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20 October 2020, Volume 42 Issue 5

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Research paper

GEOLOGICAL AND GEOMORPHIC EVIDENCES FOR THE HOLOCENE ACTIVITY OF THE NW ZHEDUOTANG BRANCH WITHIN THE XIANSHUIHE FAULT SYSTEM

MA Jun, ZHOU Ben-gang, WANG Ming-ming, AN Li-ke

2020, 42(5):  1021-1038.  DOI: 10.3969/j.issn.0253-4967.2020.05.001

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Xianshuihe Fault is an active fault which originated from the eastern margin of the Tibetan plateau and formed by the orogenic events in Songpang-Ganzi area. The origin of Xianshuihe Fault is discovered in the NW of Ganzi, then it extends to the SE, passing through Luhuo, Daofu, Qianning, Kangding, Luding, Moxi and disappears after passing through Shimian. Based on previous studies, Xianshihe Fault is a sinistral strike-slip fault. According to GPS and InSAR data, the horizontal component of average slip rate for Xianshuihe Fault is approximately 7.5~16.7mm/a. As a crucial member of the regional earthquake zone, Xianshuihe Fault separates Sichuan-Yunnan block and Bayankala block. More importantly, Xianshuihe Fault is responsible for a great number of large magnitude earthquakes especially in the Qianning-Kangding segment, a segment of Xianshuihe Fault which consists of three branches. From east to west, they are Yalahe Fault, Selaha Fault and Zheduotang Fault which are all active since Holocene. Yalahe Fault is responsible for a M7 earthquake that occurred around 1700AD. Selaha Fault is responsible for another M7 earthquake which occurred around 1725AD. Around 1955AD, a M7.5 earthquake occurred which was related to Zheduotang Fault.
According to the 1:50k Xianshihe Active Faults Map(1995) and relevant researches, it is discovered that, from north to south, the Holocene active Zheduotang segment starts from Kangding airport to Zheduotang village. The total length of Zheduotang segment is around 30km which includes the surface rupture zone of the 1955 M7.5 earthquake. Due to the absence of researches, the northern part of the Zheduotang Fault, which is to the north of the Kangding airport, remains unstudied. Based on satellite image, we discovered that there are signs of faults to the north of Kangding airport. Therefore, we selected four sites to carry out field investigations and trench analysis. The first site is to the NW of the Duoriagamo village. Based on satellite image and DEM data, many typical faulted geomorphologic features are discovered. To the NW of this site, both the fan and the terrace are offset. By analyzing the DEM data, the offset of T1 terrace is around 7.8m and the offset of Fan1 is around 15.6m. To the SE of this site, the fan is also offset by sinistral movement which has an offset value of 21.7m. The second site is to the NW of the Muyazuqing school where 2.6m of sinistral offset between the fan and the T1 terrace are measured. To the SE of this site, obvious offset of fan and floodplain are observed which both have sinistral offset of 2.5m. The third site is to the south of first Duoriagamo village. The fault here shows two parallel branches. The fourth site is near the Tonglilongba and there are 37.5m of horizontal offset of the fan.
Based on trench analysis, 17 stratigraphic units are defined from which carbon samples are acquired for geochronological analysis. By constraining the age of each stratigraphic unit, the age of four deformation events are defined. Event 1 is the youngest which occurred between 5 821~3 148a BP. Event 2 occurred between 13 060~10 745a BP, Event 3 occurred between 13 687~11 420a BP and Event 4 occurred between 41 443~13 715a BP. According to the integration results of our analysis, the location of northwestern segment of Zheduotang Fault is defined. It is discovered that, the NW segment of Zheduotang Fault is located between the Kangding airport and Duoriagamo village with a total length of 15km. The trace of Zheduotang Fault is also defined. From north to south, Zheduotang Fault passes through Duoriagamo village, Tonglilongba, Kangding airport, Zheduoshan nek, Ertaizidaoban and disappears near Zheduotang village. Moreover, after Holocene, the Zheduotang Fault is dominated sinistral slip movement along with minor vertical component. Different from previous researches, we believe that the Holocene active Zheduotang segment extends 15km further to the NW. This discovery provides some basis for perfecting the plane geometric images of the three active faults in Qianning-Kangding segment of Xianshuihe fault zone, such as Zheduotang Fault, Selaha Fault and Yalahe Fault, and is of great significance for understanding the strain distribution and strong earthquake rupture mode of each branch fault in Qianning-Kangding segment of Xianshuihe fault zone.

STUDY ON PALEO-SEISMIC EVENTS IN TRENCHES OF THE EASTERN QIULITAGE ANTICLINAL BELT

ZHANG Ling, YANG Xiao-ping, LI Sheng-qiang, HUANG Wei-liang, YANG Hai-bo

2020, 42(5):  1039-1057.  DOI: 10.3969/j.issn.0253-4967.2020.05.002

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In response to the ongoing far-field effects of the India-Eurasia collision, the Tianshan Mountains experience rapid NS convergence, and most of the present N-S shortening is absorbed along the southern and northern edges. The resultant frequent large earthquakes have inspired many scientists to explore the neotectonic activity of the Tianshan Mountains. The eastern Qiulitage anticlinal belt located in the Kuqa depression, on the southern piedmont of the Tianshan Mountains, is a typical blind fault-related fold. The Kuqa M7$\frac{1}{4}$ earthquake in 1949 as typical folding earthquake once occurred on the northern limb of the eastern segment of the Qiulitage anticline, and the epicenter was near the Village of Kang which is sparsely populated. This earthquake is a typical folding earthquake whose dominant fault did not thrust onto the earth surface. Although many tectonic-induced scarps and deformed Quaternary strata have been reported, there are still no direct evidences for the surface ruptures and corresponding causative faults of this earthquake at present. And systematic understanding of paleoseismic events in Qiulitage area is also limited by the lack of relevant chronological researches.
We conducted 1︰50 000 scale geological mapping in the Qiulitage anticline area. The local surface geological characteristics are investigated based on interpretation of Google Earth image and confirmation in the field. Together with interpreted subsurface structure by petroleum seismic reflection profiles, the relationship between the active faults thrust on the surface, low-dip-angle decollement faults in deep, and fold deformation are subsequently qualitatively analyzed. In this study, the active faults which have thrust to the surface and generated fault scarps are focused on.
Totally five trenches were chosen and cleared up, two of which are located on the southern limb of the eastern Qiulitage anticline and the others are on its northern limb. And all excavation sites are situated on fresh fault scarps. We carefully interpreted different characteristics of tectonic deformation and sedimentary process which are correlated with paleo-seismic events from trenches. According to the OSL(Optically Stimulated Luminescence)and ¹⁴C dating results, a reliable chronological framework for the deformed stratigraphic sequences was established. Based on the classic successive limiting method, six paleoseismic events were finally constrained.
Some of these interpreted paleo-seismic events produced surface ruptures on the breakthrough faults simultaneously on the southern and northern limbs of the Qiulitage anticline, and others only caused local surface ruptures on its northern limb. In a broad sense, the surface ruptures caused by these paleoseismic events have similar characteristics to those which are popular among the low-dip-angle thrust faults on the southern piedmont of the Tianshan Mountains. And the two common phenomena are that multiple ruptures may occur a single fault and multiple faults may rupture simultaneously. We speculate that only when the displacement of master faults at depth is big enough, multiple shallow secondary faults can be triggered at the same time. Conversely, only one fault is active at one time. In other words, constrained by the length and displacement of dominant faults, not all paloseismic events can cause surface ruptures on the northern and southern limbs of the Qiulitage anticline at the same time.
The revealed paleoearthquakes may have a clustering feature since ~7.4ka. They behaved as follows: 1)Three events occurred during 5.7~7.4ka. 2)one event occurred during 3.3~4.7ka. 3)the latest cluster of events may be marked by the 1949 M_W7$\frac{1}{4}$ Kuqa earthquake. Thus, the earthquake sequences have a recurrence period of about 2.5~4ka.
Significantly, the incompleteness of the paloseismic events recorded in trenches and the quality and intrinsic error of the OSL dating samples can mislead judgments. It is inevitable that the time of paloseismic event cannot be constrained strictly. In our research area, because of the lack of seismic events between event E5 and event E6(7.25~19.1ka), there is a gap in seismic event records for up to~11.85ka. However, our result offers a relatively systemic event sequence to fill the gap in studies on paleoseismicity in this area. Whether there will be a strong shock after the 1949 M_W7$\frac{1}{4}$ Kuqa earthquake remains to be further studied in detail.

LATE QUATERNARY ACTIVITY CHARACTERISTICS OF THE TANGBAL-TASDUN FAULT ZONE IN THE EASTERN TIENSHAN AREA

HUANG Shuai-tang, HU Wei-hua, YANG Pan-xin, LI Shuai, Yiliyaer

2020, 42(5):  1058-1071.  DOI: 10.3969/j.issn.0253-4967.2020.05.003

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There were several strong earthquakes of M_S≥7.0 occurring in the eastern Tianshan in the history, which is an important part of Tianshan earthquake zone. The Tangbal-Tasdun Fault is a left-lateral strike-slip fault zone of Late Pleistocene in the northwest of Barkol Basin. The study of the characteristics of its late Quaternary activities is one of the important basic work to understand the risk of strong earthquakes in Barkol area. Due to the low level of research in the eastern Tianshan region, there is a lot of controversy over the historical earthquakes. But there is no doubt that this area has the ability of generating earthquakes of magnitude greater than 7.0. Current GPS monitoring data on both sides and inside of Tianshan Mountains shows an about 20mm/a northward movement of the Pamir and Tarim plates, but a 4mm/a crustal movement rate of eastern Tianshan. This indicates that the tectonic activity of the western section of Tianshan Mountains is obviously stronger than that of the eastern section. However, according to the historical earthquake records of eastern Tianshan, there are at least two earthquakes of magnitude 7 or above happening in Barkol region. This indicates that the tectonic activity in the Barcol area is intense and the area has the condition for generating strong earthquakes.
In this paper, the methods of high resolution satellite image interpretation, field observation and analysis, micro-geographic survey and trenching are used. The geometric distribution characteristics of the Tangbal-Tasdun Fault are determined, which reveals the movement and activity of the fault zone. The activity parameters of the fault since late Pleistocene are preliminarily obtained. The results show that the fault is left-handed strike-slip with thrust motion. A surface rupture zone with a length of about 50km is developed in the east of Jijitaizi Village. The fault offset the T2 terrace with a vertical displacement of about 0.9m and a horizontal displacement between 9m and 11m. The vertical displacement of T3 terrace is about 1.6m, and the horizontal displacement is between 13m and 20m. To the west of Hongjingzi Town and Tashbastawu Village, the fault is distributed in a straight line on satellite images. The fault offset the latest geomorphic surface, with the minimum vertical displacement of about 0.1m, the maximum vertical displacement of 2m, and the horizontal displacement of 1.8~4.3m. The horizontal displacement of the fault is larger than the vertical displacement of the same period. The excavation of a trench near Kutaizi village shows that the fault has obvious characteristics of strike-slip movement. According to the phenomena of water spraying and sand emitting along the fault and the relation of cut and cover between the fault and strata, two ancient seismic events are revealed in the trench. The most recent event ruptured the ground surface. According to the empirical formula for magnitude estimation, M=7.13+0.68lgD, it is calculated and inferred that this fault section is qualified for the occurrence of M7.3~7.4 earthquake.

STUDY ON SOFT-SEDIMENTARY DEFORMATION STRUCTURES OF XIGEDA FORMATION IN YONGSHENG, MIDDLE REACHES OF JINSHA RIVER

WANG Li-bin, YIN Gong-ming, YUAN Ren-mao, WANG Ying, SU Gang

2020, 42(5):  1072-1090.  DOI: 10.3969/j.issn.0253-4967.2020.05.004

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The neotectonic movement in the middle reaches of the Jinsha River is active and the earthquakes occur frequently. Lacustrine sediments are commonly distributed on both sides of the river with stable sedimentary environment, good horizontal continuity and relatively developed stratification, which are good carriers for recording paleo-seismic events. In this study, a large number of soft sedimentary deformation structures are found in the riverside lacustrine sediments in the Taoyuan Town area in the middle reaches of Jinsha River, with strong deformation and large scale. We focus on the comprehensive analysis of four soft-sedimentary deformation profiles. In which the profiled strata are mainly medium-fine sand and clay. And the soft sedimentary deformation structures mainly include sand liquefaction, rootless faults, clay lumps and folds.
Causes analysis: In the profiles of soft sedimentary deformation structures, there are medium and fine sand layers whose thickness is from thick to super thick. Sedimentary bedding has not been observed in the sand layer; and a large number of clay debris or lumps are involved in the sand layer, which are often filled between the adjacent clay lumps; and there are quicksand channels in the sand layer. All the features indicate that the sand layer in the study profiles has been liquefied. In the study profile, we found that the soft sedimentary deformation structure has the following characteristics: The faults found in the study profile extend downward and terminate in the lower liquefied sand layer and a large number of clay lumps. There are clay lumps in the place where the clay fold structure develops, and a large number of liquefied sand bodies are filled between the fold structures. The deformation structures in the profiles are not contrastive in terms of extension, chaotic deformation characteristics and obvious stress direction. Based on the characteristics of sand liquefaction and clay deformation in the above profile, it is inferred that the deformation structure in the profile is mainly due to sand liquefaction. The liquefaction strength of sand layer determines the deformation degree of clay layer.
Trigger factors analysis: There are many factors that can trigger the liquefaction deformation of the unconsolidated sediment, such as flood, freeze-thaw, collapse and earthquake, which can cause the liquefaction deformation of the sediment under certain conditions. In this paper, the possible trigger factors are analyzed based on the combination of the structural characteristics of soft sedimentary deformation, sedimentary environment and geological background of the area. First the stratigraphic characteristics also reflect the hydrostatic sedimentary environment at that time. The soft sedimentary deformation on such a large scale could not be mainly caused by the disturbance of lake waves. The research profiles are located at a sheltered bay with weak hydrodynamics, and no alluvial strata have been found in the upper part of the soft sedimentary deformation stratum. Moreover, the soft sedimentary deformation structure caused by flooding is often a small-scale curly layered structure, which has a large difference with the deformation structure and scale in the study profiles. This suggests that alluvial and diluvial events are not the main triggering factors of the deformation. Although the landslide is likely to occur near the study area, no trace of bedrock landslide is found near the study profiles. Therefore, the invasion of bedrock landslide into the sedimentary layer cannot be the triggering factor. Moreover, the occurrence of lacustrine sedimentary layer is nearly horizontal, which is a relatively stable sedimentary state, and it is impossible to form such a large-scale slump structure due to its own gravity effect. And we don't find any sliding surface in the profiles. Therefore, the collapse is ruled out. According to the geological background and geological survey of the study area, this area does not have the conditions triggered by volcanism, glaciation and freeze-thaw. Because of the active neotectonic movement and frequent earthquakes in the study area, and seismic actions are the main trigger factors for liquefaction. So it is considered that seismic action may be the main trigger factor for the strong liquefaction deformation in the study area. According to the previous studies, the relationship between the soft sedimentary deformation structure, the liquefaction thickness and the seismic strength is discussed, the magnitude of this ancient seismic event probably reached 7 or higher.
There are sand layers in the section of “soft sedimentary deformation structure” caused by earthquake, the lower stratum is sand layer and the upper stratum is clay layer. The thickness and deformation strength of the lower sand layer determine the strength of the deformation structure of the overlying clay layer. The upper and lower surface of the sand layer are undulating, and there are clay lumps in the sand layer. The deformation structure of clay layer is complex and there is no obvious deformation rule.

THE STATIC COULOMB STRESS INFLUENCE OF THE MAINLING M6.9 EARTHQUAKE IN TIBET ON NOVEMBER 18, 2017 TO THE SUBSEQUENT EARTHQUAKES

LI Zhen-yue, WAN Yong-ge, JIN Zhi-tong, YANG Fan, HU Xiao-hui, LI Ze-xiao

2020, 42(5):  1091-1108.  DOI: 10.3969/j.issn.0253-4967.2020.05.005

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Based on the rupture model of Mainling M6.9 earthquake in Tibet on November 18, 2017, the spatial distribution of static Coulomb failure stress change at different depths are calculated respectively according to two different receiving fault selection schemes. The one scheme is that we set the parameters of receiving fault at different position to be consistent with the main shock; The other scheme is on the assumption that fault's orientation is randomly distributed under the ground, and we select the receiving fault which is most prone to slide under the influence of coseismic stress field produced by main shock. Therefore, the geometrical orientation of receiving fault will vary with space. According to the above two results of static Coulomb failure stress change, we discussed the static Coulomb stress influence produced by the main shock to short-term aftershocks and the Medog M6.3 earthquake in Tibet on April 24, 2019, respectively. The result shows that: 1)When the parameters of receiving fault are same with the main shock, the proportion of aftershocks in the positive zone of static Coulomb failure stress change is small at each depth. The focal mechanisms of aftershocks in the positive zone of static coulomb fracture stress are deemed similar to the main shock. We thought that they are motivated by the continuous rupture of the main shock. 2)Most of the aftershocks are in the negative zone of static Coulomb failure stress change at each depth. We inferred that this phenomenon which may be on account of the focal mechanisms of these aftershocks is quite different with the main shock. From the result of receiving fault to choose the most prone to slide under the coseismic stress field produced by main shock, we can clearly see that all the aftershocks are within the zone of static Coulomb failure stress change greater than the trigger threshold of 0.01MPa at different depths. It indicates that all the aftershocks are likely to be triggered. It was speculated that the aftershocks in the negative zone of static Coulomb failure stress change occurred in the crushed zone caused by violent rupture of the main shock. There are countless cracks in the crushed zone, and the orientation of these cracks is abundant. Perhaps, because most aftershocks occurred on these various cracks, their focal mechanisms are quite different from the main shock. The value of elastic constants will be reduced significantly in the crushed zone. All the results in this paper also indicate that considering the elastic constants difference between in and out of the source region is beneficial to accurately estimate the static Coulomb stress influence between earthquakes in the source region. 3)Different institutes and authors used different data and methods to get several different focal mechanisms of the Medog earthquake. According to these results, we calculated a central focal mechanism solution, which has a minimum difference with these focal mechanisms. On the basis of this central focal mechanism solution, the static Coulomb stress influence of the Mainling earthquake to the Medog earthquake is calculated quantitatively. Result indicates that the magnitude of static Coulomb failure stress change generated by the Mainling earthquake is quite small on both two nodal planes of the central focal mechanism solution of the Medog earthquake, this means that the Medog earthquake is independent of the Mainling earthquake.

COMPREHENSIVE INTERPRETATION OF THE UPPER CRUSTAL VELOCITY STRUCTURE AND CRYSTALLINE BASEMENT OF THE CENTRAL YANGTZE FAULT ZONE FROM AIR-GUN SOURCE DATA

DENG Xiao-guo, TIAN Xiao-feng, YANG Zhuo-xin, WANG Fu-yun, LIU Bao-feng, GAO Zhan-yong, ZHENG Cheng-long

2020, 42(5):  1109-1128.  DOI: 10.3969/j.issn.0253-4967.2020.05.006

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The Yangtze fault zone is a typical tectonic regime transition zone of the eastern China. Tectonically, it is characterized by alternated rifts and uplifts and “several crystalline basements with one sediment cover”. Abundant metal metallogenic deposits are developed. Improvement of the velocity model and basement structure will benefit our understanding and knowledge about the regional tectonics. Large volume airgun sources have been broadly applied to seismic surveys due to significant advantages. For instance, they are environmentally friendly, use lower frequencies, and are repeatable. Several seismic and geological research institutions, such as China Earthquake Administration, carried out a three-dimensional comprehensive sounding using the large volume airgun as the seismic source which was fired at the channel of the Yangtze River in 2015. The source-receiver geometry of this seismic experiment covered the whole Anhui Province which locates at the Middle-Lower Yangtze River. The densest observational area is in the Middle-Lower Yangtze River Metallogenic Belt which is a narrow area along the Yangtze River and consists of the Luzong, Tongling, Ningwu, and Anqing-Guichi ore deposits. The Tanlu fault zone, a giant strike-slip fault of more than 2 000km long, passes through the northwestern margin of this area. Geophysical studies have demonstrated copious geological evidences for the Yangtze fault zone, which is approximately 450km long and crosses central China, extending to the eastern coastal area. The present fault and fold systems are the consequences of the repeated tectonic events since the Mesozoic. We collected and analyzed the seismic data of 20 fixed airgun shot points, then utilized tomography, time term method and head wave traveltime inversion based on ray tracing techniques to model the upper crustal velocity and crystalline basement structure of the Anqing-Maanshan segment beneath the Yangtze fault zone. The profile along the Yangtze River consists of 100 PDS-2 seismometers with a spacing of 2km. We applied the linear and phase weighted stack methods to improve the signal-to-noise ratio of the weak seismic phases from the airgun source. According to the comparison between the linear and phase weighted stack results, the phase weighted stack method significantly improves the quality of the stacked data. We applied the band-pass filter to the stacked data to improve the onset of the first arrival, then picked up the seismic phases and assessed the errors of the picked traveltime. The comprehensive results reveal that the upper crust velocity structure and crystalline basement images show a tectonic feature of alternating rifts and uplifts. The upper crust of the Huaining Basin is the thickest area along the Yangtze River. The basement of the Huaining Basin is around 4.5km and there are Mesozoic lacustrine sedimentary layers whose thickness is about 2km. The crystalline basement depth of the Luzong Basin is 4.1km and the consolidated basin shows clear depression basin shape. This feature of the Luzong Basin reveals that it experienced extensional depression. There is a high-velocity zone beneath the crystalline basement of the Luzong Basin, and the velocity is higher than other areas along the Yangtze River. This high velocity zone shows an arc shape, which agrees with the Paleozoic reflection images by the seismic reflection survey. The profile crosses the Yangtze River in Tongling area and there are obvious velocity differences between the two sides of the Yangtze River. The velocity differences show that the Yangtze faults cut the crystalline basement in Tongling. The upper crust velocity structure of the Tongling area shows clear uplift features and its crystalline basement depth is about 2.2km, which agrees with the arc-reflection structures of the upper crust from the seismic reflection data. This uplift image reveals that the upper crust of the Tongling area has experienced extrusion deformations. The consistency of the seismic reflection imaging results with the near surface geology demonstrates that the large volume air-gun source is applicable to land-based seismic survey.

RAYLEIGH WAVE PHASE VELOCITY AND AZIMUTHAL ANISOTROPY OF THE MIDDLE-SOUTHERN SEGMENT OF THE TAN-LU FAULT ZONE AND ADJACENT REGIONS FROM AMBIENT NOISE TOMOGRAPHY

GU Qin-ping, KANG Qing-qing, ZHANG Peng, MENG Ke, WU Shan-shan, LI Zheng-kai, WANG Jun-fei, HUANG Qun, JIANG Xin, LI Da-hu

2020, 42(5):  1129-1152.  DOI: 10.3969/j.issn.0253-4967.2020.05.007

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The middle-southern segment of the Tan-Lu fault zone and its adjacent area is located in the joint zone of the North China craton and Yangtze craton. It is a natural test ground for studying the problems of intracontinental collision, continental convergence and growth, geodynamics and lithospheric deformation. Although early research involved the central-south section of the Tan-Lu fault zone and its neighboring areas, it is difficult to carry out a detailed discussion on the S-wave velocity and azimuthal anisotropy in the middle and south section of the Tan-Lu fault zone and its adjacent areas, due to different research purposes and objects, the limitation in selecting research scope or the lack of resolution.
To obtain more detailed crust-mantle velocity structure and azimuthal anisotropy distribution characteristics in the study area, this paper uses waveform data recorded by 261 fixed wideband seismic stations in the middle-southern segment of the Tan-Lu fault zone and its adjacent zone for two consecutive years. The phase velocity dispersion curve of Rayleigh surface wave with 5~50s period was extracted by time-frequency analysis. Then, the study area was divided into 0.25°×0.25°grids, and the two-dimensional Rayleigh phase velocity and azimuthal anisotropy distribution image in the area was retrieved using the Tarantola method.
The phase velocity and azimuthal anisotropy distribution images of 6 representative periods were analyzed. These images reveal the lateral heterogeneity of the crust-mantle velocity structure and spatial differences in azimuthal anisotropy in the middle-southern segment of the Tan-Lu Fault and its adjacent areas. The results show that the distribution characteristics of phase velocity have a good correspondence with geological tectonic units. In the shallow part of the earth's crust, the basins covered by thick unconsolidated sedimentary layers and the bedrock exposed orogenic belts show low and high velocity anomalies, respectively. With the increase of the period(15~20s), the influence of the shallow sedimentary layer is weakened, and the high-speed anomaly appears in some plain areas such as the Hehuai Basin and Subei Basin. The distribution of phase velocity in the lower crust and upper mantle(25~30s)is affected by the thickness of the crust, which is inversely related to the burial depth of Moho surface. For example, the Dabie orogenic belt with a thickness of 40km changes from a short period high-speed to a low-speed distribution.
Due to the differences in the tectonic environment of each geological structural unit in the study area, the azimuthal anisotropy of Rayleigh waves has obvious spatial differences. In general, the strength of anisotropy increases with increasing period(depth), and the direction of fast wave is more regular and followable. Based on the consistent distribution of low velocity and azimuthal anisotropy from the shallow crust to the lithospheric mantle in the Subei Basin, we believe that there may be a strong crust-mantle coupling phenomenon. The results obtained by different seismic anisotropy observation methods are different manifestations of anisotropy. However, due to the one-sided and low-resolution problems of single observation method, it is necessary to carry out joint inversion or comprehensive multiple observation methods.

THE STUDY OF CRUSTAL VELOCITY STRUCTURE AND SEISMICITY IN YANGJIANG AREA OF GUANGDONG PROVINCE

WANG Xiao-na, DENG Zhi-hui, YE Xiu-wei, WANG Li-wei

2020, 42(5):  1153-1171.  DOI: 10.3969/j.issn.0253-4967.2020.05.008

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This paper collects 43 225 absolute first arrival P wave arrival times and 422 956 high quality relative P arrival times of 6 390 events occurring in Yangjiang and its adjacent area from Jan. 1990 to Aug, 2019. These seismic data is recorded by 49 stations from Guangdong seismic network, Guangxi seismic network and Hainan seismic network. Based on the seismic data above, we simultaneously determine the crustal 3-D P wave velocity structure and the hypocenter parameters of 6 255 events in Yangjiang and its adjacent area by applying double-difference seismic tomography. The result shows that shallow P wave velocity in Yangjiang area is higher due to the thinner sedimentary layer and widely exposed Yanshanian granite, Indosinian granite and Cambrian metamorphic rocks. There are obvious correspondences between the distribution of shallow velocity and fault structure as well as geological structure. The velocity transfer zone along Mashui-Pubai correlates with the NE strike of Yangchun-Zhilong Fault, and the low velocity anomaly on the west corresponds to the Yanshanian granite system, while the high velocity anomaly on the east corresponds to the Cambrian epimetamorphic rock system. The Yangjiang M6.4 earthquake locates at the high velocity seismogenic body among the low velocity anomalies due to Yanshanian granite system. Besides, there is a low velocity anomaly existing below the high velocity seismogenic body as mentioned above, we speculate the low velocity anomaly is a ductile shear zone due to partial melting of lower and middle crust caused by mantle wedge melting and basaltic underplating. Moreover, a wide range of low velocity anomaly exists in 20km depth, which verifies the low velocity layer in the middle crust at Yangjiang area of South China continent. The velocity image from land to ocean in 30km depth shows low velocity in NW side and high velocity in SE side, which verifies the characteristic of crust thinning in South China coastal continent. The NEE seismic belt from Yangbianhai to Pinggang is speculated to locate in a buried fault in the southwest segment of Pinggang Fault. The buried thrust fault is a N78°E strike fault, dipping to NW with a dip angle of 85°. In addition, the buried fault locates in the abnormal junction of high velocity on the NW side and low velocity on the SE side, which reflects the tectonic activity characteristic of NW plate uplifting and SE plate declining from Miocene period. The characteristic of activity in the buried fault shows thrust movement with a small strike-slip component, which is consistent with the focal mechanism of the M4.9 earthquake occurring in 2004. Finally, there is a large difference of formation occurrence between the southwest buried fault of Pinggang Fault and the northeast segment of Pinggang Fault. We speculate that, the formation occurrence of Pinggang Fault changes near Pinggang area in the form of “dough-twist”, which causes different velocity structure and movement characteristic.

THE EFFECTS OF LATERAL INHOMOGENEITY ON ANISOTROPIC CHANGES OF APPARENT RESISTIVITY AND THE DEPTH OF RESISTIVITY CHANGES BEFORE EARTHQUAKES

XIE Tao, LU Jun

2020, 42(5):  1172-1187.  DOI: 10.3969/j.issn.0253-4967.2020.05.009

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Crustal medium has varying degrees of electrical conductivity. Electrical resistivity is an important physical property for geomaterials. Electrical resistivity is commonly used in geophysics to investigate the deformation of the crust. Resistivity of sedimentary rock and soil is a combination of resistivity of both solid matrix and crack/pore fluid, and also depends on the degree of fluid saturation, crack ratio or porosity, and crack shape. Since the electrical resistivity is related to mechanical properties, build-up of strain ought to be accompanied by resistivity changes, which might warn of an impending earthquake. Many studies on electrical resistivity changes of rock and soil have been carried out in an attempt to find a physical basis for earthquake prediction. Rock resistivity changes under deformation up to failure in uniaxial and triaxial experiments have been measured in laboratory. For water-bearing rocks, resistivity decreases or increases at low stress and decreases greatly at high stress, just before failure. A series of midpoint Schlumberger arrays were placed on rock samples. Apparent resistivity of the perpendicular array has the maximum decrease magnitude, while the parallel array has the minimum magnitude. The decrease magnitude of the oblique array falls within the upper and lower bounds of the other two arrays. In-situ experiments show the similar anisotropic changes in apparent resistivity. Apparent resistivity has been continuously monitored at fixed stations in China for more than 50 years, using Schlumberger arrays. Apparent resistivity has been monitored the same pattern of anisotropic changes before great earthquakes as the results from experiments. Surface DC apparent resistivity observation using a Schlumberger array has a depth detection range approximately equal to the electrode spacing of AB. There may also be lateral electrical inhomogeneity under the survey area, i.e. the resistivity of the medium varies in the horizontal direction within the same depth range. Lateral inhomogeneity within the detection range will cause anisotropy in apparent resistivity, but whether it will cause anisotropic changes is still uncertain. The depth range of stratum resistivity affected by tectonic stress before earthquakes has not been fully discussed. In this paper, we use finite element method to calculate the anisotropic changes in apparent resistivity caused by resistivity changes from different depth range. Lateral homogeneity and lateral inhomogeneity models are taken into account, respectively. The results show that for Schlumberger array with spacing of AB=1 000m, anisotropic changes in apparent resistivity caused by resistivity changes of stratum below 300m is inconsistent with the observed results in experiments and seismic examples. Under this situation, apparent resistivity of the perpendicular array has the minimum decrease magnitude, while the parallel array has the maximum magnitude. On the other hand, the apparent resistivity decreases in a small range, and the difference between the two monitoring directions is not significant. The depth of stratum where resistivity changes take place needs to rise to a range of tens of meters from the surface. Then the magnitude and feature of anisotropic changes are consistent with the experiments and field observations. For stratum with lateral inhomogeneity, apparent resistivity dose not display anisotropic changes when resistivity of different stratum units under the same depth has the same magnitude of isotropic variation. Anisotropic changes in apparent resistivity take place only when stratum resistivity shows anisotropic variations. However, lateral heterogeneity has little effect on anisotropic changes of apparent resistivity.

INVERSION OF P-WAVE THREE-DIMENSIONAL VELOCITY STRUCTURE AND ANALYSIS OF SEISMOGENIC STRUCTURE OF EARTHQUAKE SWARM IN 2017 IN MIAODAO ISLANDS, SHANDONG PROVINCE

LI Xia, CHEN Shi-jun, ZHANG Zheng-shuai, DAI Zong-hui, LI Xiao-han, LU Zhong-bin

2020, 42(5):  1188-1204.  DOI: 10.3969/j.issn.0253-4967.2020.05.010

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On March 3, 2017, an earthquake swarm of M_L 4.5 occurred near Dazhushan Island in Miaodao archipelago, Shandong Province, as of December 31, 2017, 2 453 aftershocks were recorded, including 46 earthquakes of M_L3 and above and 4 earthquakes of M_L4 and above. It is the most active and frequent earthquake swarm activity in Miaodao Islands area in the regional network records. On September 2, 2017, another earthquake swarm of M_L3.0 occurred near Beichangshan Island, about 15km away from the south of Dazhushan Island. More than 300 earthquakes were recorded, including one earthquake of M_L3 or above. According to the seismic data, two earthquake swarms were also recorded near Daheishan Island and Tuoji Island in Miaodao archipelago from February to March 1976. It is believed that these two swarms may be the “precursory earthquake swarms” of Tangshan strong earthquake with M=7.8 in 1976. The differences in spatial location, energy release and focal depth between the two swarm events are very similar to those in 2017. Therefore, in this paper, the three-dimensional velocity structure of P-wave in Miaodao archipelago area and the results of seismic precise relocation are obtained by using the double-difference tomography method, and the deep structural environment factors of the preparation of the earthquake swarms and the differences in the characteristics of the earthquake swarms are analyzed in combination with the fault activity and medium characteristics.
The velocity structure provides important information related to earthquake location and focal medium, and provides important basis for understanding the background of earthquake preparation and the mechanism of earthquake occurrence. Based on the observation report data of Shandong and Liaoning seismic networks, this paper selects 4 766 seismic events recorded clearly from January 2008 to December 2017 in Miaodao archipelago and nearby areas, and excludes the data with the difference of P-wave and S-wave travel time and time distance curve larger than 5s. After the difference grouping of earthquake events, 4 555 events recorded by 65 stations are finally selected for double-difference tomography inversion, and there are 26 430 P-wave absolute arrival data, 513 299 difference arrival data, 26 356 S-wave absolute arrival data and 508 482 difference arrival data. Limited by geographical conditions, the ray density is dense in the south and sparse in the north. After repeated test and selection of inversion parameters and model recovery test, high-resolution P-wave three-dimensional velocity structure image and high-precision earthquake positioning results are obtained in Miaodao archipelago, Shandong Province, and the following conclusions are obtained:
(1)The results of seismic precise relocation show that the convergence of seismic distribution near Miaodao islands is good, the NW direction zonal distribution of earthquake swarm activity is obvious, and the focal depth is mainly concentrated in the middle and upper crust. The characteristics of swarm activity show group occurrence in a short period of time, and there are obvious differences in the form of expression: the swarm near Tuoji Island has deep focal point, high frequency, large release energy, and wide distribution of focal area; the swarm activity characteristics near Daheishan Island and Beichangshan Island are just the opposite.
(2)The horizontal velocity structure shows that the lateral heterogeneity of velocity structure exists in every depth layer, which reflects the unbalanced uplift of crystalline basement and the zone filling of igneous rock. The velocity structure of the shallow crust is in good agreement with the known geological structure; the middle and shallow layers clearly reveal the basement uplifting area and the subsidence zone on both sides of Miaodao Islands; the velocity structure of the middle and lower layers is obviously affected by the deep faults and magmatic activities, and near the Dazhushan Island-Weihai North Fault, it shows obvious low-velocity characteristics, while the northern sea area of Jiaodong Peninsula is characterized by independent high-velocty abnormal blocks.
(3)The velocity structure profile shows that there is a certain correlation between the activities of faults and earthquake swarm and the velocity structure of P wave. There is an obvious low velocity region in the middle and upper crust in the profile passing through the Dazhushan Island earthquake swarm, and the earthquake swarm is nearly vertical and layered scattered in the relatively high velocity medium between the upper and lower low velocity bodies or near the velocity conversion zone, which is consistent with the characteristics of the Dazhushan Island-Weihai North Fault. The profile across the Beichangshan Island earthquake swarm reveals that there are small high-speed bodies in the middle and upper layers of the crust, and the earthquake swarm occurs at the edge of the high-velocity body.
(4)Earthquake swarm often occurs in places with dense fracture distribution, relatively weak medium and low strength. Based on the analysis of the characteristics of earthquake swarm and the three-dimensional velocity structure of P-wave in Miaodao archipelago area, it is considered that the two significant earthquake swarm activities in 1976 and 2017 are the energy release caused by the inhomogeneity of local medium and the low stress friction of regional fault during the process of regional background stress enhancement(adjustment).

CROSS-FAULT SHORT-TERM AND IMPENDING ANOMALIES BEFORE THE MINXIAN-ZHANGXIAN STRONG EARTHQUAKE AND THE CHARACTERISTICS OF META-INSTABLE STATE

ZHANG Xi, JIA Peng, LIU Xia, XU Jing, WAN Yong-kui

2020, 42(5):  1205-1217.  DOI: 10.3969/j.issn.0253-4967.2020.05.011

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2~4 days before the Minxian-Zhangxian M_S6.6 earthquake on July 22, 2013, short-term and impending anomalies dominated by normal fault type abrupt change were observed at mobile cross-fault short-leveling sites, Sidian, Maoyugou and Majietan, within 37~70km from the epicenter, especially at the Sidian site, where the amplitude of the anomaly reached up to 5.5mm. In this paper, based on the analysis of cross-fault short-leveling observations in the western Qinling Mountains tectonic area, where the above three sites are located, and the experimental results of structural mechanics for the existence of meta-instable stress state before fault instability, the anomaly mechanism is discussed, combined with the information of GPS continuous observation and small earthquakes activities. The results show that: 1)Several days before the earthquake, obvious “changes” in the micro-dynamic variation magnitude and characteristics of fault activities were observed, which revealed that the regional stress had changed. New synchronous anomalies occurred at Sidian, Maoyugou and Majietan sites, especially at Sidian site where the anomaly was large in amplitude, and the number and magnitude of the anomalies increased obviously; Moreover, the dynamic distribution of anomalies of the observational curves showed an obvious change process of the characteristics of tectonic fault activities. In the western Qinling tectonic area, the change was mainly dominated by reverse faulting, but this reverse fault type change was not dominant several months before the earthquake, and till a few days before the earthquake, the normal fault type variation was absolutely dominant, which is contrary to the long-term background of the reverse fault favorable for stress concentration in this tectonic area; In addition, temporal variation of trend accumulation rate, which reflects the feature and magnitude of stress-strain accumulation, showed the accumulation firstly slowing down in several months and turning obviously in several days before the earthquake. 2)Obvious anomaly variation was observed at the sites near the earthquake region several days before the earthquake, the crustal stress evolved from “regional change” to “localized enhancement”. During the four observational periods before the earthquake, it was observed that the cross-fault anomalies were gradually concentrated to the local tectonic region where the earthquake occurred, also the grey relativity index which reflects the general trend of fault activities displayed a larger turning magnitude in this local tectonic region; Next, the local fault segment mostly near the epicenter not only maintained the reverse fault type change consistent with the earthquake dislocation, significant accelerating anomalies were also observed at Majietan site, which may be the relatively strong stress concentration position. The auxiliary data of continuous GPS observation and small earthquake activities were also of some supporting significance to above phenomena. The comprehensive analysis shows that the temporal and spatial evolution process of fault activities and anomalies obtained from the cross-fault short-leveling observation in the western Qinling tectonic area before the strong earthquake accorded with the characteristics of meta-stable(deviating from linear stage)-meta-instable state revealed by the structural mechanics experiment.

STUDY ON SPATIAL CORRELATION OF GROUND-MOTION: A CASE STUDY OF NAPA EARTHQUAKE

CHEN Kun, YU Yan-xiang, GAO Meng-tan, KANG Chuan-chuan

2020, 42(5):  1218-1228.  DOI: 10.3969/j.issn.0253-4967.2020.05.012

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The probability of joint exceedance of ground motion intensities at multiple sites during the same earthquake needs to be quantified in aggregated seismic hazard analysis or loss assessment of spatially distributed infrastructure systems. The spatial correlation of ground motion at multiple sites cannot be considered in conventional specific-site seismic hazard analysis method. In this paper, the spatial correlation of ground motion is preliminarily studied using 344 sets of strong earthquake records from the M_W6.0 Napa earthquake, California, US on August 24, 2014. The results of finite fault inversion for Napa earthquake(Dreger et al., 2015)is used as the surface projection of fault rupture. The ground motion attenuation relationship of Boore in NGA-West2 in the Pacific Seismic Engineering Research Center, which is referred to as BSSA14 in this paper, is selected in this research. The local site amplification effect of ground motion is obtained by using the correlation between topographic slope and $V^{30}_{S}$(average shear wave velocity of rock and soil layer from surface to 30 meters underground)to obtain the site amplification coefficient of amplitude and frequency. The ground motion parameters actually observed by the station are converted to the reference surface of bedrock by using the site amplification coefficient, and the fault projection distance between each station and the surface projection of fault rupture is calculated. Comparison of strong ground motion recordings of the Napa earthquake with BSSA14 in the Next Generation Attenuation(NGA)-West2 Ground-Motion Models, indicates that the ground motion of the high frequency components of the Napa earthquake was underestimated in the attenuation relationship of BSSA14. And the peak ground acceleration residuals are mostly negative. The residuals of acceleration response spectrum for the 3-second period are basically distributed around the 0 value. The spatial correlation functions of the geometric mean for the peak ground acceleration(PGA), the peak ground velocity(PGV)and spectral acceleration at three specific periods(0.3s, 1.0s and 3.0s)of the two horizontal components are derived using the method of semivariogram function. 2km and 1km of the distance between stations is used in this paper respectively, which can guarantee the reliability of data calculation statistics. However, we found that the decrease of the distance between stations did not significantly improve the statistical results of the spatial correlation of strong ground motion in the observed data of Napa earthquake. The corresponding continuous spatial correlation function is fitted with exponential model and compared with the past studies of ground-motion correlation, which has been widely researched in the past. The analysis results show that the ground motion parameters are spatially correlated, and these spatial correlations approximate exponential decay as the distance increases. Secondly, the spatial correlation of ground motion increases with the increase of response spectrum period. It may be because the similarity is reduced by the scattering of waves during propagation, and that this reduction is greater for high-frequency waves. The short￣wavelength waves tend to be more affected or changed by small-scale heterogeneities in the process of propagation, so the spatial correlation of high-frequency seismic waves is smaller than that of long-period seismic waves. Finally, there is a regional characteristic in the spatial correlation of seismic ground motion. The spatial correlation of peak ground acceleration is weaker in southern California than in Japan and Taiwan. Results of this research in this paper can provide theoretical basis and reference for aggregated seismic hazard analysis or loss assessment. And uncertainty will be reduced in ShakeMap considering the spatial correlation of ground motion.

EXPERIMENTAL STUDIES ON ELASTIC PROPERTIES OF MONTMORILLONITE

ZHANG Ming-yang, YANG Xiao-song

2020, 42(5):  1229-1239.  DOI: 10.3969/j.issn.0253-4967.2020.05.013

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As clay minerals, montmorillonite is usually enriched in fault rocks, mudstones and clastic rocks. It is important to measure its elastic properties accurately for seismic sounding and interpretation of sonic logging. Many researches on the elastic properties of clay minerals have been carried out by predecessors, including the theoretical modelling of the elastic modulus of clay, the experimental determination of the elastic modulus of aggregates of clay minerals by cold pressing and the calculation of clay elastic modulus based on the corresponding elastic modulus of shale and two-phase bodies composed of mixture of clay-epoxy, and so forth. The comparison results of these modulus obtained by different methods show a huge difference. Previous investigations display a massive gap on the bulk modulus of montmorillonite ranging from 6GPa to 63GPa. Here, we present the new results of bulk modulus of montmorillonite based on uniaxial compression experiments on the two-phase bodies composed of mixture of montmorillonite and LR-White glue(hereinafter referred to as LRWG)and of quartz-LRWG performed at the pressure of 0.1~600MPa and at room temperature. On the base of the changes of sample length and density with pressure, the uniaxial compressibility of LRWG, quartz and montmorillonite are obtained. The internal porosity in these starting materials(two-phase bodies)were checked by scanning electron microscope to ensure the porosity within the range of allowable error. Observations show that the porosity in the starting materials is extremely low. Although very small amount of cracks inevitably exist at the boundaries between the montmorillonite aggregate and LRWG, attributed to drying shrinkage during sample preparation(heating and solidification), the pores disappear at the pressure increasing to 200MPa approximately. Hence, the samples could be regarded as pure two-phase bodies when the pressure is higher than 200MPa. After the uniaxial compression, the mineral particles in the specimen exhibit uniform distribution and no anisotropies are observed. In the initial stage of pressurization, the non-linear shortening of the montmorillonite-LRWG sample is explained to rapid reduction of the pores which is expected to be between the starting materials and sample chamber and between montmorillonite aggregate and LRWG. During the pressure range from 250MPa to 600MPa, the length of specimen linearly decreases with increasing pressure. The calculated bulk modulus of quartz is 37.49GPa which is consistent with previous report of 37GPa. The bulk modulus of montmorillonite of 10.44GPa, 6.36GPa, 8.34GPa and 7.86GPa with the average of 8.25GPa are obtained. The variation range of the bulk modulus of montmorillonite is about ±2GPa, which is significantly better than the previous experimental results. The experiments provide both reliable data and an effective technique for investigating the elastic properties of clay reservoir.

Application of new technique

PRACTICAL APPLICATION OF CLOSE-RANGE PHOTOGRAM-METRY TO EXTRACTING GEOLOGICAL INFORMATION FROM TRENCH: TAKING THE MIAODIAN 4^# LANDSLIDE AT THE SOUTHERN TABLELAND IN JINGYANG COUNTY AS AN EXAMPLE

WEI Yong, XU Qiang, DONG Xiu-jun, GUO Peng, LI Song-lin, LI Hua-jin

2020, 42(5):  1240-1254.  DOI: 10.3969/j.issn.0253-4967.2020.05.014

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Trench exposure is probably the most intuitive technological means of geology surveying work, and it is the key material to the right cognition of the delicate geological information. At the same time, trench is usually difficult to reserve, it needs to record the geological information of trench timely and accurately. The conventional methods of recording the delicate geological information of trenches are geology sketch and image mosaics technology, which mainly acquires the two-dimensional geological information. New data acquisition methods need to be explored for acquiring three-dimensional geological information. With the development of three-dimensional image technology, it is a new idea and method that terrestrial laser scanner and digital photogrammetry are applied to recording geological information of trenches.
This paper discusses the above problem by Miaodian 4^# landslide at the southern tableland in Jingyang County. The method of close-range photogrammetry to record the geological information is described as follows: 1)The photo-control-points on cleaned trench walls are established; 2)The photos from the surface of trench are collected by a Digital SLR Camera in the field. At the same time, measuring instrument can be used to measure the coordinates of the photo-control points; 3)The 3D model of the trench is reconstructed using the photogrammetric software(based on the processed photographs after the screening process and the coordinates of the photo-control points). In order to promote the application of this method, the paper also introduces the key technical problems of determining and setting up photo-control-points and scheme of photo acquisition. The 3D spatial data were acquired by methods described previously, which mainly include point cloud data, mesh, texture, orthomosaic, etc. The spatial resolution of the orthomosaic was 1.48mm/pix, and this high accuracy is enough to record millimeter-sized geological information. In order to verify the reliability of the results, the accuracy of close-range photogrammetry results is also verified and analyzed. The results show that the accuracy of close-range photogrammetry reached the centimeter level, and the level is close to the measurement equipment used in measuring the coordinates of photo-control-points. Therefore, the spatial resolution and accuracy of outcome data fully meet the requirements of recording the geological information.
Based on those 3D spatial data, the 2D and 3D geological applications of the data results were discussed in detail. First, the 2D geological application was discussed by taking the interpretation of trench profiles as an example. The technological processes mainly include creating orthomosaic, vectorization and interpretation. In the example, the interpretations of trench profiles were successfully completed using this method(Typically, the process of interpretation demands a combination of outdoor data recording and indoor experimental data). Specifically, the interpretations of small scale folds and thrust faults in the landslide deposit are obtained. Then, the 3D geological application was discussed by taking the data extraction of occurrence as an example. Extracting the feature points of 3D spatial data and computational method were described in detail. The occurrences of 3D model were contrasted to measurements of geological compass, and the two results are in good agreement. Based on the data of occurrence, they can be properly analyzed and further research can be developed from them. In this example, one can speculate about the forces and the motion state of the landslide after the data extraction of occurrence was synthesized and analyzed. And the results were normalized to the actual situation of the landslide.
Therefore, the 3D spatial data can be acquired quickly and accurately by extracting geological information from trench using close-range photogrammetry. Those outcome data include point cloud data, mesh, texture, orthomosaic, etc. The applications of those data could solve many complex engineering geological issues. Compared with the traditional methods, efficiency and security of field work are improved using photogrammetry technology, and it retains richer texture details than terrestrial laser scanning. Data results have traditional two-dimensional application functions. Moreover this data can achieve corresponding three-dimensional applications, which is deserved to be applied.

MONITORING SIGNAL OF AIRGUN SOURCE WITH DISTRIBUTED ACOUSTIC SENSING

LI Xiao-bin, SONG Zheng-hong, YANG Jun, ZENG Xiang-fang, WANG Bao-shan

2020, 42(5):  1255-1265.  DOI: 10.3969/j.issn.0253-4967.2020.05.015

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The large-volume airgun system was introduced to excite highly repetitive seismic signal for medium change monitoring. Using dense seismic array to record the seismic wavefield will be helpful to high spatial resolution time-lapse tomography. However, most dense arrays employ the nodal short-period geophone with built-in battery that is not suitable for permanent monitoring. The novel distributed acoustic sensing(DAS)technology uses fiber-optic itself as sensor that providing small station spacing. The incident seismic wavefiled induces tiny strain of the fiber-optic that leads to phase change of the Rayleigh backscattered optical signal. Therefore, measuring the phase difference between two signals scattered at two nearby scatterers can be used to recover seismic signal. Since the scatter is randomly distributed in the fiber-optic, it is possible to record seismic wavefield with spacing down to sub-meters. Each optical signal is processed in the interrogator. Therefore, the DAS array is easily maintained as a permanent dense array for seismic monitoring. We conducted a pilot experiment to test feasibility of using DAS array to record airgun signal in Binchuan, Yunnan Province.
The Binchuan Fixed Airgun Signal Transmission Station built in 2011 is the first inland large-volume airgun in China. The airgun system consists of four Bolt LL 1500 airguns and fires at 10m depth in a reservoir. The seismic energy released by one airgun shot is close to the one of M_L0.7 earthquake. During this pilot experiment, the airgun was continuously shot after midnight with an interval of 15 minute. The DAS array is a micro-structured fiber-optic buried in an “L-shape” trench, which is about 9.8km away from the airgun. To enhance SNR of the optical signal used for recover seismic signal, a series of ultra-weak fiber Bragg gratings were built in the fiber with 2m spacing. The 180m fiber-optic is buried at about 20cm depth and the trench is backfilled with sand. The channel spacing is 4m and the interrogator continuously records at 1 000Hz.
The signal is barely visible on the record of single shot due to strong ambient seismic noise and optical noise. Since the seismic signal excited by the airgun is highly repetitive, we used the time-frequency phase weighted stacking method to stack records of multiple shots. The signals clearly emerge on the stack traces and the arrival time agrees well with the records of a co-located seismometer. Compared with the seismometer's record, the DAS records concentrate in a higher frequency band(5~8Hz). Since the DAS and seismometer record the seismic wavefield in dynamic strain and particle velocity, respectively, the frequency-wavenumber scaling algorithm was used to convert DAS's strain record to particle velocity record that shows clear phase difference from seismometer's record. The difference between records of DAS and seismometer was analyzed in time-frequency domain. The largest difference occurs between 3 and 6Hz in the airgun signal wave train, which may due to lower sensitivity in lower frequency band of DAS.
The bootstrapping resample method was used to evaluate the stacking converge rate of two datasets. Comparing to the reference trace that is stacked with 24 shots, the cross-correlation coefficient reaches 0.9 with only four shots for the seismometer dataset. At the meantime, the cross-correlation coefficient is only 0.8 with 20 shots for the DAS dataset. To improve the stacking efficiency, we also tried the array stacking method. The records of 26 channels on the X lag of the array were stacked. The one-shot stacking suppressed the traffic noise from a nearby street and the airgun signal clearly emerges on the one-shot stacking trace. The airgun signals on the stacking traces of multiple shots and multiple channels are comparable, which suggest the multiple channels stacking can be used to improve time resolution for time-lapse tomography/monitoring.
In summary, the airgun signal is successfully recorded by a DAS array with an engineered fiber-optic cable. Comparing with the seismometer, DAS dataset is strongly affected by the traffic noise and lower sensitive to lower frequency band. The dense spacing also provides opportunity to stack multiple channels’ records that improves SNR of airgun signal. Since the lack of reliable vertical component records, the phase identification cannot be done via particle motion analysis. The aperture of our DAS array is too small to estimate the apparent velocity to identify seismic phase too. In the future, it is worth to use telecom fiber-optic cables as sensor for time-lapse tomography, which have been widely deployed in urban area and significantly reduced deployment cost.
The clear variation of waveforms across one lag arising from un-uniform coupling was also observed. To comprehensively evaluate the monitor capability, it is important to deploy large aperture DAS array for seismic signal attenuation analysis. Our result suggests that the stronger lower frequency system noise of the DAS integrator reduces the sensitivity to seismic signal. More attention should be paid to approaches such as environmental vibration isolation and optical noise reduction. Another issue is accurate response function. Calibration with co-located seismometers and numerical modeling are helpful to provide accurate sensitivity and response function, which is important in seismology studies.