Regional characteristics of stress field in the southern part of the north－south seismic belt in China and its relation with plate movement

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Modern tectonic stress field in the Chinese mainland inverted from focal mechanism solutions

IntroductionTheinversionapproachofregionalstressfielddevelopedinrecent10to20yearsprovidesausefultoolforstudyingthemeanstressinagivenregion(Angeller,1979;Ellsworth,1981,Xu,Ge,1984).Becauseitusesmultitudinousfaultsinsteadofsinglefault,itcanremovetheinhomogeneityoflocalmediumsoastorevealtheregionalstressinformation.Besides,thismethodproducesaRvalue,whichisdefinedby(O-2--q)/(q--q),andmaydescribestherelativemagnitUdeofintermediateprincipalstress,whereq,acand%arethemaximum,theintermediateandthemi…     

Moment tensor inversion for focal mechanism of the Beibuwan earthquakes

IntroductionOnDecember31,1994andJanuary10,1995,twoearthquakesoccurredconsecutivelyinBeibuwanarea,China.TheirmagnitudewereMs=6.0andMs=6.1respectively.ThehypocentralparametersoftheseearthquakesarelistedinTable1.Thetimeintervalofthetwoewthquakesis10dandtheirepicentrallocationsareneartoeachotherTheyarethestrongestewthquakesinBeibuwanregiononrecord.Since1980,just12earthquakeshadoccurredinthe50kmrangearoundtheepicentresofthesetwoearthquakes(Wu,etal,1996).Thelargestearthquakeamongthe12earthquake…     

安徽北淮阳构造变形带特征与应力状态研究

在分析安徽北淮阳构造变形带变形特征和活断层作用的基础上,利用该区现代中小地震资料,采用近震直达P波、S波垂直向最大振幅比方法、震源机制的系统聚类和统计分析法,反演部分地震震源机制,并分析现代中小地震活动特征及应力场状态。结果表明,北淮阳构造变形带上主要断层现今处于活动状态;现代中小地震震源总体较浅,在2次6级地震震中附近形成小震密集区,可能反映了震源区介质相对破碎。震源机制的系统聚类和统计分析结果显示,该区断层以走滑或斜滑型破裂为主,但存在倾滑的逆断层;区内应力作用方向以水平为主,也存在部分垂向力作用;主压应力P轴的优势分布为近EW向,其中既有华北应力场,也有华南应力场作用;有少量近NS向应力场作用存在。反映该区处于应力场过渡区,因此将其作为华北、南华地块区的边界是合理的。     

Temporal and spatial variation characteristics of present stress field in the northern area of North China

From focal mechanism solutions of the earthquakes in the northern part of North China during the period of 2002～2006,the directions of principal stress axes in 4 stress sub-districts are analyzed using a grid test method.The characteristic of present crustal stress directions is discussed.Based on this result and on the focal mechanism solutions calculated for some events in the period of 1977～1998,in combination with some other study results,the temporal variation of present crustal stress directions in the northern part of North China is investigated.The re-sults confirm that the direction of crustal principal stress in some regions had somewhat rotated after the 1976 Tangshan M7.8 earthquake.The mean P axes of the focal mechanism solutions rotated clockwise not only in Tangshan sub-district,but also in Beijing and Xingtai sub-districts after the Tangshan earthquake.In Beijing and Xingtai sub-districts the orientations of principal stress axes in the period of 2002～2006 are consistent with that before the Tangshan earthquake,implying that the stress orientations has rotated back to the state before the Tang-shan earthquake in these two sub-districts.The directions of the mean P axes are nearly E-W in Tangshan sub-dis-trict since the M7.8 earthquake.The present stress field in the sub-district northwest to Beijing,or in the western part of the Zhangjiakou-Bohai fault zone,is relatively stable during the time period concerned in this study.Because of the limitation of data,this paper only states a possible variation of stress field in the northern part of North China in the recent decades.     

利用多个震源机制解求东大别地区平均应力场

利用地震的震源机制解资料,得到应力张量在地理坐标系下的表达式,进而可计算平均应力张量.通过求平均应力张量的本征向量,即可得到其主轴方向,并由此推断区域应力场方向.利用东大别地区219次地震的震源机制解资料,比较了不同相似程度和不同起算震级资料得到的主轴方向,认为该方法计算结果非常稳定,主轴方位角的误差小于5°,倾角的误...     

Preliminary research to determine stress districts from focal mechanism solutions in Southwest China and its adjacent area

The method of sliding direction fitting is used to determine stress districts, taking the shear stress directions and ratios of shear stress to stress on fault planes given by focal mechanism solutions as the criteria to select focal mechanism solutions of one region and sorting out the earthquakes controlled by different tectonic stress fields, and then determining the stress districts from epicenter distribution of earthquakes. We call this method as step by step convergence method. By inversion analyzing of 297 focal mechanism solutions, we consider that Southwest China and its adjacent area can be divided into 5 stress districts, and we worked out directions of the three principal stresses and values of shape factor φin 5 stress districts.     

鄂尔多斯块体西南缘地区震源机制解与构造应力场特征分析

计算了2001年1月至2018年9月鄂尔多斯块体西南缘地区(33°—38°N,103°—109°E)M_L≥3.0地震的震源机制,分析了该区域震源机制解特征,在此基础上,采用平均矩张量技术反演了该区域的构造应力场特征。结果表明,研究区震源机制类型以走滑型和逆冲型为主,其空间分布基本与区域构造动力背景和断裂性质一致,少量不符合区域构造性质的震源机制则反映了局部地质构造的复杂性。区域构造变形主体为NE向压缩,NW向相对扩张,反映了青藏高原块体对鄂尔多斯块体的直接作用。     

2003年6月青岛震群地震震源机制与震源区应力场特征

利用山东数字地震台网记录的2003年6月青岛震群波形资料,由P波、SV波、SH波初动和它们之间的振幅比,联合计算了ML≥2.9地震的震源机制解。结果表明,震群发生前期,震源机制较为一致,P轴与北东东(80°)近水平方向的区域应力场主压应力方向一致,震群发生后期震源机制变化相对较大。上述现象相对于强震P轴方向与区域应力场主压应力方向一致,而余震P轴絮乱的现象有一定的相似性。但相对于强震在震后引起的主压应力方向变化(约40°~96°)来说,青岛震群地震引起震源区P轴的变化(约45°)并不显著。     

Determinations of directions of the mean stress field in Sichuan-Yunnan region from a number of focal mechanism solutions

Introduction With the model of double couple point source, both orientations of the T, B, and P axes of a single focal mechanism solution and parameters of the two nodal planes may be determined. The focal mechanism solution under ″the model of the best double couple″ can also be given by seis- mic moment tensor, such as those provided by Harvard CMT solutions. The orientations of the T, B, and P axes from one earthquake are associated with the released stress, but they cannot be re- gar…     

张家口—渤海地震带中小地震应力降分布特征

采用河北测震台网资料,分析了张家口—渤海地震带2008—2018年1220条ML≥2.0地震的应力降.结果表明,应力降可能与应力有关,并且随震级的增加而出现不同尺度的变化,该区域应力降优势分布为0.70—3.00 MPa.     

Types of focal mechanism solutions and parameter consistency of the sub-blocks in Sichuan and Yunnan Provinces

Based on P- and S-wave amplitudes and some clear initial P-wave motion data, we calculated focal mechanism solutions of 928 M≥2.5 earthquakes （1994-2005） in four sub-blocks of Sichuan and Yunnan Provinces, namely Sichuan-Qinghai, Yajiang, Central Sichuan and Central Yunnan blocks. Combining these calculation results with those of the focal mechanism solutions of moderately strong earthquakes, we analyzed the stress field characteristics and dislocation types of seismogenic faults that are distributed in the four sub-blocks. The orientation of principal compressive stress for each block is： EW in Sichuan-Qinghai, ESE or SE in Yajiang, Central Sichuan and Central Yunnan blocks. Based on a great deal of focal mechanism data, we designed a program and calculated the directions of the principal stress tensors, σ1, σ2 and σ3, for the four blocks. Meanwhile, we estimated the difference （also referred to as consistency parameter θ^- ） between the force axis direction of focal mechanism solution and the direction of the mean stress tensor of each block. Then we further analyzed the variation of θ^- versus time and the dislocation types of seismogenic faults. Through determination of focal mechanism solutions for each block, we present information on the variation in θ^- value and dislocation types of seismogenic faults.     

综合震源机制解法反演湖南地区构造应力场的初步结果

基于435个地震的1580条P波数据,通过综合震源机制解方法,根据地震与网格点之间的距离不一样来确定P波符号的权重,反演得到了湖南地区0.2°×0.2°的二维构造应力场.相较以往利用震源机制求解应力场的方法,该方法不仅能直接采用P波初动资料反演应力场而略去计算震源机制的中间步骤,而且能够尽可能多地使用本地区小震P波数据对应力场方向进行限定.初步研究结果表明：湖南地区综合震源机制解总体分布特点为逆断型和不确定型;P轴倾角近水平,方位具有一定的分区性,总体上呈NWW-SEE向;T轴倾角偏高,方位在局部地区变化较大,统一性相对较差;湖南地区整体上受NWW向的挤压应力,对该区的活动断裂具有一定的控制,这或许也是该区地震多发生于郴州、娄邵、湘西州所构成的NW向空间条带的主要原因之一;湖南地区主要受菲律宾海板块NWW向的推挤作用,其次才受太平洋板块向西俯冲的影响,西面虽受印度洋与欧亚板块的水平碰撞,但由于碰撞主要引起青藏高原向东扩张,同时又受到云贵高原的阻隔,其影响至本区已大为减弱.

     

1998年张北地震序列构造应力张量变化特征的研究

根据张北地震序列的地震活动特征,利用张北震区1998年1月～1999年3月间145个3 级左右地震的震源机制解资料,应用Gephart(1990)的应力张量反演方法,研究张北地震序列构造应力张量的总体变化特征和时序变化特征;研究发现强余震发生前,应力张量的波动幅度不是很大,震源及附近地区的构造应力作用较强,强余震发生后,应力因发生破裂而重新分布。而且震源区的构造应力场方向与华北地区的构造应力场方向基本一致,说明张北主震和强余震都是在华北统一构造应力场的作用下发生的。     

综合震源机制解法反演湖南地区构造应力场的初步结果

基于435个地震的1580条P波数据,通过综合震源机制解方法,根据地震与网格点之间的距离不一样来确定P波符号的权重,反演得到了湖南地区0.2°×0.2°的二维构造应力场.相较以往利用震源机制求解应力场的方法,该方法不仅能直接采用P波初动资料反演应力场而略去计算震源机制的中间步骤,而且能够尽可能多地使用本地区小震P波数据对应力场方向进行限定.初步研究结果表明：湖南地区综合震源机制解总体分布特点为逆断型和不确定型;P轴倾角近水平,方位具有一定的分区性,总体上呈NWW-SEE向;T轴倾角偏高,方位在局部地区变化较大,统一性相对较差;湖南地区整体上受NWW向的挤压应力,对该区的活动断裂具有一定的控制,这或许也是该区地震多发生于郴州、娄邵、湘西州所构成的NW向空间条带的主要原因之一;湖南地区主要受菲律宾海板块NWW向的推挤作用,其次才受太平洋板块向西俯冲的影响,西面虽受印度洋与欧亚板块的水平碰撞,但由于碰撞主要引起青藏高原向东扩张,同时又受到云贵高原的阻隔,其影响至本区已大为减弱.     

河套地震带的b值时空变化特征分析

本文以b值作为研究对象,在b值由应力状态和介质性质共同决定的理论框架下,开展了河套地震带的b值时空变化特征分析。选取1970年发生在河套地震带的ML≥1.5地震作为考察对象,运用EMR方法定量检测了最小完整性震级Mc的时序非均匀变化,并根据研究区地震活动实况,确定b值的起算震级为ML2.0。运用最小二乘法进行了b值时间扫描计算,结果显示,b值时序变化较好地对应了研究时段内的中强地震活动、台网分布和余震剔除等变化因素。运用极大似然法进行了b值空间扫描计算,结果显示,河套地震带的b值空间差异可能由地壳介质体性质和应力环境共同引起,相对来讲,b值的构造相依特征更为明显。此外,满足各震级档有足够多的地震样本依然是提高b值计算精度的关键环节。     

Preliminary research to determine stress districts form focal mechanism solutions in Southwest China and its adjacent area

IntroductionManystudies(Kan,etal,1977;Cheng,1981;Xu,etal,1987;Xie,etal,1993,Mulled,etal,1997)indicatedthatthedirectionsofcrustaltectonicstresseshaveregionalvariation.SimpleanalysesbymeansOfthreeprincipalaxesoffocalmechanismsolutions,whichexpressedasP...     

2014年云南盈江Ms6.1地震震源机制研究

文章搜集了盈江地区多个地震序列震源机制解共计162个,对从苏典断裂到大盈江断裂的区域进行了构造应力场反演。研究结果表明,该区域总的主压应力轴方向为NNE—SSW向。并从内在动力学演化的角度分析了2008年3月21日—2014年5月30日发生的6次震群活动,发现其存在明显的相互联系性。研究还发现,局部也出现应力场偏转的现象,苏典断裂南端的主压应力轴向北偏转角度变大,这可能与该区形成的三角凹陷有关。此外,本文还搜集到2016—2020年之间发生于盈江地区的1003个地震目录,通过分析发现昔马—盘龙山断裂到大盈江断裂之间的区域可能存在应力场闭锁,因此,加强该区域的危险性防范至关重要。