Two-step interface and velocity inversion—— Study o e of the Tangshan seismic region

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3-D isotropic and anisotropic tomography of P-wave travel times from the Anhui airgun experiment in the Yangtze River

The Middle-Lower Yangtze River is a typical transition region between the nearly NW-oriented Tethys and NE-trending Pacific tectonic regimes.Structures of different periods and directions overlap strongly during these processes.The NE-trending Yangtze River compound structural belt and NW-trending Tongling-Hangzhou struc-tural belt both control the magmatic activities and distribu-tions of the metallogenic belts in the area.Here,we obtain 3-D high-resolution isotropic and azimuthally anisotropic velocity structures at depths of 1-10 km using the first arrivals from airgun sources.The velocity maps correspond well with the tectonic structures,with high-velocity anomalies distributed in ore-concentrated districts and low-velocity anomalies distributed along the Yangtze River.The fast directions are generally consistent with the fault strike,indicating that the azimuthal anisotropy is mainly dominated by the fault and fracture trends in the upper crust.The complicated fast directions near the Luzong and Tongling ore deposits reveal complex deformations in the upper crust,which are mainly caused by the intersection of the Yangtze River compound and Tongling-Hangzhou structural belts.The magma intrusion beneath the two ore deposits(Luzong and Tongling)are connected at depths of 5-10 km.     

Relationship between the earthquake sequences of Tangshan and Xingtai and the three dimensional velocity structure

In this paper, based on the results of tomographic image of Tangshan and Xingtai areas, the relations between the characteristics of the two strong earthquake sequences and their three-dimensional velocity structures are studied. The research results indicate that:① Mosaic distribution of low-velocity bodies and high-velocity bodies, especially the existence of high-velocity bodies with large size in crust are the common basis of development of the two earthquake sequences. ②Scale, depth, and heterogeneity of high-velocity and low-velocity bodies are the important factors to effect the characteristic of earthquake sequences.③ The depth of the high-velocity body in Tangshan area is less than that in Xingtai area, which is the principal reason why the dominant focal depth and the biggest focal depth of Tangshan earthquake sequence are less than Xingtai's.④The depth of the high-velocity bodies in Ninghe area is more than that in Tangshan-Luanxian area, which lead to the biggest magnitude and epicentral intensity are lower. These results could be helpful for predicting the main shock of strong swarm-type earthquakes and later strong aftershocks.     

四川芦山MS7.0地震震源区及其周边区域P波三维速度结构研究

利用四川数字地震台网和流动地震台站在芦山M_S7.0地震震后(2013年4月20日—6月23日)记录到的2026次区域地震事件的28188条P波到时资料,采用地震层析成像方法反演得到了芦山地震震源区及其周边区域中上地壳P波三维速度结构. 结果表明,浅部地壳的P波速度异常分布特征与地表地质构造、 地形和岩性密切相关,即成都断陷盆地表现出与第四纪沉积有关的低速异常区;犍为、 乐山一带的川中微升区和川青块体龙门山以西的邻近地带均表现为与构造抬升有关的高速异常;宝兴、 康定附近分布的基性火山岩及火山碎屑岩均呈局部高速异常分布. 芦山地震震源位于高低速异常分界线附近且偏向高速体一侧,其下方存在明显的低速异常分布,可能与流体的存在有关. 流体的作用导致中上地壳内部发震层的弱化,使孕震断层易于破裂,可能对芦山地震起到了触发作用. 芦山地震与汶川地震两次地震的余震密集区相距50 km,这50 km地震空区震源体的深度范围附近目前正处于高速异常区内,加之龙门山断裂带西南段又具有比较典型的断错地貌发育,使得该段地震空区(大邑—邛崃活动断裂破裂空段)现在所处的深浅部构造环境变得复杂,其潜在的地震危险性仍值得进一步关注.      

Three-dimensional P-wave velocity structure of the crust beneath Hainan Island and its adjacent regions, China

Using over 3 500 first P arrival times recorded by nine digital seismic stations from Hainan Digital Seismic Net-work during 1999～2005,a 3-D P-wave velocity model of the crust under Hainan Island and adjacent regions has been determined. The results show that the pattern of velocity anomalies in the shallower upper crust is somewhat associated with the surface geological tectonics in the region. A relative low-velocity anomaly appears north of the Wangwu-Wenjiao fault zone and a relative high-velocity anomaly appears south of the Wangwu-Wenjiao fault zone,corresponding to the depressed areas in north Hainan Island,where many volcanoes are frequently active and geothermal values are relatively higher,and the uplifted and stable regions in central and south of the Hainan Is-land. In the middle and lower crust velocities are relatively lower in east Hainan than those in west Hainan,possi-bly suggesting the existence of the upwelling of hot materials from the mantle in east Hainan. The pattern of veloc-ity anomalies also indicates that NW faults,i.e.,the Puqian-Qinglan fault,may be shallower,while the E-W Wangwu-Wenjiao fault may be deeper,which perhaps extends down to Moho depth or deeper.     

滇西地区地壳速度结构及其构造意义

使用2011年8月至2012年8月在南北地震带南段布设的密集流动地震台站记录的波形数据,采用绝对定位方法对滇西地区的1593个地震进行了初步定位,应用近震走时层析成像方法获得了滇西地区三维地壳P波速度结构。研究结果表明,攀枝花附近的高速异常从地表可一直延伸至中地壳,并在中下地壳深度与滇西北地区的高速异常形成大范围的高速块体。这一高速异常体对青藏高原物质向南逃逸起到一定的阻挡作用,可能是导致川滇活动块体北部次级块体快速抬升的重要因素。位于印度板块与欧亚板块俯冲边界的腾冲、保山地块在下地壳表现为明显的大范围低速异常,推测该低速异常与印度板块东向俯冲引起的地幔热物质上涌有关。金沙江-红河断裂带是川滇块体重要的南部边界,红河断裂带中段的弥渡至红河一带具有高速的地壳结构以及较弱的地震活动性,可能表明该区现阶段处于闭锁状态,是未来需要重点监测的地段之一。     

1927年古浪8级大震及其周边地区三维地壳P波速度结构特征

本文联合利用甘肃及周边测震台网记录的古浪及周边地区4592次地震的P波绝对到时资料和相对到时资料,采用双差地震层析成像方法反演了古浪震源区高分辨率的三维P波速度精细结构.结果显示,浅部P波速度分布与地表地质之间具有很好的对应关系.皇城—双塔断裂带在6 km以上深度表现为高速异常带,而在6~15 km逐渐转换为明显的低速特征,之后再次转换为高速体.震区下部在10~20 km深度有一个尺度约200 km²的低速异常体,地震发生时破裂首先在该低速体发生,与主震空间位置非常吻合.主震区的岩石结构主要由奥陶纪变质砂岩、石英岩和加里东期的花岗岩等坚硬岩体组成.这种坚硬岩体对应的P波速度结构为高速体,有利于能量积累.武威盆地在20 km以上深度表现为明显的低速异常,在25 km深度之下,整体显示为高速体,表现出稳定块体的特征.表明武威盆地中下地壳和上地幔顶部已插入到冷龙岭隆起带之下.震区小震重新定位发现,皇城—双塔断裂带东、西两段表现出不同的力学运动性质,西段以逆冲运动为主,地震主要发生在断裂的下盘.而东段地震却主要发生在上盘,断层活动以局部拉张为主.我们还首次发现在皇城—双塔断裂带的中段与主破裂呈垂直方向存在有在主震发生时新产生的一条共轭断层,基于小震的断层面参数反演显示该断裂是一高倾角运动性质以右旋为主兼具正断的断裂.

     

根据接收函数反演得到的首都圈地壳上地幔三维S波速度结构

利用2002~2003年中国地震局地质研究所台阵实验室以唐山大震区为中心布设的40个流动宽频带地震台站和首都圈数字台网的33个宽频带台站的远震数据,采用接收函数非线性反演方法得到其中72个宽频带台站下方60 km深度范围内的S波速度结构.根据得到的各台站下方地壳上地幔的S波速度结构,并综合刘启元等（1997）用接收函数非线性反演方法得到的延怀盆地15个宽频带流动台站下方的地壳上地幔S波速度结构模型,给出了39°N～41°N,114°E~119.5°E区域内沿不同走向、不同深度S波速度分布.由于综合了利用首都圈数字地震台网的宽频带台站以及流动地震台阵的观测数据,本文给出了较前人同类研究空间分辨率更好的结果.结果表明: (1)研究区的速度结构,特别是怀来以东的速度结构十分复杂.在10~20 km深度范围内,研究区地壳具有高速和低速异常块体的交错结构.研究区中上地壳速度结构主要被与张渤地震带大体重合的NW向高速条带和穿越唐山大震区的NE向高速条带所控制,而其中下地壳的速度结构主要为延怀—三河—唐山地区上地幔隆起所控制.(2)研究区内存在若干壳内S波低速体,它们主要分布在唐山,三河及延怀盆地等地区.在这些地区,壳内低速体伴随着壳幔界面的隆起和上地幔顶部速度结构的横向变化.(3)地表断层分布与地壳速度结构分区有较好的相关性,表明断层对不同块体有明显的控制作用.其中,宝坻断裂,香河断裂和唐山断裂均为超壳断裂.(4)首都圈内大地震的分布与壳内低速体及上地幔顶部的速度结构有密切关系.对于唐山大地震的成因,仅考虑板块作用引起的水平应力场是不够的,有必要充分重视由于上地幔变形引起的地壳垂直变形和上地幔物质侵入造成的热效应.     

郯庐断裂带中南段及邻区Pn波速度结构与各向异性

郯庐断裂带是一条纵贯我国大陆东部NNE走向的巨型深断裂,其中南段及邻区(115°E-122°E,29°N-38°N)跨越了华北断块区、扬子断块区和华南褶皱系三大一级构造单元,由于其重要性和复杂性,长期以来一直是地学家们研究的热点.本文从国际地震中心(ISC)、中国地震台网及区域地震台网的地震观测报告中精心挑选出6381个Pn震相数据,用Pn波时间项层析成像法反演得到了郯庐断裂带中南段及邻区上地幔顶部Pn波速度结构和各向异性.结果显示,研究区上地幔顶部具有显著的横向非均匀性,相对于7.95 km·s-1的平均速度而言,Pn波速度值在7.68~8.24 km·s-1范围内变化.Pn波速度分布在郯庐断裂带中段和南段具有分段性:沿中段及周边存在一NE向低速异常带,低速可能是由于岩石圈的减薄和软流圈的高温物质沿郯庐带上涌导致;沿南段表现为一NNE向弱高波速异常带,作为高低速的边界带清晰地勾勒出了华北与扬子这两个不同块体,该边界在江苏域向华北地块NW方向凹进.Pn波速度各向异性的强弱与速度分布存在一定的相关性.总体上,如鲁西隆起及以南等低速区、茅山断裂附近的高低速过渡带,其速度各向异性较为强烈;而在具有高速异常的苏北盆地、合肥盆地等稳定区域下方其各向异性较弱.本文通过Pn波震相基本未能探测到郯庐断裂带中段的方位各向异性,推测是上地幔顶部被"冻结"下来的各向异性痕迹被软流圈热物质上涌这一强烈构造运动削弱所导致.南段具有与断裂伸展方向近乎平行的快波速方向.Pn波速度横向变化和强震活动存在一定关联.强震主要发生在Pn波低速异常区或高低速过渡带上.郯城8.5级地震震中位于中段和南段高低速过渡带,该区域也是速度横向变化最大的地方,最容易集中应力和产生应力差.     

玉树地震震源区速度结构与余震分布的关系

利用玉树震区21个应急流动地震台站和青海省地震台网固定地震台站的观测数据,采用双差层析成像方法,对2010年4月14日至6月15期间发生的地震进行了重定位,并反演得到了玉树地震震源区的三维速度结构.重定位结果揭示余震主要沿NW向成窄带状分布在断层的两侧,表明脆性破裂应力释放主要集中于一个狭窄的区域内.在西北端,余震偏离玉树—甘孜断裂分布,在SW向也有分布,推测可能与南西向次级断裂有关.双差层析成像得到的速度结构在浅部与地表地质构造相一致,中上地壳的速度结构显示巴颜喀拉地块为高速异常,羌塘地块为低速异常.玉树地震余震分布与特定的速度结构存在相关性：主震发生在高低速过渡带偏高速体的一侧,余震主要分布在高速体外围,高速体内部几乎没有余震分布.一般说来,中上地壳的高速体通常具有较高的强度,可以积累较强的孕震能量.主震发生后,高速体内积累的弹性能量向周边释放,可能是导致高速体周边余震发生的主要原因.     

Three-dimensional crustal velocity structure model of the middle-eastern north China Craton (HBCrust1.0)

Lithosphere thinning and destruction in the middle-eastern North China Craton (NCC), a region susceptible to strong earthquakes, is one of the research hotspots in solid earth science. All 42 seismic wide-angle reflection/refraction profiles have been completed in the middle-eastern NCC. We collect all the 2-D profiling results and perform gridding of the velocity and interface depth data, building a 3-D crustal velocity structure model for the middle-eastern NCC, named HBCrust1.0, by using the Kriging interpolation method. Our result shows that the first-arrival times calculated by HBCust1.0 fit well with the observations. The result demonstrates that the upper crust is the main seismogenic layer, and the brittle-ductile transition occurs at depths near interface C (the interface between upper and lower crust). The depth of interface Moho varies beneath the source area of the Tangshan earthquake, and a low-velocity structure is found to extend from the source area to the lower crust. Based on these observations, it can be inferred that stress accumulation responsible for the Tangshan earthquake may have been closely related to the migration and deformation of the mantle materials. Comparisons of the average velocities of the whole crust, the upper and the lower crust show that the average velocity of the lower crust under the central part of the North China Basin (NCB) in the east of the craton is obviously higher than the regional average. This high-velocity probably results from long-term underplating of the mantle magma.     

前郭MS5.8和松原MS5.7地震震源区地壳速度结构与孕震环境

为了解2013年11月23日吉林前郭MS5.8和2018年5月28日吉林松原MS5.7地震震源区的地壳速度结构及其孕震环境,本研究收集了东北地区CEA固定台站、NECESSArray流动台站和五大连池WAVESArrary流动台站的连续波形数据,使用背景噪声成像方法,获得了前郭地震和松原地震震源区地壳S波速度结构.结果显示,这两个地震主要发生在相对高波速异常区,而震源区下方存在明显低波速异常,且该低速层呈北北东走向,并有向北延展的趋势.结果表明,震源区相对高波速异常区易于积累能量孕育地震,而震源区下方的低波速异常可能代表了流体作用.这种流体作用会降低断层面有效正应力从而触发地震.这种流体作用可能和太平洋板块深俯冲至我国东北地区（包括前郭地震和松原地震震源区）下方的地幔转换内形成"大地幔楔"结构与动力学密切相关.在"大地幔楔"结构中,由于地幔转换带中滞留板块脱水作用和地幔角流作用,容易形成热湿物质上涌,进而导致松辽盆地中北部岩石圈物质拆沉、地幔流体入侵至中下地壳、然后作用于断裂带,从而导致了两个震源区中强度地震的发生.     

唐山及邻区双差层析成像研究

收集唐山及邻区2010—2018年76个固定地震台站的震相数据,采用双差层析成像的方法,联合反演了唐山及其邻区中上地壳的震源位置和P波三维速度结构.结果表明:在浅层研究区P波速度的变化特征与地表地质构造和活动断裂的展布具有较好的一致性.唐山断裂两侧速度差异明显,其附近P波速度结构横向不均性显著;中地壳存在明显的低速异常...     

川滇交界东段昭通、莲峰断裂带的深部结构特征与2014年鲁甸M_S6.5地震

2014年鲁甸M_S6.5地震位于川滇菱形块体向东突出的过渡变形区大凉山次级块体南东缘的昭通、莲峰断裂带内部,属于青藏高原东南缘南北地震带的中南段,近十多年来,该断裂带及其周边中强地震的发生频次明显增多,昭通、莲峰断裂带是否具备孕育和发生强震的深部构造背景成为一个亟待研究的问题.为了研究昭通、莲峰断裂带的深部结构特征及孕震背景,探求2014年鲁甸M_S6.5地震的成因的深部动力机制,本文充分收集了四川、云南等区域数字地震台网和"中国地震科学台阵探测-南北地震带南段"("喜马拉雅"项目Ⅰ期)流动地震台阵的观测数据,应用区域震和远震联合反演的方法得到川滇地区三维速度结构图像,在此基础上重点剖析和研究了昭通、莲峰断裂带P波速度结构;再对昭通、莲峰断裂带及周边区域的重力、航磁数据进行三维视密度和视磁化强度反演,得到了壳内不同深度层视密度的横向变化特征和反映壳内磁性物质的分布范围以及结晶基底的视磁化强度异常分布情况,综合分析研究昭通、莲峰断裂带的深部结构特征及孕震动力环境.研究结果表明:川滇交界东部昭通、莲峰断裂带及其周边地区上地壳物质存在显著的横向介质差异,中下地壳深度范围大凉山次级块体西南缘存在低速异常分布,并呈现出近SN向的展布特征,2014年鲁甸M_S6.5地震位于该高低速异常的分界线附近略偏向高速体一侧.P波速度结构还揭示了鲁甸M_S6.5主震震源体下方中下地壳存在大范围低速异常分布,P波速度异常扰动与重磁异常的展布特征、梯度变化在深度和分区特征上均具有较好的联系和可比性,结合昭通、莲峰断裂带中下地壳范围内存在大范围的低密度弱磁性异常分布,综合表明了该区中下地壳物质相对较为软弱,这种特有的深部物性结构特征有利于应力在脆性的上地壳内积累和集中.研究结果还揭示了共轭断裂的深部构造形态,高低航磁异常边界与NW向的苞谷脑—小河断裂的深部展布形态相一致,苞谷脑—小河断裂处于航磁异常突变带附近,昭通断裂北段(昭通—鲁甸段)位于上地壳强磁性、高波速异常区内且具有深大断裂的深部地球物理场响应特征,因此该断裂段(昭通—鲁甸段)具备发生7级及以上强震的深部构造背景.当大凉山次级块体内部的中下地壳低速管流层自NW向SE方向运动到昭通、莲峰断裂带附近时,受到华南块体的强烈阻挡,应力在昭通、莲峰断裂附近基底性质存在差异处集中,脆性上地壳中低强度区域在横向挤压的构造应力场作用下易于破裂从而引发强震,这也正是昭通、莲峰断裂带内部鲁甸M_S6.5地震孕育和发生的深部构造环境.     

Quasi-waveform seismic tomography of crustal structures in the capital circle region of China

Seismic tomography is one of the main tools to explore the interior of the earth.In this study,the quasi-waveform seismic tomographic method is used for the first time to reveal the crustal structures in the capital region of China.3-D highresolution V_P,V_S and the Poisson’s ratio models are generated by inverting 29839 direct P-and 29972 direct S-wave traveltimes selected from 3231 local earthquakes.The results reveal strong crustal heterogeneities.The velocity anomalies at shallow depths are well consistent with surface geologic structures.The relatively low-velocity anomaly layer in the middle crust may be the result of multiple phases of tectonic activity.Earthquakes generally occurred on the boundaries of high-and low-velocity and Poisson’s ratio anomalies.There are obvious low-velocity anomalies below the hypocenters of the Tangshan earthquake and the historical Sanhe-Pinggu earthquake,implying the existence of fluids.The similar velocity structures around the hypocenters of the two earthquakes indicate that the occurrences of the two earthquakes may be related to the same mechanism.The highresolution velocity models provide important observational constraints on the small-scale heterogeneities and dynamic mechanism of the crust in the capital region of China.     

Imaging 3-D crustal P-wave velocity structure of western Yunnan with bulletin data

Western Yunnan is a region with intensive tectonic activity and serious earthquake risk. It is of significant importance to study three dimensional crustal structure of this region to understand the tectonic setting and disaster mechanism. Densification and digitalization of seismic networks in this region provides an opportunity to study the velocity structure with bulletin data. In this study, we collect P-wave data of 10 403 regional earthquakes recorded by 79 seismic stations from January 2008 to December 2010. In addition to first arrivals data (Pg with epicentral distance less than 200 km and Pn), the Pg (or P) data with epicentral distance more than 200 km are also considered as later direct arrivals in the tomographic inversion. We also compare the quantity and the quality of the seismic data before 2010 and after 2010. The test results show that adding the follow-up Pg phase can effectively improve the inversion ability of crustal imaging, and quantity and the data quality are significantly improved since 2010. The tomographic results show that: (1) The Honghe fault zone, which is the major fault systems in this region, may cut through the entire crust, and the velocity contrasts between two sides at lower crust beneath the Honghe fault are estimated at higher than 10%, while the velocity difference below Nujiang fault zone extends only in the upper crust; (2) Most of the earthquakes in the region occurred at the interface of high-velocity media and low-velocity media, i.e., the areas with high velocity gradient, which has been validated in other areas.     

S-wave velocity structure in Tangshan earthquake region and its adjacent areas from joint inversion of receiver functions and surface wave dispersion*

Using the seismic records of 83 temporary and 17 permanent broadband seismic stations deployed in Tangshan earthquake region and its adjacent areas (39°N–41.5°N, 115.5°E–119.5°E), we conducted a nonlinear joint inversion of receiver functions and surface wave dispersion. We obtained some detailed information about the Tangshan earthquake region and its adjacent areas, including sedimentary thickness, Moho depth, and crustal and upper mantle S-wave velocity. Meanwhile, we also obtained the v_P/v_S structure along two sections across the Tangshan region. The results show that: (1) the Moho depth ranges from 30 km to 38 km, and it becomes shallower from Yanshan uplift area to North China basin; (2) the thickness of sedimentary layer ranges from 0 km to 3 km, and it thickens from Yanshan uplift region to North China basin; (3) the S-wave velocity structure shows that the velocity distribution of the upper crust has obvious correlation with the surface geological structure, while the velocity characteristics of the middle and lower crust are opposite to that of the upper crust. Compared with the upper crust, the heterogeneity of the middle and lower crust is more obvious; (4) the discontinuity of Moho on the two sides of Tangshan fault suggests that Tangshan fault cut the whole crust, and the low v_S and high v_P/v_S beneath the Tangshan earthquake region may reflect the invasion of mantle thermal material through Tangshan fault.     

2017年九寨沟MS7.0地震震源区速度结构与余震分布

采用九寨沟M_S7.0 （M_W6.5）地震的余震直达P波、S波走时数据,通过体波走时层析成像方法,获得了震源区及其邻区的P波和S波速度结构,并利用成像结果对余震进行了重定位。结果显示:余震主要集中分布于高、低速异常交界处偏低速异常一侧,呈走向NNW,倾向SW,倾角较高的分布特征;余震序列两侧的P波、S波速度结构揭示了发震断层两侧介质性质的差异,即上盘为刚性较强的高地震波速度区,下盘为刚性较弱的低地震波速度区。由余震分布特征和地震波速度结构推断:九寨沟地震发生在上地壳底部,发震断层具有上盘地震波速度高、下盘地震波速度低的特征;主震引起的后续破裂在上地壳内部的剧烈形变区内传播,破裂能量终止于25 km深度附近。      

Tomographic inversion for the three-dimensional seismic velocity structure of the Aswan region,Egypt

The Thurber iterative simultaneous inversion program is used to determine the three-dimensionalP-wave velocity structure in the Aswan seismic region of Egypt. The tomographic inversion presented in this study is based on 1131P-phase observations at 13 stations from 89 local earthquakes, all of which occurred within the Kalabsha fault zone. The assumed initial velocity model is that deduced from local explosion experiments. The results indicate that the Aswan region is characterized by a heterogeneous crust, consisting of a shallow, low-velocity zone and a deeper high-velocity anomaly. Seismic velocity structure within the shallow part demonstrates that the inferred change in velocity exists primarily across the east-west trending Kalabsha fault scarp, whereas the high-velocity zone is located south of this fault. Two well-resolved, low-velocity zones appear within the upper 6 km of the crust. The first coincides with a graben structure located between the Kalabsha and Seiyal faults and the second exists between the N-S Kurkur fault and the main axis of Lake Aswan. Both low-velocity zones occupy an area of approximately 30×40 km, located along the western bank of the lake. The most significant result of this study is that the location of the deeper, high-velocity anomaly coincides with the concentration of seismic activity in the lower crustal layer.     

Structural Setting of Strong Earthquakes in the Huabei Area of China

— Using P and S arrival times, which occurred within the Huabei seismic network, we carried out a tomographic inversion and compared results with the earthquake catalogue of the last 1000?years in the area. The results are as follows:¶1) The hypocenters of most of the strong shocks are distributed in the transitional zones between high- and low-velocity areas in the crust, especially at edges of high-velocity blocks.¶2) Strong shocks predominantly lie above low-velocity blocks, or in transitional zones between low- and high-velocity areas, in the lower crust.¶3) The tectonic settings for the Tangshan and the Sanhe-Pingu earthquakes are similar. Both are not known near large fault belts, and in zones with a sharp lateral velocity gradient.¶4) The Ninghe, Tangshan and Luanxia earthquakes are located in high-velocity blocks that differ in size and depth. This difference can explain the focal depth distribution of the Tangshan earthquake sequence, i.e., earthquakes are shallower in the northeastern Luanxian area but deeper in the southwestern Ninghe area.