The Yecheng, Xinjiang, China, earthquake of February 14, 1980 —A destructive intermediate earthquake

In this study, we have checked the location and focal depth of the Yecheng earthquake (m _b = 6.0, maximum intensity VII) of February 14, 1980. The result shows that this is an intermediate event with a focal depth of 90 km. The microepicenter is located at 36. 4°N, 76, 9°E, while the macroepicenter is at 37.3°N, 76.9°E, 90 km to the north of the microepicenter. This is the first destructive intermediate event in China which led to a damage as severe as of intensity VII. The focal mechanism of the event is determined to be of thrust type. Combined with the analysis of seismological and geological data in surrounding area, the possible relation between the event and plate movement has been discussed. The result in this paper indicates that in some particular place, the destructive effect of intermediate event should be considered in seismic hazard assessment. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 137–143, 1992.     

LIMITATION OF CURRENT TECTONIC DEFORMATION MODES IN THE WESTERN MARGIN OF ORDOS BASED ON SEISMIC ACTIVITY CHARACTERISTICS

Due to the interaction between the Tibetan plateau, the Alxa block and the Ordos block, the western margin of Ordos(33.5°~39°N, 104°~108°E)has complex tectonic features and deformation patterns with strong tectonic activities and active faults. Active faults with different strikes and characteristics have been developed, including the Haiyuan Fault, the Xiangshan-Tianjingshan Fault, the Liupanshan Fault, the Yunwushan Fault, the Yantongshan Fault, the eastern Luoshan Fault, the Sanguankou-Niushoushan Fault, the Yellow River Fault, the west Qinling Fault, and the Xiaoguanshan Fault. In this study, 7 845 earthquakes(M≥1.0)from January 1st, 1990 to June 30th, 2018 were relocated using the double-difference location algorithm, and finally, we got valid locations for 4 417 earthquakes. Meanwhile, we determined focal mechanism solutions for 54 earthquakes(M≥3.5)from February 28th, 2009 to September 2nd, 2017 by the Cut and Paste(CAP)method and collected 15 focal mechanism solutions from previous studies. The spatial distribution law of the earthquake, the main active fault geometry and the regional tectonic stress field characteristics are studied comprehensively. We found that the earthquakes are more spatially concentrated after the relocation, and the epicenters of larger earthquakes(M≥3.5) are located at the edge of main active faults. The average hypocenter depth is about 8km and the seismogenic layer ranges from 0 to 20km. The spatial distributions and geometry structures of the faults and the regional deformation feature are clearly mapped with the relocated earthquakes and vertical profiles. The complex focal mechanism solutions indicate that the arc-shaped tectonic belt consisting of Haiyuan Fault, Xiangshan-Tianjingshan Fault and Yantongshan Fault is dominated by compression and torsion; the Yellow River Fault is mainly by stretching; the west Qinling Fault is characterized by shear and compression. The structural properties of the fault structure are dominated by strike-slip and thrust, with a larger strike-slip component. The near-north-south Yellow River Fault is characterized by high angle NW dipping and normal fault motion. Based on small earthquake relocation and focal mechanism solution results, and in combination with published active structures and geophysical data in the study area, it is confirmed that the western margin of Ordos is affected by the three blocks of the Tibetan plateau, the Alax and the Ordos, presenting different tectonic deformation modes, and there are also obvious differences in motion among the secondary blocks between the active faults. The area south of the Xiangshan-Tianjingshan Fault has moved southeastward since the early Quaternary; the Yinchuan Basin and the block in the eastern margin of the Yellow River Fault move toward the SE direction.     

STUDY ON SOURCE PARAMETERS OF THE 8 AUGUST 2017 M7.0 JIUZHAIGOU EARTHQUAKE AND ITS AFTERSHOCKS,NORTHERN SICHUAN

On August 8, 2017, a strong earthquake of M7.0 occurred in Jiuzhaigou County, Aba Prefecture, northern Sichuan. The earthquake occurred on a branch fault at the southern end of the eastern section of the East Kunlun fault zone. In the northwest of the aftershock area is the Maqu-Maqin seismic gap, which is in a locking state under high stress. Destructive earthquakes are frequent along the southeast direction of the aftershocks area. In Songpan-Pingwu area, only 50~80km away from the Jiuzhaigou earthquake, two M7.2 earthquakes and one M6.7 earthquake occurred from August 16 to 23, 1976. Therefore, the Jiuzhaigou earthquake was an earthquake that occurred at the transition part between the historical earthquake fracture gap and the neotectonic active area. Compared with other M7.0 earthquakes, there are few moderate-strong aftershocks following this Jiuzhaigou earthquake, and the maximum magnitude of aftershocks is much smaller than the main shock. There is no surface rupture zone discovered corresponding to the M7.0 earthquake. In order to understand the feature of source structure and the tectonic environment of the source region, we calculate the parameters of the initial earthquake catalogue by Loc3D based on the digital waveform data recorded by Sichuan seismic network and seismic phase data collected by the China Earthquake Networks Center. Smaller events in the sequence are relocated using double-difference algorithm; source mechanism solutions and centroid depths of 29 earthquakes with M_L≥3.4 are obtained by CAP method. Moreover, the source spectrum of 186 earthquakes with 2.0≤M_L≤5.5 is restored and the spatial distribution of source stress drop along faults is obtained. According to the relocations and focal mechanism results, the Jiuzhaigou M7.0 earthquake is a high-angle left-lateral strike-slip event. The earthquake sequence mainly extends along the NW-SE direction, with the dominant focal depth of 4~18km. There are few shallow earthquakes and few earthquakes with depth greater than 20km. The relocation results show that the distribution of aftershocks is bounded by the M7.0 main shock, which shows obvious segmental characteristics in space, and the aftershock area is divided into NW segment and SE segment. The NW segment is about 16km long and 12km wide, with scattered and less earthquakes, the dominant focal depth is 4~12km, the source stress drop is large, and the type of focal mechanism is complicated. The SE segment is about 20km long and 8km wide, with concentrated earthquakes, the dominant depth is 4~12km, most moderate-strong earthquakes occurred in the depth between 11~14km. Aftershock activity extends eastward from the start point of the M7.0 main earthquake. The middle-late-stage aftershocks are released intensively on this segment, most of them are strike-slip earthquakes. The stress drop of the aftershock sequence gradually decreases with time. Principal stress axis distribution also shows segmentation characteristics. On the NW segment, the dominant azimuth of P axis is about 91.39°, the average elevation angle is about 20.80°, the dominant azimuth of T axis is NE-SW, and the average elevation angle is about 58.44°. On the SE segment, the dominant azimuth of P axis is about 103.66°, the average elevation angle is about 19.03°, the dominant azimuth of T axis is NNE-SSW, and the average elevation angle is about 15.44°. According to the fault profile inferred from the focal mechanism solution, the main controlling structure in the source area is in NW-SE direction, which may be a concealed fault or the north extension of Huya Fault. The northwest end of the fault is limited to the horsetail structure at the east end of the East Kunlun Fault, and the SE extension requires clear seismic geological evidence. The dip angle of the NW segment of the seismogenic fault is about 65°, which may be a reverse fault striking NNW and dipping NE. According to the basic characteristics of inverse fault ruptures, the rupture often extends short along the strike, the rupture length is often disproportionate to the magnitude of the earthquake, and it is not easy to form a rupture zone on the surface. The dip angle of the SE segment of the seismogenic fault is about 82°, which may be a strike-slip fault that strikes NW and dips SW. The fault plane solution shows significant change on the north and south sides of the main earthquake, and turns gradually from compressional thrust to strike-slip movement, with a certain degree of rotation.     

逆康普顿散射对γ暴余辉的影响

γ暴余辉的发现是γ暴研究史上的一个重大突破,火球模型几乎可以较好地解释γ暴余辉的观测特性。但在标准的火球模型中,通常只考虑电子的同步加速辐射,没有考虑电子逆康普顿散射的贡献。这里我们详细计算了逆康普顿散射对γ暴余辉的影响,发现在一定的条件下,逆康普顿散射的影响是很重要的,它可以显著地改变辐射能谱,进而改变γ暴余辉的光变特性。     

Stochastic Fermi acceleration in turbulent fields with non-vanishing wave helicities

In the previous paper [Astropart. Phys. 10 (1999) 121] we showed that the opposite helicity circularly polarized Alfvén waves of finite amplitudes provide conditions to forward–backward asymmetry of particle scattering. In this letter we present an analytic solution of kinematic equation proving the enhancement of stochastic acceleration efficiency due to regular (asymmetry) term. The process is controlled by the ratio of the regular and the ordinary diffusion term.     

全球大地震时空分布与动力学机制的初步研究

文中根据最新的全球大震资料划分了 10 0a来全球特大地震的时空演化过程 ,表现为环太平洋带与近纬向活动带两种强震分布图像 ,以 2 0a左右的时间段交替出现。逆冲和引张型地震释放了全球强震能量的主要部分。逆冲型地震主要发生在环太平洋地震带上 ,在喜马拉雅碰撞边界和巽他弧也有发生。引张型地震主要出现在洋脊和裂谷地区。而走滑型地震主要分布在东亚大陆内部、北美板块西边界和加勒比板块的周边、西南太平洋带、以及地中海 -喜马拉雅带部分区域 ,呈近纬向分布。地幔对流的垂直运动与水平运动分量的比率 ,决定了逆冲、引张与走滑型强震的能量释放份额。地幔运动在地表体现在逆冲、引张与走滑型强震空间分布的区域差异性。这种差异在时间上的演化形成了全球强震活动环太平带与近纬向分布图像的交替出现。     

Changes in strain trajectories in three different types of plate tectonic boundary deduced from earthquake focal mechanisms

Principal strain orientations (minimum horizontal compression—ex and maximum horizontal compression—ey) were established at three different types of plate tectonic boundary: two transform faults, an oceanic ridge located on the Southeast Indian Ridge and a trench located close to the South Sandwich Archipelago. To establish the strain patterns in each zone, 104 earthquake focal mechanisms (centroid-moment tensor solutions for earthquakes with mb≥4; Harvard seismology data, CMT) were examined by fault population analysis. Despite the existence of only one tectonic process that controlled deformation in these zones (divergence, convergence or passive displacement), and only one main strain tensor, several coeval strain ellipsoids were found. These differed from the main strain tensor in the location of the principal strains. In general, permutations were observed between the principal strains, i.e., interchanges between the location of the principal strain axes maintaining the strain ellipsoids in the same 3D orientation. Only in some cases were changes in the ellipsoid orientation associated with major structures.     

Stress fields in the Iberian-Maghrebi region

This study concerns the present stressfield between the Eurasian and Africanplates in the Iberian-Maghrebi region(Portugal, Spain, Morocco, Algeria andTunisia). In addition to an up-to-datecatalogue of earthquakes in this area, acatalogue of the focal mechanisms composedof 486 solutions of fault planes,standardized in terms of notation andinformation type, was used. These data wereused applying the right-dihedron method ofAngelier and Mechler (1977), to obtaindifferent zones with homogeneous stress.The results obtained for shallowearthquakes (h < 30 km) coincide, in themajority of cases, with the general stressfields proposed by numerous authors forthis region, according to which there isNW-SE compression. However, the stressorientation appears to vary in certainareas, perhaps perturbed by the opening ofthe Atlantic Ocean, the approach of Iberiaand Africa, or the extension of the AlboranSea.For the intermediate earthquakes (30 < h< 150 km) no general pattern was found,and the P and T axes seem to be randomlyoriented for the depth intervalsconsidered. For the very deep earthquakes(h > 600 km), however, the P axis lies ina NNW-SSE direction, dipping towards theSSE, while the T axis is subhorizontal in aNE-SW direction.The determinations from the focalmechanisms highlight the existence of aregional stress field with a subhorizontalcompression axis trending NW-SE. Superimposed are others that specificallyaffect particular sectors; these arerelated to the opening of the AtlanticOcean, the extension of the BeticCordillera and the Alboran Sea, and eventhe present compression between the Iberianand European plates.     

2003年8月16日内蒙古5.9级地震概述

2003年8月16日内蒙古赤峰发生5．9级地震，造成生命和财产较严重损失。这是赤峰市历史上700多年来最大的一次地震。文章对该震的基本参数、震源机制、破坏伤亡和救援等作了概述。     

关中陕南地区一些小地震震源参数和环境剪应力的研究

对2001年9月～2002年6月间发生在关中陕南地区42次小地震,利用数字地震记录资料,进行了波谱分析和震源参数的测定,并分析了环境剪应力分布特征。结果显示关中陕南地区环境剪应力处于较低水平,应力值3.3×105～32.2×105Pa之间,西部12×105～32×105Pa略高于东部8×105～17×105Pa,中部最低。西部有两个应力值较高区,一个是宝鸡、眉县、凤翔附近区域较高值区,应力值20×105～32×105Pa,另一个是留坝、凤县附近区域较高值区,应力值20×105～37×105Pa之间。东部的较高值区是高陵、耀县、三原附近地区,应力值在16×105～18×105Pa之间,该区域曾发生1998年1月5日泾阳MS4.8级地震。