由初至P震相重新定位2017年九寨沟地震序列的主震与M_L≥3.0余震并分析发震构造

综合利用区域台站和流动台站(近台)的记录,基于初至P震相重新测定了2017年九寨沟序列M S7.0主震和M L≥3.0余震的震源位置,并利用较高精度的定位结果分析余震分布与地震构造的关系,解释发震断裂带的结构.获得的新认识有:(1)九寨沟主震震源深度为16 km,位于余震带中段的南缘;余震主要分布深度为4~17 km.(2)沿余震带的走向,余震分布与主震同震位错大小的分布明显相关.余震带中段8~16 km深度存在的余震稀疏区与同震位错的高值区相吻合,应是发震断裂带主凹凸体的部位,也是主震时应变释放较充分的部位;余震带南东段10~18 km深度的余震密集区对应了同震位错的亏损区之一,三次M L≥5.0余震都发生于此;余震带西北段在5~10 km之下既缺少余震,又属同震位错的亏损区,可能与那里多条断裂的交汇或合并造成的构造复杂性有关;余震带中-北西段3~5 km深度的也缺少余震,也对应了浅部的同震位错亏损区.(3)证实了九寨沟地震的发震构造为虎牙断裂带北段,同时新揭示出发震断裂带表现为由主断裂和分支断裂构成的、向上分叉的花状结构,尺度约为4.5 km宽(最大)、35 km长,主断裂朝SW陡倾.这些反映主震破裂可能不只受控于单一的断裂,而有可能是沿主断裂发生主破裂,而沿分支断裂发生次要破裂.另外,本文对发震断裂带结构的分段解释,是遵循构造地质学原理去综合震源排列、震源机制解、地表断层已知位置、相邻剖面断层解释结果等信息的分析结果,而不仅仅依据余震的密集分布进行推断.

Late Quaternary Slip-rates and Slip Partitioning on the Southeastern Xianshuihe Fault System, Eastern Tibetan Plateau

Quantitative analysis of the kinematics of the active faults distributed around the QinghaiTibetan Plateau is critical to understand current tectonic processes of the plateau. Chronological analysis, based on the comparison among regional climate and geomorphology, digital photogrammetry, offset landforms, and the tectonics were adopted in this study on the Xianshuihe fault in the eastern Tibetan plateau. Two or more offset-age data were obtained for each segment of the Xianshuihe and theYunongxi faults. The offset landforms, including river terrace, alluvial fan and glacial moraine, provide constraints for the late Quaternary slip rate of the Xianshuihe fault. The left-lateral strike slip rate of the Xianshuihe fault decreases from 17 mm/a on the northwest segment to 9.3 mm/a on the southeast segment. Regarding the Xianshuihe fault zone and its adjacent blocks as a regional tectonic system, vector analysis was used to quantitatively analyze the longitudinal kinematical transformation and transversal slip partitioning on the fault zone in terms of the kinematical parameters of the main faults within the zone. The results show that there is a distributed vertical uplift at a rate of 6.1 mm/yr caused by shortening across the Gongga Mountains region. Based on these results, we established a model of the slip partitioning for the southeastern segment of the Xianshuihe fault zone.

Active Fault Exploration and Seismic Hazard Assessment in Fuzhou City

It has been proven by a number of earthquake case studies that an active fault-induced earthquake beneath a city can be devastating. It is an urgent issue for seismic hazard reduction to explore the distribution of active faults beneath the urban area and identify the seismic source and the risks underneath. As a pilot project of active fault exploration in China, the project, entitled “Active fault exploration and seismic hazard assessment in Fuzhou City", started in early 2001 and passed the check before acceptance of China Earthquake Administration in August 2004. The project was aimed to solve a series of scientific issues such as fault location, dating, movement nature, deep settings, seismic risk and hazard, preparedness of earthquake prevention and disaster reduction, and etc. by means of exploration and assessment of active faults by stages, i.e., the preliminary survey and identification of active faults in target area, the exploration of deep seismotectonic settings, the risk evaluation of active seismogenic faults, the construction of geographic information system of active faults, and so on. A lot of exploration methods were employed in the project such as the detection of absorbed mercury, free mercury and radon in soil, the geological radar, multi-channel DC electrical method, tsansient electromagnetic method, shallow seismic refraction and reflection, effect contrast of explored sources, and various sounding experiments, to establish the buried Quaternary standard section of the Fuzhou basin. By summing up, the above explorations and experiments have achieved the following results and conclusions: (1)The results of the synthetic pilot project of active fault exploration in Fuzhou City demonstrate that, on the basis of sufficient collection, sorting out and analysis of geological, geophysical and borehole data, the best method for active fault exploration (location) and seismic risk assessment (dating and characterizing) in urban area is the combination of various approaches, that is, the possible location of active fault determined by using geochemical exploration as a guide “scout", the shallow seismic sounding as the main tool, the electromagnetic method as a supplement, establishing the standard Quaternary profile or stratigraphic sequence from drilling and various geophysical parameters from borehole logs as methods to correct and verify the data above. And in addition, the method also includes the field surveys on fault exposures, trenching, paleoearthquake investigation, dating and comparison of lithology, strata sequence, absolute or relative ages of the cores on the two sides of buried faults. (2)The Fuzhou basin locates under the regional seismotectonic settings which have the potential of moderate earthquake. Comparatively, the region is less affected by the “Taiwan dynamic Antenna"; (3)The activity of the major faults in Fuzhou basin is weak in general. All the six identified target faults are not Holocene faults, among which the Bayi Reservoir-Shanggan fault and the Minhou-Nanyu fault are dormant at least since the mid Epipleistocene time, and the rest are dormant since the Epipleistocene time; (4)In terms of deep-seated structures beneath the basin, there is no evidence indicating the possible occurrence of the underneath strong destructive earthquakes. The adjacent Changle-Zhao’an fault zone is the potential seismic source which may possibly affect Fuzhou City; (5)There exists potential of moderate-strong earthquake on the major faults of the region, but the probability is low; (6)The seismic hazards are weak in the region and the surface earthquake fractures are not likely to occur; (7)The first geographic information system of active faults is developed with functions of information query and display, data management, analysis and processing, etc.

A Review on Researches of Active Tectonics—History,Progress and Suggestions

This paper reviews the history and progress of research on active tectonics in China and overseas.By giving a brief introduction on the history of active tectonic research in China and other countries,the paper sums up the process and development of quantitative investigation of active tectonics since the 1980s.The focus is on the main efforts and progress made in China on certain aspects of research,such as basic surveys and applied investigation of active tectonics,the study of theories related to regional active tectonics and their kinematics and geodynamics,surveys on coupling relations between deep and shallow structures,active fault surveys and prospecting and seismic hazard assessment in urban areas,as well as the efforts made using Quaternary geochronology.Furthermore,the paper looks back on Chinese quantitative investigation of active tectonics in China and sums up cognitions derived from studies on the determination of several basic and measurable parameters of active tectonics.These parameters include the length of fault and fault segmentation,coseismic slip and cumulative slip,fault slip rate,the sequence of paleoearthquake events and the time elapsed since the most recent event.At the same time,efforts and progress made in China on assessing the long-term seismic potential for active faults and evaluating the risk from potential active fault movement have been reviewed by summarizing research on developing theories,models,methods and the application of time-dependent seismic potential to probabilistic assessment,magnitude estimation for potential earthquakes on active faults,and the forecast of potential risk caused by active fault movement.Finally,in consideration of the realities and problems in the research of active tectonics in China,the authors put forward several suggestions for issues worthy of more attention for further investigation in the future.

Study on Potential Strong Earthquake Risks Around the Mabian Area,Southern Sichuan

Based on seismic data from the regional network for the last 34 years,we analyzed the present fault behavior of major fault zones around the Mabian area,southern Sichuan,and identified the risky fault-segments for potential strong and large earthquakes in the future.The method of analysis is a combination of spatial distribution of b-values with activity background of historical strong earthquakes and current seismicity.Our results mainly show:(1) The spatial distribution of b-values indicates significant heterogeneity in the studied area,which reflects the spatial difference of cumulative stress levels along various fault zones and segments.(2) Three anomalously low b-value areas with different dimensions were identified along the Mabian-Yanjin fault zone.These anomalies can be asperities under relatively high cumulated stress levels.Two asperities are located in the north of Mabian county,in Lidian town in western Muchuan county,and near Yanjin at the south end of the fault zone.These two areas represent potential large earthquake seismogenic sites around the Mabian area in the near future.Besides them,the third relatively smaller asperity is identified at southern Suijiang,as another potential strongearthquake source.(3) An asperity along the southwestern segment of the Longquanshan fault zone indicates the site of potential moderate-to-strong earthquakes.(4) The asperity along the segment between Huangmu town in Hanyuan county and Longchi town in Emeishan city on Jinkouhe-Meigu fault has potential for a moderate-strong earthquake.

Basic Characteristics of Active Structures in Western Sichuan and Its Vicinity and Strong Earthquake Recurrence Model

Western Sichuan and its vicinity are located in the juncture of three big active blocks, namely, the Chuandian (Sichuan and Yunnan), the Bayan Har and the South China blocks, on the eastern margin of the Qinghai-Xizang(Tibet) Plateau. Many groups of active faults that are capable of generating earthquakes are developed there. Because there exist lateral secondary active faults, the Chuandian block can be further divided into the central Yunnan and northwestern Sichuan sub_blocks; while the Longmenshan sub_block can be divided on the east end of the Bayan Har block. Joint exploration of deep crustal structure shows that there exist low-velocity and high-conductivity layers in the crust of the Chuandian and Bayan Har blocks, which are one of the important factors that make the upper crust prone to earthquake. The results of geological study and modern GPS observation show that blocks of different orders all have SE-or SSE-trending sliding, clockwise rotation and upwelling movement; but there are some differences in amplitude. This paper has also given the geological or GPS slip rates of main active fault zones and discussed the main scientific problems still existing now.

Discovery of the Longriba Fault Zone in Eastern Bayan Har Block, China and its tectonic implication

Re-measured GPS data have recently revealed that a broad NE trending dextral shear zone exists in the eastern Bayan Har block about 200 km northwest of the Longmenshan thrust on the eastern margin of the Qinghai-Tibet Plateau. The strain rate along this shear zone may reach up to 4-6 mm/a. Our interpretation of satellite images and field observations indicate that this dextral shear zone corresponds to a newly generated NE trending Longriba fault zone that has been ignored before. The northeast segment of the Longriba fault zone consists of two subparallel N54°±5°E trending branch faults about 30 km apart, and late Quaternary offset landforms are well developed along the strands of these two branch faults. The northern branch fault, the Longriqu fault, has relatively large reverse component, while the southern branch fault, the Maoergai fault, is a pure right-lateral strike slip fault. According to vector synthesizing principle, the average right-lateral strike slip rate along the Longriba fault zone in the late Quaternary is calculated to be 5.4±2.0 mm/a, the vertical slip rate to be 0.7 mm/a, and the rate of crustal shortening to be 0.55 mm/a. The discovery of the Longriba fault zone may provide a new insight into the tectonics and dynamics of the eastern margin of the Qinghai-Tibet Plateau. Taken the Longriba fault zone as a boundary, the Bayan Har block is divided into two sub-blocks: the Ahba sub-block in the west and the Longmenshan sub-block in the east. The shortening and uplifting of the Longmenshan sub-block as a whole reflects that both the Longmenshan thrust and Longriba fault zone are subordinated to a back propagated nappe tectonic system that was formed during the southeastward motion of the Bayan Har block owing to intense resistance of the South China block. This nappe tectonic system has become a boundary tectonic type of an active block supporting crustal deformation along the eastern margin of the Qinghai-Tibet Plateau from late Cenozoic till now. The Longriba fault zone is just an active fault zone newly-generated in late Quaternary along this tectonic system.

Lushan M_S7.0 earthquake:A blind reserve-fault event

In the epicenter of the Lushan MS7.0 earthquake there are several imbricate active reverse faults lying from northwest to southeast,namely the Gengda-Longdong,Yanjing-Wulong,Shuangshi-Dachuan and Dayi faults.Emergency field investigations have indicated that no apparent earthquake surface rupture zones were located along these active faults or their adjacent areas.Only brittle compressive ruptures in the cement-covered pavements can be seen in Shuangshi,Taiping,Longxing and Longmen Townships,and these ruptures show that a local crustal shortening occurred in the region during the earthquake.Combining spatial distribution of the relocated aftershocks and focal mechanism solutions,it is inferred that the Lushan earthquake is classified as a typical blind reverse-fault earthquake,and it is advised that the relevant departments should pay great attention to other historically un-ruptured segments along the Longmenshan thrust belt and throughout its adjacent areas.

Rupture segmentation and slip partitioning of the mid-eastern part of the Kunlun Fault,north Tibetan Plateau

The Kunlun Fault,an active fault on the border between the Bayan Har and Kunlun-Qaidam blocks,is one of the major left lateral strike-slip faults in the Tibetan Plateau.Previous research has not reached a consensus on agreeable slip rates along much of its length and the slip rate gradient along the eastern part,both of which play critical roles in a range of models for the eastward extrusion and thickened crust of the Tibetan Plateau.New slip rates have been determined at sites along the eastern part of the Kunlun Fault by dating deposits and measuring atop displaced fluvial terrace risers.Field investigations and interpretation of satellite images reveal geometrical features of the fault and the late Quaternary offset,new earthquake ruptures and surface-rupturing segmentation,from which long-term slip rates and earthquake recurrence intervals on the fault are estimated.The tectonic geomorphology method has determined that the long-term horizontal slip rates on the Tuosuohu,Maqin and Maqu segments from west to east are 11.2±1,9.3±2,and 4.9±1.3 mm/a while their vertical slip rates are 1.2±0.2,0.7±0.1,and 0.3 mm/a in the late Quaternary.Results indicate that the slip rates regularly decrease along the eastern ～300 km of the fault from >10 to <5 mm/a.This is consistent with the decrease in the gradient such that at the slip rate break point is at the triple point intersection with the transverse fault,which in turn is transformed to the Awancang Fault.The vector decomposition for this tectonic transformation shows that the western and eastern branches of the Awancang Fault fit the slip-partitioning mode.The slip rate of the southwestern wall is 4.6 mm/a relative to the northeastern wall and the slip direction is 112.1°.The mid-eastern part of the Kunlun Fault can be divided into three independent segments by the A'nyêmaqên double restraining bend and the Xigongzhou intersection zone,which compose the surface rupture segmentation indicators for themselves as well as the ending point of the 1937 M7.5 Tuosuohu earthquake.The average recurrence interval of the characteristic earthquakes are estimated to be 500-1000 a,respectively.The latest earthquake ruptures occurred in AD 1937 on the western Tuosuohu segment,as compared to ～514-534 a BP on the Maqin segment,and ～1055 to 1524 a BP on the Maqu segment.This may indicate a unidirectional migration for surface rupturing earthquakes along the mid-eastern Kunlun Fault related to stress triggered between these segments.Meanwhile,the long-term slip rate is obtained through the single event offset and the recurrence interval,which turn out to be the same results as those determined by the offset tectonic geomorphology method,i.e.,the decreasing gradient corresponds to the geometrical bending and the fault's intersection with the transverse fault.Therefore,the falling slip rate gradient of the mid-eastern Kunlun Fault is mainly caused by eastward extension of the fault and its intersection with the transverse fault.

Tectonic dynamics and correlation of major earthquake sequences of the Xiaojiang and Qujiang-Shiping fault systems, Yunnan, China

The N-S trending Xiaojiang fault zone and the NW-SE trending Qujiang-Shiping fault zone are adjacent active fault systems and seismogenic zones associated with strong and major earthquakes in Yunnan, China. To understand the interaction of the two fault systems, and its probable influence on earthquake occurrences, this paper conducts a synthetic study based on data of active tectonics, historical earthquakes, relocated small earthquakes, GPS station velocities and focal mechanism resolutions. The study makes several conclusions. (1) The active southward motion of the western side of the Xiaojiang fault zone (i.e. the side of the Sichuan-Yunnan block) has a persistent and intensive effect on the Qujiang-Shiping fault zone. The later fault zone has absorbed and transformed the southward motion of the western side of the former fault zone through dextral strike-slip/shearing as well as transverse shortening/thrusting. (2) Along the Xiaojiang fault zone, the present sinistral strike-slip/shearing rate decreases from 10 and 8 mm/a on the northern, central and central-southern segments to 4 mm/a on the southern segment. The decreased rate has been adjusted in the area along and surrounding the Qujiang-Shiping fault zone through reverse-dextral faulting and distributed shearing and shortening. (3) The tectonic-dynamic relation between the Xiaojiang fault zone and the Qujiang-Shiping fault zone is also manifested by a close correlation of earthquake occurrences on the two fault zones. From 1500 to 1850 a sequence of strong and major earthquakes occurred along the Xiaojiang fault zone and its northern neighbor, the Zemuhe fault zone, which was characterized by gradually accelerating strain release, gradually shortening intervals between M≥7 events, and major releases occurring in the mid to later stages of the sequence. As a response to this sequence, after an 88-year delay, another sequence of 383 years (from 1588 to 1970) of strong and major earthquakes occurred on the Qujiang-Shiping fault zone, and had the same features in accelerating strain release and its temporal course. (4) Since there has been no M≥7 event for 177 years on the Xiaojiang fault zone, the potential risk of a strong or major earthquake occurring on this fault zone in the future should be noticed and studied further.