Episodes of high correlation between annual rates of earthquakes: The Baikal Rift Zone

Correlation analysis techniques were used to study variations in the annual rates N of completely reported earthquakes with energy class K ≥ 8 that occurred from 1964 to 2001 in the Baikal Rift Zone (BRZ), in three subregions within that zone, and in six areas. This correlation analysis of samples of annual rates of earthquakes N with different observation periods revealed two statistically significant episodes of short-lived synchronization between the seismic processes in the BRZ, in the late 1960s and in the late 1970s to the early 1980s. The 1970–1980 episode stands out because of its duration and the highest correlation level; this makes it the dominant phenomenon in the Baikal Rift seismicity synchronization. The observed synchronization episodes between annual rates of earthquakes show that the seismic process was activated at about the same time in different subregions of the BRZ, thus producing short-lived coherent increases in seismicity rates.     

Searching for an Earthquake Precursor—A Case Study of Precursory Swarm as a Real Seismic Pattern Before Major Shocks

A long-range correlation between earthquakes is indicated by some phenomena precursory to strong earthquakes. Most of the major earthquakes show prior seismic activity that in hindsight seems anomalous. The features include changes in regional activity rate and changes in the pattern of small earthquakes, including alignments on unmapped linear features near the (future) main shock. It has long been suggested that large earthquakes are preceded by observable variations in regional seismicity. Studies on seismic precursors preceding large to great earthquakes with M ≥ 7.5 were carried out in the northeast India region bounded by the area 20°–32°N and 88°–100°E using the earthquake database from 1853 to 1988. It is observed that all earthquakes of M ≥ 7.5, including the two great earthquakes of 1897 and 1950, were preceded by abnormally low anomalous seismicity phases some 11–27 years prior to their occurrence. On the other hand, precursory time periods ranged from 440 to 1,768 days for main shocks with M 5.6–6.5 for the period from 1963 to 1988. Furthermore, the 6 August, 1988 main shock of M 7.5 in the Arakan Yoma fold belt was preceded by well-defined patterns of anomalous seismicity that occurred during 1963–1964, about 25.2 years prior to its occurrence. The pattern of anomalous seismicity in the form of earthquake swarms preceding major earthquakes in the northeast India region can be regarded as one of the potential seismic precursors. Database constraints have been the main barrier to searching for this precursor preceding smaller earthquakes, which otherwise might have provided additional information on its existence. The entire exercise indicates that anomalous seismicity preceding major shocks is a common seismic pattern for the northeast India region, and can be employed for long-range earthquake prediction when better quality seismological data sets covering a wide range of magnitudes are available. Anomalous seismic activity is distinguished by a much higher annual frequency of earthquake occurrence than in the preceding normal and the following gap episodes.     

论发震构造特性在潜在震源区参数确定中的应用

发震构造特性是潜在震源区划分及其地震年发生率确定的重要依据。潜在震源区除了反映“未来具有发生破坏性地震的地区”的内涵外,还应反映高震级档地震具有相似复发特征的涵义。由于在地震活动性参数统计单元内,有一些具有不同本底地震的活动构造块体,为更好地反映地震活动的空间不均匀性,考虑潜在震源区的三级划分是有必要的。通过分析潜在震源区内高震级档地震的复发特征,计算预测时段内潜在震源区的高震级档地震的发震概率,采用预测时段内概率等效转换获得地震年平均发生率的方法,有助于在中国地震危险性分析框架内考虑潜在震源区的强震复发特性。另外,文中还对潜在震源区内特征地震次级震级档频度不足的特性和发震构造上强震非均匀性在地震危险性分析中的应用问题进行了探讨     

2020年1月16日新疆库车5.6级地震总结

系统梳理了2020年1月16日新疆库车5.6级地震前出现的地震活动和地球物理观测等异常,结果如下：①地震活动：震前存在地震平静、地震高频、带状分布、地震窗等中短期异常;②地球物理观测：震中350 km范围内均为形变异常,分别体现在固体潮、地倾斜、钻孔应变测项;③综合方法主要识别出年尺度异常。对库车5.6级地震序列进行分析,可知主震震源机制为走滑型破裂,余震不丰富,序列衰减不明显,序列发展过程中地震活动起伏变化明显。综合分析认为,此次地震前地震活动中短期异常较为突出,地球物理观测异常相对较少,发震前2年异常数量出现先增加后减少的现象,对地震的发震时间具有一定指示意义。     

强震前大范围地震活动性参数的时空扫描

在大地震发生之前, 伴随着应力的集中加强及其在震源附近区域引起的非弹性变形, 区域地震活动性可能呈现某些异常特征.本文选择一些有明显物理意义并得到岩石破裂实验支持的地震活动性参数, 例如有震面积数 A、平均释放能量E、地震累加频度 N 和大小地震比例系数 b 值等, 采用扫描方法在大范围内搜索, 识别孕育强震的危险地区, 并对假想孕震区进一步做时间扫描计算, 寻找并判断大震前由中期异常过渡到中短期异常的阶段, 在对华北、西南及东北若干强震资料扫描计算的基础上, 着重剖析唐山地震前华北地区区域地震活动异常变化的一些特征.讨论了上述地震活动性参数在强震中期预报中的不同效能, 认为这种时空扫描方法, 特别是 b 值扫描应用于强震的中期预报是有一定意义的.      

台湾海峡7.3级地震及其震后地震趋势研究

通过对台海海峡７．３级地震序列研究表明，该地震是一次板内的主－余震型地震，震源机制解确定此次地震是ＮＷ走向断裂的走滑正断层错动；震区地震构造背景和震前的地震活动异常图像与我国东南沿海成一体性，表明属于我国东南涡海地震带的地震活动震前出现多项地震学异常。震兆演化过程存在明显的阶段性，本次地震有一寂的可预测性，根据地震活动分幕结果，认为近期东南沿海的一系列中强震活动属于该带第五活跃幕的地震群体活动并预     

1900年以来全球地震活动的初步分析

20世纪50年代和60年代，是全球地震矩的大释放阶段。从60－80年代地震矩释放速率呈下降的趋势。90年代后，地震矩释放速率有所回升。从50－80年代，中国大陆及东半球的地震矩释放速率有明显的下降趋势。一次9级左右地震的成丛是20世纪地震活动的明显特点。     

祁连山地震带小震活动增强指标提取与预报效能分析

总结2015年以来、2010年以来和1980年以来祁连山地震带活动增强指标,得到祁连山地震带5年、10年以及40年尺度的小震频度预报效能。祁连山地震带M_L2.0以上地震月频度持续增强的异常出现时间在3个月以上,异常出现后1~3个月,祁连山地震带及边邻地区发生5级以上地震以及青藏高原东北缘发生6级以上地震的概率较高。作为短期预报指标,祁连山地震带的地震活动增强对甘肃及边邻地区的地震预报工作具有重要的指示意义。     

Seismic gaps and source zones of recent large earthquakes in coastal Peru

The earthquakes of central coastal Peru occur principally in two distinct zones of shallow earthquake activity that are inland of and parallel to the axis of the Peru Trench. The interface-thrust (IT) zone includes the great thrust-fault earthquakes of 17 October 1966 and 3 October 1974. The coastal-plate interior (CPI) zone includes the great earthquake of 31 May 1970, and is located about 50 km inland of and 30 km deeper than the interface thrust zone. The occurrence of a large earthquake in one zone may not relieve elastic strain in the adjoining zone, thus complicating the application of the seismic gap concept to central coastal Peru. However, recognition of two seismic zones may facilitate detection of seismicity precursory to a large earthquake in a given zone; removal of probable CPI-zone earthquakes from plots of seismicity prior to the 1974 main shock dramatically emphasizes the high seismic activity near the rupture zone of that earthquake in the five years preceding the main shock. Other conclusions on the seismicity of coastal Peru that affect the application of the seismic gap concept to this region are: (1) Aftershocks of the great earthquakes of 1966, 1970, and 1974 occurred in spatially separated clusters. Some clusters may represent distinct small source regions triggered by the main shock rather than delimiting the total extent of main-shock rupture. The uncertainty in the interpretation of aftershock clusters results in corresponding uncertainties in estimates of stress drop and estimates of the dimensions of the seismic gap that has been filled by a major earthquake. (2) Aftershocks of the great thrust-fault earthquakes of 1966 and 1974 generally did not extend seaward as far as the Peru Trench. (3) None of the three great earthquakes produced significant teleseismic activity in the following month in the source regions of the other two earthquakes. The earthquake hypocenters that form the basis of this study were relocated using station adjustments computed by the method of joint hypocenter determination.     

Prediction and verification of earthquakes induced by the Xiluodu hydropower station reservoir

Research has been conducted on reservoir-induced earthquakes in China since the Xinfengjiang reservoir-induced earthquakes in the 1960s. Regulations now require the risk of reservoir-induced earthquakes to be evaluated in the pre-research stage of all hydropower projects. Although nearly 40 cases of reservoir-induced earthquakes have been reported in China, analyses comparing the changes in seismic activity following reservoir impoundment with predictions are rare. In this study, we compared seismic activities observed in the reservoir area before and after the impoundment of the Xiluodu hydropower station in terms of the spatial distribution, frequency, and focal depths of the earthquakes, and clarified the correlation between their frequency/timing and reservoir level after impoundment. We then concluded that the seismic events in the head region were karst-type earthquakes, while those in the second segment of the reservoir were tectonic earthquakes. The spatial distribution of the earthquake epicenters and the seismic intensities validated some of the results for the reservoir-induced seismic risk assessment for the Xiluodu hydropower station, indicating that the proposed earthquake triggers and predictive models are reasonable. This study can provide a valuable reference for investigating the mechanism (s) of reservoir-induced earthquakes, revising reservoir-induced earthquake hazard assessment codes, and predicting the hazard zones of reservoir-induced seismicity under similar conditions.     

1995—1996年云南地区三次强震前地震活动的短期异常

在地震学参量年度异常的背景＾（３）,阐述了１９９５￣１９９６年云南地区发生的孟连、武定、丽江三次强震前的地震活动短期异常,包括地震平静、小震群、窗口地震、调制地震、地震波参数以及４级左右地震向震中迁移等。结果表明,这些异常特征表现突出,其演化进程显示了强震孕育的阶段性。     

地震活动性层次结构分析及危险区预测方法

对地震活动性进行层次结构分析可划分出不同的地震区带,中国大陆的地震活动可分为9个地震区带,分析各个区带的地震活动性可作出某一活跃期强震发生主体地区的判别,对某一个地震区带的地震活动进行层次结构分析可作出地震危险区预测。用震源面投影图代替点投影的震中分布图可更合理地解释一个构造带上强震的发生过程,更清楚地勾画出未来强震的孕震范围。对强震破裂区闭锁段、背景空区、孕震空区的关系用实测震例进行了解释。     

震级转换关系及其对地震活动性参数的影响研究

本文根据1990-2007年《中国地震年报》中同时给出Ms和ML、且震源深度〈70km的6577个浅源地震资料,经统计回归得到了全国和各地震区Ms与ML之间的经验关系。新的震级转换关系接近于Ms=ML,本文建议在需要进行震级标度转换时,对于没有测定Ms的低震级地震可直接使用Ms=ML进行转换。与目前广泛使用的震级转换关系相比,采用本文建议的震级转换方法后,全国各地震区5级以上地震的数量基本没有变化,但5级以下地震的数量有明显增加,导致由此统计得到的各地震区震级．频度关系中的b值有不同程度的增大。此外,用地震数量直接得到的4级以上地震的年平均发生率V4有明显增高,且部分地震区甚至增加了50％以上。在高震级地震发生率不变的情况下,地震活动性参数的上述变化反映了对地震区地震活动水平的估计有提高,可能导致概率地震危险性分析结果的提高,对地震区划和工程场地地震安全性评价有重要意义。     

An analysis of the earthquake time series at Mount St. Helens, Washington, in the framework of recent volcanic activity

The analysis of the earthquake time distribution at Mount St. Helens reveals a good correlation between the physical state of the volcano and statistical parameters of earthquake sequence. There are three main seismic phases in the whole 1980–1986 period. The first one precedes the main eruption of May 18, 1980. It begins with a sudden increase of the seismicity level in late March and continues with an Utsu (1961) type decay of the seismic occurrence rate, characterized by a small value of the decay coefficient, β. The second phase lasts from the cataclysmic eruption on May 18, 1980 until the continuous dome building episode in 1983 and is characterized by a very slow exponential increase of the background level of seismicity. The third phase covers the remaining part of the sample and is characterized by a stationary earthquake clustering process episodically interrupted by peaks of activity related to eruptions. The trends in seismic occurrence rate within each phase, as well as the statistical parameter variations at each transition, are analyzed and discussed in the framework of volcanic activity. This leads to the conclusion that statistical techniques may give a significant contribution in understanding changes in volcanic processes such as those at Mount St. Helens.     

地震活动性的统计分析:由过去推测将来的可能性研究

为了由过去的地震活动性推测将来的地震活动性,引入了地震（震级≥m）的期望年发生率v（≥m）来描述一个地区的地震活动性．根据全球地震目录（1964-1994年）以及南加州（1932-1995年）和华北（1970-1994年）两个区域地震目录资料,以统计样本量作为目录记录时间长短的相对量度,对由不同的统计样本量计算得出的地震实际年发生率v（≥m,T,t）进行了统计分析,得到三点结沦：①在统计样本量n足够大的情况下,地震实际年发生率表现出准平稳时间过程的特征,可近似地看作地震期望年发生率,本文给出了这种近似的误差（离差系数）与统计样本量之间的定量关系;②离差系数与统计样本量之间的关系与震级无关,表现出不同震级层次的相似性;③统计样本量相同时,不同震级的地震期望年发生率之间满足logv（≥）=a－bm的关系,形式上与G－R关系相似,但它给出了由小地震的统计特征估计大地震的期望年发生率及其统计误差的方法．基于上述结论,进一步讨论了地震活动性的统计特征在地震危险性分析中的潜在应用。     

Precursory seismic crustal deformation in the area of southern Albanides

On the basis of growing evidence thatstrong earthquakes are preceded by a periodof accelerating seismicity of moderatemagnitude earthquakes, an attempt is madeto search for such seismicity pattern in NWAegean area. Accelerating seismic crustaldeformation has been identified in the areaof southern Albanides mountain range(border region between Greece, formerYugoslavia and Albania). Based on certainproperties of this activity and on itssimilarity with accelerating seismicdeformation observed before a strongearthquake which occurred in the sameregion on 26 May 1960 (M = 6.5), we canconclude that a similar earthquake may begenerated in the same region during thenext few years. This conclusion is inagreement with independent results whichhave been derived on the basis of the timepredictable model.     

华北地区强震的背景地震活动性的研究

7级以上的强震并不是在一片空白上突然发生的,而是在一种间歇性的地震活动背景下发生的,研究这种背景地震活动的规律,为判定一个活动期的强震的潜在震源区提供信息。本文通过对华北历史上记录到的18次7级以上强震的震前史和震后史的研究,提出了华北地区强震背景地震活动的四条规律:①强震前多有6级以上的背景地震发生;②并且多有信息地震发生;③短时间近距离的两个强震之间可能出现缓震现象;④强震具有一定的减震作用。文中根据这四条规律,对华北地区1501——1730年的强震潜在震源区做了判别分析。      

新疆于田地区地震活动与强震间的应力转移

新疆于田地区位于青藏高原与塔里木盆地相交会的地方,强震频发。2008年以来,该地区陆续发生的4次M6以上强震都位于阿尔金断裂带南分支的龙木错—郭扎错断裂带附近。这些地震的发生为研究于田地区中强震活动的空间分布及其之间的相互作用及提供了很好的实例。首先,利用小震目录对区域地震活动进行分析,给出了地震活动性参数b值的空间分布。然后,采用分层黏弹性模型研究了区域4次中强地震所造成的区域应力转移。我们发现,于田地区近年来强震活动的时空分布特征可能与区域地壳应力水平的差异有关。康西瓦断裂带上历史强震活跃,但是现今地震较平静,b值较高;而龙木错—郭扎错断裂带附近的b值相对较低,现今强震活动频繁。于田地区4次中强震造成的应力转移则比较复杂,2012年地震受到2008年地震的卸载作用,而2014年和2020年2次地震的发生均受到前序地震的促进作用。综合区域地震活动和强震之间应力转移的分析,可以认为阿尔金断裂带南支的龙木错—郭扎错断裂带上,应力水平仍然相对较高,未来的地震危险性仍然较大。     

宜宾北4.7级地震活动背景、特征及其与鲁甸5.6级强震呼应成因分析

通过对宜宾北4．7级地震震中及附近地区地震地质、地震活动背景、区域地震活动性、水位资料的分析，表明宜宾北4．7级地震震前经历了区域地震活跃至平静、再发震的过程，其部分地震学参数存在异常，川12井水位也出现较大异常，并对宜宾北所处的华蓥山断裂带上的中强震与川滇交界附近地区南北地震带上的强震存在较高的对应关系成因进行了分析，认为华蓥山断裂带上中强地震活动对川滇交界地区南北地震带上强震的发生存在中短期预报意义。