1995年4－6月的全球地震动态

１９９５年第二季度，全球地震活动恢复到１９９４年的高水平态势：浅源地震频数猛增，并出现两次Ｍｓ７．７以上巨大地震。大洋岛弧带成为本季度地震活动中心。所罗门群岛有两次中强震。菲律宾萨马岛附近发生大震群。千岛群岛大震的余震未息。秘鲁北部和缅印边界各有一次中深源地震。希腊发生中强地震两次。萨哈林岛地震显示全球地震信息。     

2009年7月24日西藏尼玛西南MS5.6地震研究

主要研究2009年7月24日西藏尼玛西南MS5.6地震的基本参数、地震序列特征、震源参数、发震构造等;利用震中附近600km范围内台站测定参数研究地震的震源机制解,与哈佛大学给出的震源机制解较一致,且与通过现场考察的发震断层走向具有一致性。研究认为本次地震发生在冈底斯山—拉萨块体内部,断裂为NNW向,主要受张应力作用产生左旋走滑正断层活动。此外还分析了震前地震学条带异常特征,结束表明,震前1年出现NW向条带非常显著,研究结论为该地区今后地震预测提供科学依据。     

2013年8月河北蔚县小震群遗漏地震检测与发震构造分析

华北地区近年来小震群活动频繁, 在有数字波形记录的中强地震相对缺乏的背景下, 小震群发震构造精细研究可为华北地区地震危险性分析和地震趋势判断提供重要依据. 本文利用匹配滤波技术对2013年8月22—25日河北蔚县小震群遗漏地震事件进行检测, 并通过地震精定位和震源机制求解分析此次震群的发震构造. 计算结果显示, 通过互相关扫描检测到18次被地震台网常规分析遗漏的地震, 约为地震目录给出的13次地震事件的1.38倍. 该震群发震构造有北东向和北西向两组断裂, 震群活动前期以北东向构造活动为主, 后期地震主要发生在北西向构造, 北西向构造在此次震群活动中地震频度和强度均高于北东向构造. 震源机制计算结果显示北西向构造发震机制以正断拉张为主.      

Seismicity and water level variations in the Lake Aswan area in Egypt 1982–1997

The data described here are obtained from the continuous record of earthquakeactivity and lake water-level variation in the Lake Aswan area in Egypt between 1982 and 1997. The seismicity is monitored by a local telemetered seismograph network. The hypocentral parameters of earthquakes have been determined using Hypo71. The earthquake foci are distributed in two seismic zones, shallow and deep in the crust. Shallow events have focal depths of less than 10 km. Deep events extend from 10 to 30 km. The temporal sequence of seismicity iscorrelated with both the water-level variation and the average daily change of the water level in the lake Aswan. The temporal variation of the seismicityindicates that there are only six sequences of increased seismic activity during 1982–1997. The correlation between the seismic activity and the daily variation of the lake water level is poorly observed except with the June 1987 events swarm, which was accompanied by the presence of an anomaly in rate of water level decreasing. It is concluded that the increase in seismic activity in the Aswan reservoir is demonstrating an example of rapid reservoir-triggered seismicity. The deeper seismic sequence in this area, which was associated with the November 14, 1981 mainshock (M_D = 5.7), and the earlier seismicity (1981–82), has been correlated with a deeper high velocity anomaly (Awad and Mizoue, 1995-b).     

1998年泾阳4.8级地震的一些特征

分析了 1 998年泾阳MS4.8地震的一些特征 .该次地震序列类型属孤立型 ,与陕西关中地区历史强震多以主 余震型为主的特征不同 .震源机制解显示 ,该次地震的断层错动以左旋走滑为主 ,主压应力方向为NE～SW ,与大华北区域应力场的方向吻合     

Earthquake deformation in the northwestern Sierras Pampeanas of Argentina based on seismic waveform modelling

We investigate the seismic properties of modern crustal seismicity in the northwestern Sierras Pampeanas of the Andean retroarc region of Argentina. We modelled the complete regional seismic broadband waveforms of two crustal earthquakes that occurred in the Sierra de Velasco on 28 May 2002 and in the Sierra de Ambato on 7 September 2004. For each earthquake we obtained the seismic moment tensor inversion (SMTI) and tested for its focal depth. Our results indicate mainly thrust focal mechanism solutions of magnitudes M_w 5.8 and 6.2 and focal depths of 10 and 8 km, respectively. These results represent the larger seismicity and shallower focal depths in the last 100 years in this region. The SMTI 2002 and 2004 solutions are consistent with previous determinations for crustal seismicity in this region that also used seismic waveform modelling. Taken together, the results for crustal seismicity of magnitudes ≥5.0 in the last 30 years are consistent with an average P-axis horizontally oriented by an azimuth of 125° and T-axis orientation of azimuth 241° and plunge 58°. This modern crustal seismicity and the historical earthquakes are associated with two active reverse faulting systems of opposite vergences bounding the eastern margin of the Sierra de Velasco in the south and the southwestern margin of the Sierra de Ambato in the north. Strain recorded by focal mechanisms of the larger seismicity is very consistent over this region and is in good agreement with neotectonic activity during the last 11,000 years by Costa (2008) and Casa et al. (in press); this shows that the dominant deformation in this part of the Sierras Pampeanas is mainly controlled by contraction. Seismic deformation related to propagation of thrusts and long-lived shear zones of this area permit to disregard previous proposals, which suggested an extensional or sinistral regime for the geomorphic evolution since Pleistocene.     

小浪底库区近场地震活动特征分析

统计整理小浪底库区近场地震,对地震位置进行精确定位,并计算较大地震的震源机制解,结合水库地震的活动特点,对地震活动和震源机制特征进行分析,认为小浪底库区地震活动具有水库地震特征,且同时受区域地质构造影响。     

The seismicity and tectonic stress field characteristics of the Longmenshan fault zone before the Wenchuan MS8.0 earthquake

The seismicity of Longmenshan fault zone and its vicinities before the 12 May 2008 Wenchuan M_S8.0 earthquake is studied. Based on the digital seismic waveform data observed from regional seismic networks and mobile stations, the focal mechanism solutions are determined. Our analysis results show that the seismicities of Longmenshan fault zone before the 12 May 2008 Wenchuan earthquake were in stable state. No obvious phenomena of seismic activity intensifying appeared. According to focal mechanism solutions of some small earthquakes before the 12 May 2008 Wenchuan earthquake, the direction of principal compressive stress P-axis is WNW-ESE. The two hypocenter fault planes are NE-striking and NW-striking. The plane of NE direction is among N50°?70°E, the dip angles of fault planes are 60°?70° and it is very steep. The faultings of most earthquakes are dominantly characterized by dip-slip reverse and small part of faultings present strike-slip. The azimuths of principal compressive stress, the strikes of source fault planes and the dislocation types calculated from some small earthquakes before the 12 May 2008 Wenchuan earthquake are in accordance with that of the main shock. The average stress field of micro-rupture along the Longmenshan fault zone before the great earthquake is also consistent with that calculated from main shock. Zipingpu dam is located in the east side 20 km from the initial rupture area of the 12 May 2008 Wenchuan earthquake. The activity increment of small earthquakes in the Zipingpu dam is in the period of water discharging. The source parameter results of the small earthquakes which occurred near the initial rupture area of the 12 May 2008 Wenchuan earthquake indicate that the focal depths are 5 to 14 km and the source parameters are identical with that of earthquake.     

Seismic analysis of the Xiluodu reservoir area and insights into the geometry of seismogenic faults

The Xiluodu (XLD) reservoir is the second largest reservoir in China and the largest in the Jinsha River basin. The occurrence of two M > 5 earthquakes after reservoir impoundment has aroused great interest among seismologists and plant operators. We comprehensively analyzed the seismicity of the XLD reservoir area using precise earthquake relocation results and focal mechanism solutions and found that the seismicity of this area was weak before impoundment. Following impoundment, earthquake activity increased significantly. The occurrence of M ≥ 3.5 earthquakes within five years of impoundment also appear to be closely related to rapid rises and falls in water level, though this correlation weakened after five years because earthquake activity was far from the reservoir area. Earthquakes in the XLD reservoir area are clustered; near the dam (Area A), small faults are intermittently distributed along the river, while Area B is composed of multiple NW-trending left-lateral strike-slip faults and a thrust fault and Area C is composed of a NW-trending left-lateral strike-slip main fault and a nearly EW-trending right-lateral strike-slip minor fault. The geometries of the deep and the shallow parts of the NW-trending fault differ. Under the action of the NW-trending background stress field, a series of NW-trending left-lateral strike-slip faults and NE-trending thrust faults in critical stress states were dislocated due to the stress caused by reservoir impoundment. The two largest earthquakes in the XLD reservoir area were tectonic earthquakes that were directly triggered by impoundment.     

华北地区强震前的地震活动图象

本文分析了华北地区(30——42N,105——124E)自16世纪以来8次7.5级以上大地震发生之前的地震活动图象.发现在这些大地震发生之前的若干年(绝大多数不超过20年),都表现出一个M4(3/4)地震在某一范围內相对较集中活动的区域,然后在该区中部或边缘发生大地震。 唐山7.8级地震前,随着大地震发生时间的逼近,Ms4.0地震的震中分布,在大范围活动的基础上,分阶段地逐步自外围向主震震中收缩,然后发生大地震。这种地震活动图象的演变过程可分成若干阶段的现象,对于研究震源性质、地震孕育过程和中长期地震预报等,可能具有一定意义。      

Earthquake activity in the Aswan region,Egypt

The November 14, 1981 Aswan earthquake (M _L= 5.7), which was related to the impoundment of Lake Aswan, was followed by an extended sequence of earthquakes, and is investigated in this study. Earthquake data from June 1982 to late 1991, collected from the Aswan network, are classified into two sets on the basis of focal depth (i.e., shallow, or deeper than 10 km). It is determined that (a) shallow seismicity is characterized by swarm activity, whereas deep seismicity is characterized by a foreshock-main shock-aftershock sequence; (b) the b value is equal to 0.77 and 0.99 for the shallow and deep sequences, respectively; and (c) observations clearly indicate that the temporal variations of shallow seismic activity were associated with a high rate of water-level fluctuation in Lake Aswan; a correlation with the deeper earthquake sequence, however, is not evident. These features, as well as the tomographic characteristics of the Aswan region (Awad andMizoue, this issue), imply that the Aswan seismic activity must be regarded as consisting of two distinct earthquake groups.We also relocated the largest 500 earthquakes to determine their seismotectonic characteristics. The results reveal that the epicenters are well distributed along four fault segments, which constitute a conjugate pattern in the region. Moreover, fault-plane solutions are determined for several earthquakes selected from each segment, which, along with the 14 November 1981 main shock, demonstrate a prominent E-W compressional stress.     

基于模板匹配定位法的江苏盐城附近微震检测和构造分析

利用基于GPU加速的匹配定位法和双差定位法,对江苏盐城及邻区18个台站记录的2009~2018年共10年的连续地震资料进行分析。首先从台网目录中挑选211个地震事件作为模板事件,使用匹配定位技术对江苏盐城附近连续10年的地震进行检测和识别,共识别出1349个地震事件,约为台网目录地震事件的3倍,最小完备震级由台网目录的M_L1.9降为M_L1.2。然后利用双差定位法对检测到的地震事件进行精定位,精定位的结果揭示：建湖地区的地震密集带与洪泽-沟墩断裂有关,震源深度优势分布为5~20km,断裂两侧震源深度有显著差异,断裂带倾向NW;射阳震群震源深度比建湖震群有所加深,优势分布为10~25km,震源深度由南东向西北逐渐变浅;宝应地区地震丛集分布;东台地区由于模板事件相对较少,扫描定位后,地震事件在陈家堡-小海断裂带附近零星分布。研究结果为研究盐城地区的地震活动性、发震断层的深部构造提供了基础数据支撑。     

Ring structures of seismicity in central Tien Shan and Dzungaria: Possible precursory processes of large earthquakes

It is shown that episodes of comparative seismic quiescence that lasted about 20–25 years in the areas of study alternated with intervals of sharply increased seismicity as series of large (M ≥ 6.9) earthquakes occurred during two to three decades. Since no M ≥ 6.6 earthquake has occurred in the area for as long as 21 years after the 1992 Susamyr event, middle-term prediction would require identification of zones of imminent large earthquakes. More reliable identification of such zones rests on data relating to inhomogeneities in the field of S-wave attenuation in the lithosphere, as well as on the characteristics of ring structures of seismicity. Such structures are formed as zones of seismic quiescence that are bounded by M ? Mth earthquake epicenters, where Mth is the threshold magnitude value. Correlative relationships were previously derived, lgL(Mw) and Mth(Mw), for events with different focal mechanisms (L is the length of the longer axis of a seismicity ring and Mw is the magnitude of the associated large earthquake). These relationships were used to estimate the Mw of large events that can occur in these ring structures. The greatest earthquake with Mw ? 7.5 is probably about to occur in southern Tien Shan, east of the 1949 Khait earthquake rupture. A smaller event (Mw ～ 7.0) can occur in the Kyrgyz Range area. Still smaller earthquakes probably have their precursory areas north and east of Lake Issyk-Kul, as well as in Dzungaria.     

Characteristics of seismic activity before the MS8.0 Wenchuan earthquake

The characteristics of spatio-temporal seismicity evolution before the Wenchuan earthquake are studied. The results mainly involve in the trend abnormal features and its relation to the Wenchuan earthquake. The western Chinese mainland and its adjacent area has been in the seismically active period since 2001, while the seismic activity shows the obvious quiescence of M≥?7.0, M≥?6.0 and M?≥5.0 earthquakes in Chinese mainland. A quiescence area with M?≥7.0 has been formed in the middle of the North-South seismic zone since 1988, and the Wenchuan earthquake occurred just within this area. There are a background seismicity gap of M?≥5.0 earthquakes and a seismogenic gap of M_L?≥4.0 earthquakes in the area of Longmenshan fault zone and its vicinity prior to the Wenchuan earthquake. The seismic activity obviously strengthened and a doughnut-shape pattern of M?≥4.6 earthquakes is formed in the middle and southern part of the North-South seismic zone after the 2003 Dayao, Yunnan, earthquake. Sichuan and its vicinity in the middle of the doughnut-shape pattern show abnormal quiescence. At the same time, the seismicity of earthquake swarms is significant and shows heterogeneity in the temporal and spatial process. A swarm gap appears in the M4.6 seismically quiet area, and the Wenchuan earthquake occurred just on the margin of the gap. In addition, in the short term before the Wenchuan earthquake, the quiescence of earthquake with M_L≥?4.0 appears in Qinghai-Tibet block and a seismic belt of M_L?≥3.0 earthquakes, with NW striking and oblique with Longmenshan fault zone, is formed.     

基于活动地块边界带的测震学参数Mf值对丽江地震的预测应用

地震构造是强震发生的必要基础,强震的孕育和发生与构造密切相关。强震的孕育和发生不仅与震中周围构造有关,而且与孕震区所在的整个构造带有关。这就表明整个构造带的地震活动性与未来强震都有关联。因此,在做测震学参数异常预测地震时,必须考虑整个构造带的地震活动。过去人们常以震中周围地区的地震活动资料来做测震学参数的异常分析,可能会丢失部分信息。文中以金沙江—红河边界带和地震学参量Mf值的结合为例,对基于活动地块边界带的测震学参数强震预测进行了探索,其结果对丽江地震预测效果很好,这对边界带的地震危险性判定有某种参考价值。     

成组中强震前后动态小震的时空活动特征

分析了进入活跃时段以来,新疆及邻近地区自１９９５年３月１９日和硕Ｍｓ５．０地震后,相继发生的１２次Ｍｓ≥４．７地震前后调制小震和动态小震的时空活动特征,据此提出了孕系统失稳模式;     

On the correlation between seismicity characteristics and S-wave attenuation in the ring structures that appear before large earthquakes

We discuss seismicity characteristics in the source zones of two great earthquakes: the December 26, 2004 Sumatra (M_w = 9.0) and the November 14, 2001 Kunlun (M_w = 7.8) events. Ring structures of low magnitude seismicity have been forming prior to these earthquakes for several decades. We studied the short period shear-wave attenuation field in the area of these ring structures. The method we used is based on the analysis of the rate of attenuation for the early Sn and Lg codas to detect attenuation inhomogeneities in the uppermost mantle. We show that the ring structures have comparatively high attenuation of shear waves compared with the crustal volumes inside the rings. The fact that there is no recent volcanism in the area of the seismicity rings shows that this effect is due to a high content of free fluids in the uppermost mantle. Proceeding by analogy with our results, we identified a zone in northern Tien Shan that is anomalous for these parameters; the zone may be related to the precursory process of a large earthquake. We discuss the geodynamic mechanisms that may be responsible for fluid concentration in the seismicity rings.     

Earthquake swarms to the west of Mt Ruapehu preceding its 1995 eruption

Some months prior to the 1995 eruption of Mt Ruapehu (New Zealand), a series of shallow earthquake swarms occurred about 15–20 km west of the summit of Ruapehu. Several earthquakes in these swarms were felt, and the largest event was M_L 4.8. Crustal earthquakes of M_L≥3.0 within 20 km of the summit of Ruapehu have been rather uncommon in recent years. Furthermore, the two periods of strongest activity were both just before times when the temperature of Crater Lake showed rapid increases. The second of these rapid heating phases was immediately followed by increases in the Mg²⁺ ion concentration in Crater Lake, indicating that chemical interactions were occurring between fresh magmatic material and the lake water. The coincidence between seismicity and lake changes suggested a link with the following eruption. A 1-D simultaneous inversion to locate the earthquakes more accurately showed that most of the earthquakes fell into three spatial clusters, each cluster having a small horizontal cross-section. The predominant depth was about 10–16 km. The b-value of this swarm was 0.74, quite compatible with ordinary tectonic earthquakes. Each cluster of earthquakes lies close to the normal Raurimu Fault which runs predominantly north–south to the west of Ruapehu, with an east-trending branch splaying off near its northern end (see Fig. 1b). Composite focal mechanisms of events in the two more southern clusters are oblique-normal, while the other cluster to the north has an oblique-reverse mechanism. The two oblique-normal mechanisms suggest that extension has occurred on part of the fault. This stress pattern was also observed in the focal mechanism solutions of events that occurred after the eruption, when a denser network of portable seismographs covered the region. Although we cannot definitely connect the occurrence of these swarms to the eruptions later in 1995, there is a strong suggestion that the seismicity was connected to the process of magma movement, which temperature and chemical changes in Crater Lake suggest was occurring during the first half of 1995.     

大地震前后区域地震活动聚散及其讨论

本文初步讨论了中国八次大地震(M_s≥7.0)前后区域地震活动的时空变化过程。在这些大地震前后区域地震活动可分为三个阶段:(Ⅰ)地震活动主要分布在远震源区内;(Ⅱ)活动迁移集中到震源区及近震源区内,接着,大地震发生;(Ⅲ)大地震发生之后,特别是早期强余震活动期过后,区域地震活动逐渐扩散到远震源区中去,扩散地点往往是(Ⅰ)阶段中不甚活动的地点。此外,(Ⅱ)阶段中区域地震活动的持续时间,以及(Ⅲ)阶段中地震活动开始向远震源区扩散的时间似乎和大震震级的大小无关。 最后,尝试应用地震包体理论和大震后区域应力场调整的研究结果去理解上述大震前后区域地震活动的聚散过程。 研究大地震前后区域地震活动的特征,是探讨地壳地震的物理力学过程的一个重要途径。石田和金森博雄曾详细评述过强震前地震平静、前兆震群和地震簇集的图象。邱群等注意到唐山大地震前区域地震活动向未来震源区及其附近集中迁移的现象。作者曾指出过我国八次七级以上大地震前皆显示出区域地震活动的迁移过程。大地震发生之后,茂木曾研究过余震活动在震源区内扩散的特征。苏联卡托克曾报导过强震后区域地震活动沿断裂扩散的现象。本文将讨论大地震前后区域地震活动集中和扩散的过程和特征,并应用地震包体理论和应力调整的研究结果,去     

2019年四川长宁MS6.0地震序列重定位和震源特征分析

采用双差定位方法对2019年1月1日至2019年10月20日期间四川区域台网记录到的地震进行重定位,得到7 030个重定位事件,并获得了四川长宁M_S6.0地震序列较准确的空间分布,并据此计算了震后长宁震源区的平均b值,分析了地震序列的活动性;利用近震全波形拟合方法获得了主震及4次M_S≥5.0地震的震源机制解和矩心深度,初步分析了本次地震序列的发震构造,获得如下主要结果:① 四川长宁余震序列呈NW?SE向分布,余震深度分布整体呈现出西深东浅的趋势,且西部地区地震的频度远远高于东部地区;② b值空间分布显示,震后长宁地区呈现出明显的挤压构造环境;③ 主震和4次震级较大余震的矩心深度均较浅,尽管均为逆冲型为主的地震事件,但破裂面走向有所差异;④ 推测主震及中强余震是长宁背斜地区既有断裂或者同震过程中所产生的新生断层长期受到外力挤压而错断所致。