map shows a maximum horizontal displacement of 150m, 38m and 6.5m in the alluvial fan Fan3, Fan2 and Fan1, respectively. The geological age of Fan3 landform in the area obtained by OSL dating is(34.3±3.3)ka, the geological age of Fan2 landform is(11.6±1.0)ka, and that of Fan1 landform is(3.2±0.3)ka. Comparing with the analysis and collation results on the alluvial fans in the northern Qaidam Basin obtained by other researchers, the geological age of Fan3 alluvial fan in the northern Qaidam Basin is about 40ka, that of Fan2 is about 12ka, and the geological age of Fan1 is about 3.5ka. The age of the alluvial fan in the Xitieshan area is basically consistent with the development time of the alluvial fan in the region, indicating that the northern region of the Qaidam Basin was under a large-scale regional uplift during the same period, and the uplift activity was synchronous and recurrent.
Through GPS measurement of fault scarps across faults, the average height of the scarps formed in Fan1 is 1.2m. According to the geological dating of Fan1, the vertical movement rate is calculated to be 0.33~0.38mm/a. The average height of the scarps formed by the alluvial Fan2 is 2.35m. According to the geological dating of Fan2, the vertical movement rate is calculated to be 0.17~0.23mm/a.
We analyze the vertical displacement and related geomorphological ages of the two periods of alluvial fans at the two sites with one west to Xitieshan Town and one east to Quanjihe after measuring the horizontal and vertical displacement data of the geomorphic surface in this area. The Late Pleistocene strike-slip rate of this section is 3.55~4.72mm/a since 40ka and 2.68~3.65mm/a since 12ka, the Holocene strike-slip rate is 1.81~2.1mm/a since 3.2ka, and the Holocene vertical slip rate is 0.33~0.38mm/a. This amount of geological slip rate is consistent with the slip rate of 2~4mm/a from GPS observation.
According to the reverse “S” type structural system, natural profiles and trenching profiles of the northern margin fault of Qaidam Basin, it is believed that the fault was squeezed and uplifted by the Qilian Mountains block in the early stage, and the fault activity was mainly thrust, and in the latter stage, due to the impact of the Altun Tagh Fault, the fault activity takes the form of strike-slip. Controlled by the Altun Tagh Fault, North Qaidam Fault, the Elashan Mountain Fault and East Kunlun Fault, the Qaidam Basin behaves as a block rotating clockwise.

STUDY ON CO-SEISMIC DEFORMATION AND SLIP DISTRIBUTION OF THE AKETAO M_S6.7 EARTHQUAKE DERIVED FROM INSAR DATA

WEN Shao-yan, SHAN Xin-jian, ZHANG Ying-feng, LIU Yun-hua, WANG Chi-sheng, SONG Chun-yan

2020, 42(6):  1401-1416.  DOI: 10.3969/j.issn.0253-4967.2020.06.009

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The Aketao M_S6.7 earthquake occurred on November 25, 2016, which was located at the intersection of Gongur extensional system and Pamir frontal thrust. This region is characterized by complex fault structure, high altitude, complex terrain conditions, sparsely populated and few observed data, so the conventional geodetic survey technology is difficult to obtain comprehensive surface deformation information, while InSAR can take advantage of its all-weather, all-day, large-area and high-density continuous monitoring of ground motion. Therefore, this study takes M_S6.7 earthquake as the research object to carry out the co-seismic deformation field extraction and fault static slip distribution inversion. Firstly, the co-seismic deformation field was obtained by using ascending and descending data of Sentinel-1A. The results indicate that the interferogram spatial decorrelation is more serious in the north side of fault, which is affected by the steep terrain. The fringes in the south side of fault were distributed as elliptical semi-petal shapes, and the fringes are smooth and clear. The northern and southern part of the fault was asymmetric: The interferogram fringes of the southern part were dense while fewer fringes were formed in the northern part, and the fringes were semi butterfly-shaped on the surface. The horizontal displacements dominated the co-seismic deformation in this event, with magnitude of 12cm in ascending and -21cm in descending. The deformation occurred mainly on the south wall of fault. Based on the right view imaging of Radar, the co-seismic deformation is consistent with the movement features of dextral strike-slip fault and the focal mechanism provided by USGS and GCMT. The cross section of aftershocks after precisely positioning showed that the dip angle of fault is larger above the depth of 15km, which is generally manifested as the shovel-like structure with the dominant tendency of southward dip. By conducting a comprehensive analysis of deformation feature and aftershocks profile, we proposed that the southwest-dipping Muji Fault is the seismogenic fault. Secondly, a large area of continuous deformation images obtained by InSAR technology contains millions of data points and there is a high correlation between them. In order to ensure the calculation efficiency and inversion feasibility in the inversion process, the quadtree sampling method was used to reduce the number of data points and the datasets were finally obtained that can be received by the inversion system on the basis of retaining the original details of the deformation field. The two tracks InSAR datasets which were down-sampled by quadtree method were used to constrain the inversion to retrieve the fault geometry parameters and slip distribution. The single-segment and two-segment static slip distribution on the fault plane based on uniform elastic half space model were constructed during inversion process. The F-test of fitting residuals based on single-segment and double-segment fault model show that the population variance of the two models was significantly different at the confidence level of 95%, and the variance of the double-fault model was smaller. Through the comprehensive analysis of predicted deformation field, residuals and F-test, it is considered that the simulated results of double fault model are better than that of the single, and the observation data can be better interpreted. The result shows that the simulated co-seismic deformation field and its corresponding observed values were consistent in morphology and magnitude, and the correlation between observed and modeled is up to 0.99. In addition, as can be seen from the spatial distribution and frequency histogram of residuals, the overall residual was not large, mainly concentrated in the range of -0.2~0.2cm with the characteristics of normal distribution. However, there were still some larger residuals on the near fault in ascending track, which may be related to the simplified model. There were two patches with significant slip distribution on each segment and the rupture basically reached the surface. The slip was mainly distributed along the downdip range of 0~20km and was about 50km along the fault strike. The rupture reached the surface and the peak slip of 0.7m was at the depth of 9km. The western segment is dominated by the right-lateral strike-slip and the eastern segment is dominated by the right-lateral strike-slip with slightly normal faulting. The seismic moment derived from inversion was 8.81×10¹⁸N·m, which is equivalent to M_W6.57. The average slip angle obtained by inversion is -175° in the west section and -160° in the east section. The synthetic analysis holds that the source characteristics of the M_S6.7 earthquake was characterized by dextral strike-slip with a slightly normal component, which was composed of two sub-seismic events. The western section was basically pure right-lateral strike-slip with a dip angle of 75°, while the eastern was characterized by dextral strike-slip with a small amount of normal component with a dip angle of 55°. The Aketao earthquake occurred on the northern Pamir salient and its tectonic deformation was mainly characterized by crustal shortening, strike-slip and internal extension of the frontal edge observed by GPS. Generally speaking, the Pamir salient was blocked by nearly east-west South Tian Shan in the process of strong northward pushing under the action of NE direction pushing of Indian plate, and “hard and hard collision” occurred between them. The eastern part of Pamir salient extruded eastward along the nearly NS trending Gongur extensional system, and at the same time rotated clockwise, which caused the nearly EW extension since the Late Quaternary. The Aketao earthquake is a tectonic event occurring at Gongur Shan extensional system, which shows that the pushing of the Indian plate in the NE direction is continuously strengthened, and also implies that the internal crustal deformation of the Pamir Plateau is still dominated by extension in EW direction, which is basically consistent with the present observation of GPS.

FOCAL MECHANISM AND TSUNAMI NUMERICAL SIMULATION OF THE NOVEMBER 14, 2019 MOLUCCA SEA M_W7.1 EARTHQUAKE

XU Zhi-guo, WANG Jun-cheng, WANG Zong-chen, LIANG Shan-shan, SHI Jian-yu

2020, 42(6):  1417-1431.  DOI: 10.3969/j.issn.0253-4967.2020.06.010

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A strong earthquake with magnitude M_W=7.1 occurred in the area of Molucca Sea, Indonesia on November 14, 2019(Coordinated Universal Time, UTC), and then generated a small-scale local tsunami. In order to better understand the earthquake source characteristics and seismogenic structure, as well as to assess the hazard of tsunami caused by earthquake, this paper mainly focuses on the regional tectonic background, the focal mechanism, and tsunami numerical simulation for the Molucca Sea M_W7.1 earthquake. The broadband seismic waveforms from IRIS Data Management Center are used to estimate the moment tensor solution of this earthquake by W phase method. The result shows that the Molucca Sea earthquake occurred at a shallow depth on a high dip-angle, right-lateral reverse fault, the aftershocks were distributed along the SSW-NNE direction and concentrated near the main shock. These results indicate the Molucca Sea earthquake with characteristic of compressional rupture occurred in the complex plate boundary region of eastern Indonesia, which is dominated mostly by the collision interaction of the Halmahera slab and the Sangihe slab in the east and west sides of Molucca Sea under control of current regional stress field. The coseismic displacements of Molucca Sea M_W7.1 earthquake calculated using Okada's model of rectangular dislocation in a uniform elastic half-space show that the Molucca Sea earthquake generated vertical coseismic deformation with a maximum uplift of 0.15m when the rupture occurred along the high dip-angle reverse fault. The synthetic tsunami waveforms are provided by COMCOT tsunami modelling package solving the nonlinear shallow water wave equations based on the determined fault geometry from W phase inversion. These studies indicate the vertical coseismic deformation resulting in the sudden uplift of water volume above the earthquake source, and finally inducing a small-scale local tsunami. The energy of tsunami mainly propagates to both side of the fault, and part of energy propagates to Sula Islands of Indonesia along the fault dislocation direction; and compared with the first cycle of tsunami records observed by tide gauges deployed along the coastal line of earthquake source region, the observed tsunami head wave fits well with the synthetic wave, both are consistent in amplitude and tsunami arrival time, but the follow-up waveforms are quite different. The numerical simulation of tsunami shows that, in combination with the fault geometry parameters obtained by W phase fast inversion, the tsunami numerical model can be used for tsunami early warning, and it provides sufficient accuracy for forecasting tsunami wave height, thus, having great practical significance for understanding the propagation process and disaster distribution of tsunami.

FITTING THE FAULT PLANE PARAMETERS WITH SMALL EARTHQUAKES AND THE CHARACTERISTICS OF STRESS FIELD OF CHANGDAO AREA

CUI Hua-wei, ZHENG Jian-chang, ZHANG Zheng-shuai, LI Dong-mei, CHAI Guang-bin

2020, 42(6):  1432-1445.  DOI: 10.3969/j.issn.0253-4967.2020.06.011

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Using seismic observation data of Shandong seismic network, we relocated 2 927 earthquakes(M_L≥0.2) recorded from Feb. 2017 to Apr. 2019 with double-difference algorithm in Changdao area. The fault plane parameters are calculated with 1 631 relocated earthquakes in the northern and southern earthquake swarms based on the simulated annealing and Gauss-Newtonian nonlinear inversion algorithms. There are two different earthquake swarms in both sides of 38°N. In order to distinguish the different earthquake swarms, we divide them into the northern earthquake swarm locating in the north of 38°N and starting from Feb. 2017, and the southern earthquake swarm locating in the south of 38°N and starting from Aug. 2017.
The stress field of Changdao area is inverted with 7 266 P wave polarities of 2 518 earthquakes in the swarms using the composite focal mechanism method. This method takes full advantage of all P wave polarities, thus avoiding the errors brought about by inverting focal mechanism with P wave polarities. The study region is divided into grids of 0.25°×0.25° before the stress field inversion for the northern and southern earthquake swarms. The rake on the fault plane of the northern and southern earthquake swarms is calculated using the stress field and fault plane parameters.
1 432 and 219 earthquakes are used to calculate the fault plane parameters for the northern and southern earthquake swarms, respectively. The result shows that the fault plane parameters are different between the northern and southern swarm. The strike, dip and rake of fault plane are 287.18°, 84.09° and -18.3° in the northern earthquake swarm, which is nearly the same with the previous results of shallow-depth acoustic reflection profiling. The fault plane parameters for the southern earthquake swarm are 269.67°, 67.46° and -3.6°. This result is similar to that of marine geophysical survey and the seismo-geological studies. The type of both fault planes is sinistral strike-slip according to the rake on the fault plane.
The stress field is inverted with a 50km radius smoothing in this paper. In general, the stress field calculated by this paper is basically identical with the previous results obtained by focal mechanism inversion and hydraulic fracturing in-situ stress measurement in Changdao area and is consistent with the stress field of the North China area. The stress field is controlled by pushing and subduction of the Pacific Plate from east to west. But there is a slight difference in the stress field between the northern and southern earthquake swarms. The compressive axis of stress field is rotated between the northern and the southern earthquake swarms. The stress field is in strike-slip regime in the northern earthquake swarm. The direction of P-axis is NEE-SWW, with a nearly horizontal plunge, and the direction of T-axis is NNW-SSE with a low plunge. In the southern earthquake swarm, the stress field is in a regime of normal faulting with a small amount of strike-slip component. The P axis is in NE-SW direction with plunges varying from 30° to 50°, and the T axis is the same as the northern swarm.
Based on the fault plane fitting, the seismogenic fault for the northern earthquake swarm is maybe the buried NW extension of the Dazhu Island-Weihaibei Fault, and the southern earthquake swarm occurred on a secondary EW-trending fault. According to the rakes of seismogenic faults, both of them are of strike-slip movement, and the stress field is in strike-slip regime in the northern earthquake swarm and normal with a small amount of strike-slip in the southern swarm. Both northern and southern earthquake swarms are controlled by the sinistral strike-slip Penglai-Weihaibei Fault, but the southern swarm is also under the influence of SN extension. We believe that the reason for the different fault plane parameters and stress fields is the different structure of the northern and southern earthquake swarms.

THE PARTICIPATORY CONSTRUCTION OF A SEISMIC SCENARIO FOR WEINAN CITY: A PILOT ACTION RESEARCH TO ADDRESS THE IMPROVEMENT OF EARTHQUAKE DISASTER RISK REDUCTION IN CHINA

SU Gui-wu, Janise Rodgers, TIAN Qing, QI Wen-hua, Philip England, Timothy Sim, John Young, WANG Dong-ming, LI Zhi-qiang, FENG Xi-jie, SUN Lei, CHEN Kun, Emily So, Barry Parsons, ZHAO Jin-li, SHI Jian-liang, YUAN Zhi-xiang, Yue Cao, ZHOU Qi, WEI Ben-yong, David Milledge, Alexander Densmore

2020, 42(6):  1446-1473.  DOI: 10.3969/j.issn.0253-4967.2020.06.012

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Earthquake disaster reduction approach in China is essentially top-down, which is highly effective in mobilizing large-scale disaster reduction activities. However, the overall resilience of a society to earthquake also heavily depends on actions from various bottom-up components/actors(e.g., family, community), pointing to the strong need for a governance model that integrates the existing top-down approach with broad bottom-up engagement of grass-roots and the public. To accumulate research evidences for developing that governance, the overall objective of the work of creating a seismic scenario for Weinan City, Shaanxi Province, China(the Weinan scenario work, in short), was thus planned to address in particular the following two major gaps in earthquake disaster risk reduction in China: (i)between top-down and bottom-up earthquake disaster risk reduction(DRR)approaches, with a particular emphasis on the weak bottom-up aspect, and(ii)between science and earthquake DRR policies and practices, especially the insufficiency in the research and associated applications relevant to various bottom-up components/actors.
Using the paradigm of trans-disciplinary, participatory action research, the Weinan scenario work delivered this objective through direct interactions and close collaborations between two different groups of people: multi-disciplinary UK-USA-China collaboration research team and various local DRR practitioners and other stakeholders. The overall progresses include: 1)Using pan-participatory methodology, the two groups worked closely together to co-identify earthquake risk, co-explore pathways to risk reduction and resilience building, and so on, which ensured the reliability of the scenario results and the local context-appropriateness and then applicability of the scenario work's DRR recommendations; meanwhile, with action research process, the two groups realized synchronous interactions and seamless connections between the three large aspects in risk science of risk assessment, risk communication, and risk reduction practice improvement, which have often been conducted separately, thereby resulted in a kind of direct, immediate, and in-situ/on-site “science research into policies and practices”; 2)By serving both governments and bottom-up actors, and by looking at earthquake DRR issues from multi-scale point of view, the two groups co-addressed how to improve both top-down and bottom-up earthquake DRR policies and practices. Especially, zooming in on community-based disaster risk reduction(CBDRR), school-based DRR, family-based DRR, and broad disaster reduction education-the broadest and most sustainable linkages between top-down approach and bottom-up pathway to earthquake DRR, a large scale of specialized surveys and other relevant investigations were conducted, a series of current baselines and future improvement directions were identified; 3)Focusing on bettering disaster reduction education and improving long-term risk communication, the two groups co-created two versions of storytelling-led and latest science-grounded scenario narratives that are different in both contents and presentation styles/formats: one for government officials, the other for the general public. By constructing the plot of the story to properly highlight the key earthquake risk problems facing Weinan, we hope non-technical readers can easily understand research findings and better follow DRR recommendations provided, further facilitating “science into policies and practices”; by unfolding and illustrating the disaster-amplification or superimposing effects produced by a distinct local vulnerable social element-poor rural family with left-behind children, we hope readers can understand earthquake risk as deeply and comprehensively as possible from multi-perspectives; by incorporating elements of sensibility, emotion, humanity, and artistic appeal into rational but often “dry” sciences, we hope to help intensify resonance, build consensus, inspire emotion, improve DRR attitudes, foster DRR values, and then motivate DRR action and participation; and most importantly to inspire long-lasting learning, reflection, and action improvement among local population-the most direct, fundamental, and broad actors for reducing local earthquake disaster risk.
The participatory action research-guided Weinan scenario work has the utility of “throwing out a brick to attract a jade” for China's earthquake DRR field, it also provides the international similar studies with valuable experience and implications from China context.

Application of new technique

INTERFEROMETRIC SOURCE IMAGING OF SICHUAN CHANGNING M_S6.0 EARTHQUAKE

ZHAO Bo, GAO Yuan, LIU Jie, LIANG Shan-shan

2020, 42(6):  1474-1491.  DOI: 10.3969/j.issn.0253-4967.2020.06.013

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Seismic interferometry imaging can accurately determine the source location. The main shock and some aftershocks of Sichuan Changning M_S6.0 earthquake occurring on 17th June, 2019 are located with seismic interferometry imaging in this study. This technique is different from the traditional earthquake location method in that it does not use the phase arrival data in the earthquake catalog. Due to the existence of seismic phase picking errors, the traditional earthquake location method has a basic limitation on the amount of location error reduction. Interferometry imaging method directly applies the waveform records for source location and gets the travel time difference information from cross-correlation calculating, thus, greatly reduces the phase reading error. There are three main processes of interferometric imaging location technique, that is, waveforms cross-correlation of onset phases between station pairs, interferometric waveforms migration, and superposition. However, the complex focal mechanism and radiation pattern of natural earthquakes will cause the polarities of the first arrival phases to be different. When performing superposition processing, the addition of the positive and negative amplitudes will reduce the superimposed energy. By calculating the characteristic functions of the original waveform records, the inconsistency of the polarity of the initial phase caused by the different radiation patterns of the source in different azimuths is eliminated. Since the seismic stations used for location are regional network, the distance between station pairs is relatively close, and waveforms cross-correlation can eliminate some velocity disturbances. Besides, there is a certain coupling between origin time and source location. Therefore, the origin time and source location should be decoupled during locating. The waveform cross-correlation between station pairs can subtract the same origin time, thus, eliminating the location error caused by the disturbance of origin time. In addition, an advantage of the interferometric imaging location method is that it increases the amount of available data. The non-repetitive pairing between stations makes the travel time difference data far more than the direct wave phase data, and these travel time difference data are independent of each other and have no correlation. In this study, the natural earthquakes are located by applying interferometric imaging technology with cross-correlation migration kernel function. After migration and superposition of interferometric waves, the horizontal position and depth of the sources are imaged. The location of the main shock is(28.38°N, 104.88°E, 8.0km), and it is on the west of the aftershock belt. We compared the result from this study with the results of USGS(28.40°N, 104.95°E, 10km), GFZ(28.43°N, 104.94°E, 10km)and CENC(28.34°N, 104.90°E, 16km). The epicenter positions of the four results are relatively consistent, and the deviation is within 0.07°. The depth from our study is consistent with the results from USGS and GFZ, and the difference is 2km. In this study, the depths of M_S4.0~4.9 aftershocks are 5km. Three M_S5.0~5.9 aftershocks are distributed in the depth of 8~10km. The Changning earthquake sequence is located at the western end of the Changning anticline, which is the main geological structure in the Changning area and trending NWW-SEE generally. The anticline is about 100km long from east to west, and about 20km wide from north to south. The Changning anticline was formed in the Mesozoic Era. It was pushed by the NE-SW trending tectonic stress at that time, and accompanied by multiple small faults, shown as high-angle compressive thrust faults. The epicenter distribution shows that the aftershock belt is distributed along the NWW direction. After interferometric imaging location, the average travel time residual is 0.6s. In this study, we used four different velocity models(CodaTomo, Crust1.0, IASPI91 and SIMPLE)for calculating the theory travel time for migration. The effects of four different velocity models on the location are tested and the results show that the seismic interferometric imaging location method is stable. The average travel time residuals of four velocity models are 0.66s, 0.68s, 0.80s, and 0.71s. By calculating the array/network response function, the influence of the station distributions and the length of the characteristic function window on the positioning result are evaluated. The network response functions with four dominant frequencies at 0.01Hz, 0.05Hz, 0.1Hz and 0.25Hz were calculated and compared. The network response functions have fewer local maximums but converge slowly at 0.01Hz, 0.05Hz, and 0.1Hz. In the depth direction, the resolution is very low. The dominant frequency of the eigenfunction calculated in this study is about 0.25s. At this frequency, the network response function shows good convergence and stability in both horizontal location and source depth.

THE APPLICATION OF GEOMORPHIC INDEXES IN SMALL-SCALE GEOMORPHOLOGY:A CASE STUDY IN DUSHANZI ANTICLINE IN THE NORTHERN CHINESE TIAN SHAN FORELAND

ZHOU Chao, HE Hong-lin, WEI Zhan-yu, SU Peng, REN Guang-xue

2020, 42(6):  1492-1508.  DOI: 10.3969/j.issn.0253-4967.2020.06.014

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Landform is the shape of the earth's surface, which is the combined influence of tectonic movement and surface erosion. Geomorphic indexes are the quantitative methods applied in geomorphology, aiming to extract the tectonic and erosion information from the surface morphology. Since the 1950s, the HI(Hypsometric Integral)had been used to quantitatively characterize the three-dimensional volume residual rate of drainage basins after erosion and to estimate the geomorphic evolution stage, and the relief had been used to evaluate the erosion response of regional tectonic uplift. Since the 1970s, with the construction of the stream power incision model, the k_sn(Steepness)based on the model has been widely used to estimate the distribution of uplift rate, and it has become an important branch of geomorphology to obtain the information contained in the landform by using geomorphic indexes. The quality of terrain data affects the research level of geomorphology. In the early stage of geomorphic research, field survey is the main method to carry out quantitative statistics of geomorphic units within a certain range. With the development of satellite remote sensing technology, DEM data are widely used in large-scale structural geomorphic research, such as the study of geomorphic parameters of orogenic belts. In recent years, with the further development of space exploration technology, a large number of high-quality DEM data have been produced. Based on these data, whether the geomorphic indexes methods which have been widely used in large-scale geomorphology research could be applied to small-scale geomorphology to extract more precise structural and geomorphic information has become an important issue of quantitative geomorphology research. In this paper, Dushanzi anticline in northern Chinese Tianshan foreland is taken as the research object to explore the application of geomorphic indexes methods to the study of small-scale geomorphology. Dushanzi anticline is a propagation fold formed in the foreland of Tian Shan Mountains as a result of the India-Eurasia collision and is still active since the Holocene. The geological outcrop of the Dushanzi anticline is about 90km². There are river channels which are well preserved on the anticline, providing an ideal area for the calculation of geomorphic indexes. Consequently, the area is an ideal place for the study of the application of geomorphic indexes methods in the small-scale geomorphology. Based on the 12.5m spatial resolution DEM from ALOS(Advanced Land Observing Satellite), we calculated the HI, k_sn and relief of the study area to explore their applicability to the study of small-scale geomorphology and then the geomorphic parameters are comprehensively analysed to discuss the structural and geomorphic information of anticline. The results indicate that: 1)In the quantitative study of small-scale geomorphology, the lower level drainage basins should be used to generate the HI on the premise of the accuracy of the data to improve the resolution of the HI results. Invalid data of small drainage basins should be eliminated in the process of calculating k_sn to ensure its accuracy although the density of the data will decrease. The smaller window should be used to calculate the relief on the premise of ensuring statistical error and research demand to improve the resolution of results. The higher resolution of DEM is helpful to improve the resolution and accuracy of the above indexes. 2)The results of geomorphic indexes indicate that the core of the anticline has higher uplift rate, larger erosion amount, smaller volume residual rate, and later stage of geomorphic evolution compared with the inclined end of the anticline and a continuous change of landform from intense down-cutting to topographic relaxation could be observed from the core to the inclined end of the anticline. The calculation results of geomorphic indexes are consistent with the geological facts of Dushanzi anticline, which shows that the geomorphic indexes methods are effective in the study of small-scale geomorphology.

SEISMIC THERMAL INFRARED ANOMALY EXTRACTION BASED ON SKEWNESS

LIU Wen-bao, MENG Qing-yan, ZHANG Ji-chao, ZHANG Ying, LU Xian, MENG Ya-fei

2020, 42(6):  1509-1524.  DOI: 10.3969/j.issn.0253-4967.2020.06.015

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The seismic thermal anomaly has been confirmed by a large number of earthquake case studies, but the weather factor is still the limiting factor for the fine extraction of thermal anomalies. In the past, in order to eliminate the short-term noise of meteorology, the spatial domain difference value was used to replace the temperature value in the original remote sensing image, but the cloud mask will affect the spatial domain mean value, which makes the spatial domain difference value not representative. This causes thermal anomalies due to differences in cloud masks and spatial distribution of temperature. In previous RST studies, the thermal anomaly thresholds were judged to be different values of 1.6, 2, 3, and 4, and there was no stable value. When the number of background fields changes under the same threshold, the thermal anomaly area will change. Therefore, when constructing the background field, the time domain difference is used first and then the spatial domain difference is calculated to remove the weather influencing factors. By introducing skewness abnormal data monitoring algorithm, different sample numbers correspond to different thresholds in thermal abnormality judgment. In this experiment, the areas of 32.70°~42.72°N and 96.37°~106.65°E in Gansu, Qinghai and Sichuan were used as the study area. The new algorithm is used to analyze the spatial and temporal distribution of thermal anomalies in Hutubi M_S6.2 earthquake on December 8, 2016, Jiuzhaigou M_S7.0 earthquake on August 8, 2017, and Jinghe M_S6.6 earthquake on August 9, 2017. Statistical analysis is made on the statistical relationship between earthquakes of magnitude 4 and above and thermal anomalies in the study area from January 1, 2003 to August 31, 2019. Experiments and results show that: 1)By analyzing the annual mean surface temperature map in 2004, it was found that the night temperature in Qinghai Province in the study area was lower than that in other areas, and the night temperature in the southern Gansu and desert areas in the northeast of the study area was relatively high. On June 26, 2004, most of the cloudless images were in the low-temperature region of Qinghai Province, which resulted in a low average value of the spatial domain. The temperature difference between the high-temperature and cloudless regions of Qinghai Province and Gansu Province was relatively large, so the spatial domain difference caused the false thermal anomalies. The difference in time domain is calculated before calculating the difference in spatial domain, and the thermal anomaly almost disappears. Using the difference in time domain and then calculating the difference in spatial domain will eliminate the false thermal anomaly caused by the difference in temperature and spatial distribution of clouds. 2)Through experiments, it is found that the skewness in different sample numbers used in this paper corresponds to different thresholds and the position and area of thermal anomalies are closest when the RST threshold is 4.5. When the RST is at a threshold of 4.5 and the number of background fields is 12, 14, and 16, respectively, the thermal anomaly area becomes smaller as the number of background fields increases, but for the skewness of different thresholds corresponding to different sample numbers in the background when the number of fields is 12, 14, and 16, respectively, the change in thermal anomaly area is small. It is obtained that under the premise that the number of samples satisfies the abnormality judgment, different sample numbers corresponding to different thresholds will basically keep the thermal anomaly area unchanged when the number of background fields changes, and the thermal anomaly threshold judgment stability gets stronger. 3)By extracting the thermal anomalies of Hutubi M_S6.2 earthquake on December 8, 2016, Jiuzhaigou M_S7.0 earthquake on August 8, 2017, and Jinghe M_S6.6 earthquake on August 9, 2017, it was found that the thermal anomaly before the Jiuzhaigou M_S7.0 earthquake moved around the epicenter from the northwest Jungong Fault to the northeast Longxian-Baoji Fault in a clockwise direction; Before the Hutubi M_S6.2 earthquake, the whole thermal anomaly moved from south to north to the epicenter; Before the Jinghe M_S6.6 earthquake, the overall thermal anomaly moved from the vicinity of the northwest Tuster-Ballake Fault to the epicenter and was “/” shaped. 4)There were 210 earthquakes with M_S≥4.0 in the study area, and a total of 98 thermal anomalies occurred. The total number of thermal anomalies corresponding to the earthquake is 46, accounting for 46.94%; the number of earthquakes with thermal anomalies found in 210 earthquakes is 53, accounting for 25.2%, among them, 73.6%were preceded by thermal anomalies. By analyzing the relationship between thermal anomalies over the years and earthquakes, it is found that the TPR value increases with the magnitude of earthquake. Although the number of strong earthquakes in the study area is small and the TPR value of strong earthquakes in this experiment is not representative, the overall trend shows that the probability of earthquake thermal anomalies increases with the increase of the earhtquake magnitude. The greater the probability of occurrence of seismic thermal anomalies during strong earthquakes, the greater the significance is for future earthquake prediction. In this paper, we use two types of data, cloud and surface temperature, to optimize the algorithm to remove the influence of weather factors on seismic thermal anomalies. However, the influence of topography and ground features on temperature still exists, and there is currently no good way to judge and rule it out.