前震研究进展综述

介绍了当前国内外前震研究领域的一些进展。 对比了不同前震定义条件下前震序列的共性特征, 对几种主要的前震机理及主要的前震识别方法进行了简要综述, 对其特点及存在的问题进行了评述和讨论。 前震指主震之前在主震断层面上、 紧邻主震破裂起始点发生的小地震。 主震发生之前的一系列前震活动构成前震序列。 在不同的前震定义条件下, 具有“直接前震”的震例比例从10%至40%不等。 理论上前震可用级联应力触发或预滑动模型进行解释。 前震空间上主要集中分布在距离主震10~75 km范围内, 但其时间分布形式复杂, 大多发生在主震前1~2天, 部分前震序列的地震活动率显示明显的加速特征, 但许多前震序列单独来看却往往显示主-余型序列的衰减特征。 序列地震震源机制一致、 序列b值偏低是前震序列最突出的特征。 前震的发生与主震破裂形式和构造环境似有一定关系, 在有限的前震震例中, 逆冲型主震似乎具有相对较多的前震。 部分震例的研究结果显示, 随主震的临近, 前震震源深度有逐渐下迁的特点。 到目前为止, 震前很难判定一次地震或一个地震序列是否为前震或前震序列, 所使用的前震识别主要有基于统计类比的方法、 基于震源机制一致及衍生的相关方法以及基于对地震成核过程精细检测的方法。 从现有不多的震例研究结果来看, 尽管地震时空丛集和震源机制高度一致是前震序列的最显著特征, 但却不是判定前震序列的充分条件。 由于成核的破裂扩展速度和滑动位移有随时间较快增大的趋势, 因而基于对地震成核过程精细检测的方法有望在前震识别中发挥更为重要的作用, 但需更多震例进行验证。     

Foreshock Occurrence Rates before Large Earthquakes Worldwide

—Global rates of foreshock occurrence involving shallow M≥ 6 and M≥ 7 mainshocks and M≥ 5 foreshocks were measured, using earthquakes listed in the Harvard CMT catalog for the period 1978–1996. These rates are similar to rates ones measured in previous worldwide and regional studies when they are normalized for the ranges of magnitude difference they each span. The observed worldwide rates were compared to a generic model of earthquake clustering, which is based on patterns of small and moderate aftershocks in California, and were found to exceed the California model by a factor of approximately 2. Significant differences in foreshock rate were found among subsets of earthquakes defined by their focal mechanism and tectonic region, with the rate before thrust events higher and the rate before strike-slip events lower than the worldwide average. Among the thrust events a large majority, composed of events located in shallow subduction zones, registered a high foreshock rate, while a minority, located in continental thrust belts, measured a low rate. These differences may explain why previous surveys have revealed low foreshock rates among thrust events in California (especially southern California), while the worldwide observations suggest the opposite: California, lacking an active subduction zone in most of its territory, and including a region of mountain-building thrusts in the south, reflects the low rate apparently typical for continental thrusts, while the worldwide observations, dominated by shallow subduction zone events, are foreshock-rich.     

The Tongliao MS5.3 Earthquake and Its Foreshock Determination

Taking the 2013 Tongliao MS5. 3 earthquake as a research subject, on the basis of statistical analysis of earthquake sequence using the HypoDD location method and focal mechanism solutions,the paper analyzes and discusses the relationship between the ML4. 4 and MS5. 3 earthquakes. The results show that the Tongliao MS5. 3 earthquake occurred under the background of medium-small earthquakes long-term quiescence and short-term enhancement in the epicentral area. The results of accurate seismic location shows that the Tongliao MS5. 3 earthquake sequence is distributed in the NW direction,extending 10 km,and the ML≥3. 0 aftershocks are concentrated south of the mainshock. The distance between the MS5. 3 mainshock and the ML4. 4 foreshock is about 1. 8 km,with a focal depth of 7. 208 km and 7. 089 km,respectively,their focal location is very close,and may have occurred on the same fault plane. The results of focal mechanism shows that the Tongliao MS5. 3 earthquake is of the strike-slip type,the focal mechanism of aftershocks are disordered,and with time lapse,the type is changed from strike-slip to thrust and normal faulting. The bigger foreshocks had similar focal mechanism and were all normal fault types,which exhibits to some extent,an obvious crustal medium anisotropy in the epicentral area before macroscopic rupturing,as represented by alignment fractures,with stress action enhanced,this"consistency"of seismic precursor regime would gestate the mainshock. According to the characteristics of temporal-spatial distribution of earthquake sequence and similarity of focal mechanism,we judge that the Tongliao MS5. 3 earthquake sequence is a foreshock-mainshock-aftershock type.     

山东乳山地区震群特征及发震背景再研究

2013年10月1日在山东省威海市乳山市发生M3.2级地震,之后发生了一系列震群活动。截至2016年5月,山东台网已经记录到了1万多次余震,其中3级以上地震9次,4级以上地震3次。频繁的地震构造活动引起了乳山市及周边地区强烈震感。为研究乳山震群的发震机理,本文利用山东台网数字化地震波资料和新建的乳山台阵资料,通过双差精定位方法重新确定了震中位置。研究结果表明:余震序列呈现出NW向的条带分布;采用CAP方法（Cut and Paste）反演震群中9次3级以上地震的震源机制解,结果显示几次较大地震的震源深度平均约为5km,与台网编目定位的结果基本相同。从得到的精定位结果并结合震源机制解的结果来看,震群的走向是NW向,倾角是NE向,与最近的乳山断裂有一定距离。由此推断该区域可能是乳山断裂的分支,或者有一条或多条隐伏断裂。     

千岛群岛地区地震序列特征

搜集到千岛群岛地区1964～1976年共6个地震序列,并分析了它们的特征．按序列震源分布与俯冲带的空间关系判定,它们都属于板缘地震序列．其中正处于俯冲带的5个地震序列的各种参数比较一致：震中分布区域长轴较长,长轴与短轴的比值高,震源深度延伸范围大,显示与俯冲方向相同的倾向,倾角中等．但有1个例外,估计与早期资料精度偏低有关．     

全球浅源巨大地震序列统计特征

系统研究了1976年以来全球58次M_W≥7.8浅源地震序列的统计特征。结果显示:(1)在58次巨大地震中,板间地震45次,板内地震13次,板内地震强度低于板间地震。(2)74.1%的板间地震为逆断层错动,61.5%的板内地震为走滑型错动。(3)58次地震序列中,82.8%为主-余型,17.2%为多震型;与5级以上地震序列不同,巨大地震没有孤立型,其余震比较活跃;板内地震中,多震型占7.7%,而板间地震中多震型占20%。(4)对于主-余型序列,75%的主震与最大余震的震级差为1.0～2.0级;震级差与主震震源错动类型有关,走滑型的震级差明显大于逆冲型;68%的最大余震发生在主震后3天内,其次为10天左右与1个月左右; 49%的D_(max-aft)(最大余震震中与主震震中间的距离)不超过余震区长轴的1/3,31%的D_(max-aft)为余震区尺度的1/3～1/2;最大余震的发生时间、最大余震震中与主震震中间的距离同主震断层错动类型间的关系不明显。(5)应用ETAS模型计算了46个序列参数后发现,b值、p值和a值均呈Beta分布,b值平均为1.164±0.211,p值平均为1.559±0.412,a值平均为1.673±0.911; p值和a值分布分散;对于不同的序列类型、震源错动类型及板内、板间地震,b值差异不显著;逆冲型序列p值明显大于走滑型和正断层型;板间地震序列a值明显小于板内地震;逆冲型序列a值明显小于走滑型和正断层型;这表明,与板内地震相比,板间地震具有较强的"余震激发余震"的能力;逆冲型破裂虽然会导致序列衰减较快,但触发次级余震的能力相对较强。(6)逆冲型巨大地震余震区长轴L的对数与主震震级M_W间的拟合关系式为lg L=(-1.399±0.306)+(0.470±0.037) M_W。     

Foreshock sequence of September 26th, 1997 Umbria-Marche earthquakes

On September 3^rd (22:07 UTC), 1997 a small earthquake with Mw=4.54 started the foreshocks sequence (1500 events with M_L <3.1) of the September 26^th seismic sequence. Two days after, three seismic stations of the University of Camerino were installed around the macroseismic epicenter of the foreshock. We present in this paper the location of foreshocks (with 2.1L<3.3) which occurred between September 3^rd and 26^th. Foreshocks location, with horizontal (ERH) and vertical (ERZ) error less than 1.5 km, define an area 4 km large. Foreshocks have been localized between the epicenters of the two major events of September 26^th, which occurred at 00:33 UTC with Mw=5.6 and at 09:40 UTC with Mw=6.0 (Amato et al., 1998; Ekström et al., 1998). In a vertical cross-section, hypocenters show a low angle (30^°) structure with SW dip-direction. Focal mechanisms for three of the major events show dip-slip fault solutions with strike direction of about N130, in agreement with the CMT solutions of September 3^rd and September 26^th earthquakes (Ekström et al., 1998). Data recorded at two stations Popola (POP) e Capodacqua (CPQ) located on the rupture area of the September 26^th faults, allowed us to calculate a mean Vp/Vs ratio of 1.84±0.03 for the foreshock. This value is lower than the Vp/Vs ratio of 1.89±0.02 calculated for the aftershock sequence occurred in the same area. Besides, the Vp/Vs ratio during the foreshocks sequence is not stable in time but it seems to increase approaching September 26^th. After September 26^th mainshocks, this value tends to stabilize around a higher value of 1.89. Following the dilatancy model, we suggest that the relative low Vp/Vs ratio before the main shocks could indicate the presence of fluid in the focal volume. The presence of fluids could have increased the effective stress on the fault plane and could be responsible for the long foreshock activity before the two main earthquakes of September 26^th. Therefore, we suggest that this foreshock activity could have also contributed to reduce the friction along the September 26^th fault plane, breaking the active structure in two smaller segments. In this hypothesis, foreshock activity could have drastically contributed to mitigate the seismic potential of the Colfiorito's active structure.     

Analysis on Double-difference Earthquake Location and the Seismicity Pattern of the Yangjiang Earthquake Sequences

The locations of about 400 earthquakes in Yangjiang, Guangdong Province are determined using the double, difference earthquake location algorithm （DDA）. The seismicity pattern becomes concentrated from discrete grids. The rupture characteristics of the Yangjiang earthquake sequence show a conjugated distribution in NW and NE directions. The major distribution trends NE and dips NE with an angle of 30^o and a length of 30km,and the minor distribution trends NW and dips SE with an angle of 30^o and a length of 20km. The focal depth is 5km - 15km. The distribution of the Enping earthquake sequence,which is not far from Yangjiang,is NW-trending. The relationship between hypocenter distribution and geological structure is discussed.     

云南中强震序列类型的划分及其空间分布特征

依据公元886～2009年云南Ms≥5.0地震资料,分析云南地区中强以上地震序列类型早期划分及空间分布特征.结果认为:云南地区Ms≥5.0地震序列以主余型为主,其次为多震型;主余型与多震型地震散布于云南省各个地区,孤立型主要分布于东川、玉溪、盈江等地区.各小区域的地震震型特点为:滇西北地区与云南地区地震类型比例一致;小...     

中国大陆地区地震序列显著地震的时间分布特征

本文对中国大陆地区１９６６年以来约１００个５级以上地震序列的系统分析表明，地震序列显著地震累积频度Ｎ（ｔ）与时间ｔ间大体可分为幂函数、指数以及线性函数等３种分布形态，它们可能较客观地描述了地震序列随时间发展的复杂性与多样性。Ｎ（ｔ）与时间ｔ分布函数的不同，可能与后续显著地震性质不同有关，即指数函数可能预示后续发生双震型序列的第二主震或多震型序列中的强震甚至续发前－主－余型序列的主震；线性函数可能意     

郯庐地震带分段特征与小震序列活动分析

2008年汶川地震后,中国东部的地震活动水平上升,郯庐地震带内的小震序列活动增强显著。根据郯庐断裂带的分段特征,将郯庐地震带划分不同分段,结合分段内不同小震序列活动变化,研究不同分段的地震活动性参数异常,分析郯庐地震带分段之间的地震活动性的相互关系。     

2010年青海玉树7.1级地震的前震识别及短临预报可能性研究

2010年4月14日青海玉树发生了7.1级地震,在震前2小时发生了一次4.7级前震。分析表明,玉树7.1级地震前一些主要的地震活动异常多有显示,例如空区、条带、增强和平静等,且与7级大震震例比较,这些前兆地震活动图像的演变具有类似的过程。跟踪这些图像及其演变有可能识别前震。最后评估了该识别方法的可信度。     

结合异年倍九律鉴别前震类型

鉴别前震的方法甚多。该文主要运用异年倍九法（或称异年倍九律）帮助鉴别前震。鉴别的方式是在欲鉴别的小震或小震群发生地区寻找组成异年倍九律的地震,其某个第九天日期若与该小震或小震群发生日期相重或相近,则认为这个小震或小震群可能是前震。文中列举了2010年玉树大震前的小震和2008年汶川大震前都江堰小震群的实例,皆被鉴别为可能是前震。     

敖汉旗地区震群精定位及其空间分布特征

以敖汉旗地区震群作为研究对象,选取内蒙古测震台网2018—2019年的震相观测报告,分别利用单纯型定位法、Hypo2000定位法和双差定位法对其进行重定位和对比分析。结果显示,双差方法的定位精度最高。根据双差方法定位结果,敖汉旗的震群在赤峰?开源断裂的南北两侧均有1个集中分布区,2个区域的震源深度范围均为4～12km;其中,73%的地震位于断裂北侧,震中位置沿NW?SE向呈条带状展布,延伸长度约6.8km,震源深度由西向东逐渐变深、由南向北逐渐变深再变浅;而断裂南侧的地震其震中位置聚集呈簇状,SN向最大扩展长度约0.98km,震源深度无明显沿EW或SN方向的变化特点。敖汉旗震群的精定位结果与对该序列地震空间分布特点的认识,可为该地区发震构造的研究与三维精细速度结构的建立提供更为精确的震源参数与参考依据。     

广西北流—广东化州5.2级地震及前震的矩震级测定

利用理论波形与实际波形拟合方法 ,测定了发生于2019年10月12日22时55分26秒和24秒的广西北流-广东化州M_S5.2级地震(主震)及其M_L4.2级前震的矩震级,得到主震矩震级为M_W 4.7、前震矩震级为MW 4.2。     

GPS基准站坐标分量噪声的时间序列与分类特征

根据地震预测对异常信息(长期、中期和短期异常)与噪声分离(白噪声、闪烁噪声和随机漫步噪声)的需要,文中应用小波方法对中国大陆的有关GPS基准站坐标分量和测站间坐标分量进行了分离,并将它们归纳为四类高频、中频、较低频和低频。通过对高频、中频和较低频的时间序列特征进行分类分析,获得如下看法:①高频类可能主要是白噪声造成的,其离散的程度较中频类略大,或许其中在某时刻包含临震前兆和震时形变信息;②中频类主要突出闪烁噪声的特征,或许其中在某时刻包含着短临前兆信息;③较低频类若剔除年和半年周期成分则显示随机漫步噪声的特点,或许其中包含着中期前兆信息;④低频类主要体现趋势性形变信息;⑤高频与中频类噪声的大小一般与观测季节明显有关,噪声最大的时段大约是8月份,噪声最小的时段大约是2月份;⑥判定不同频域信息的显著性时,其噪声要进行相应的综合或分离;⑦一定的空间域内的测站间坐标增量噪声平均小于测站相应坐标的噪声,而且噪声大小随时间的变化更加突出,甚至相差达一倍以上。这些可能说明,虽然这些噪声是GPS观测所固有的,但并非完全是独立的,测站间越近,共性的成分越突出。所有这些将为我们变形分析时信息显著性的判断、方法的选择、资料处理时权的确定、观测的最佳时段选择、跟踪监测方案制定和参考基准站的确定等提供重要的参考。