Modelling the pore fluid diffusion process in aftershock initiation for 2004 Sumatra earthquake: implications for marine geohazard estimation in the Andaman region

The role of fluid injection on the occurrence and migration path for the aftershocks of 2004 Sumatra earthquake (Mw 9.3) and January 2005 Andaman earthquake swarm within the aftershock sequence is investigated here from the viewpoint of pore fluid diffusion process. The Sumatra earthquake created a regionally extensive crustal rupture plane exceeding 1,200 km length below the Andaman Sea. The r–t plots (Shapiro et al. 1997) are constructed for these aftershocks in order to examine the role of poroelastic effects as rupturing progressed with time. Their main results are as follows: the r–t plot corresponding to first 3 h of aftershock activity (when only 44 events of mb ≥ 4.5 originated) reveals that 95% of the data points occurred below the modelled parabola with relatively high D value of 20 m²/s, whereas a significantly low D value of 3.5 m²/s characterises the aftershock activity for the first 24 h (when 420 events of mb ≥ 4.0 occurred). Here, the Coulomb stress was transferred from the main shock with a rapid imposition of normal stress, thus inducing the pore-pressure change that started diminishing almost immediately by fluid diffusion, at a rate, defined by the diminishing D value. The modelling results for fault seismicity at far off distances from the main epicentre are interpreted here as potential indicators for large-scale sub-seabed rupturing—consequent to stress changes induced by bending of the Indian Ocean plate. Bathymetric slopes under the Andaman subduction zone are particularly amenable to sub-marine slides where crustal E–W hinge faults inferred seismically cut across the N–S trending regional thrust and strike-slip faults. Seabed rupturing appears to allow deep-slab hydration in these areas, producing pressure gradients along the normal faults. These features are important since they can herald marine geohazards in the Andaman region.     

The Gornozavodsk earthquake of August 17(18), 2006, in the south of Sakhalin Island

The August 17 (18), 2006, Gornozavodsk earthquake (M_w = 5.6) in the southwestern part of Sakhalin was preceded by a number of anomalous seismological and geophysical phenomena. The extensive data recorded by a network of digital seismic stations make it possible to track the aftershock dynamics of the process within 24 hours after the main event. The paper describes various manifestations of the earthquake.     

Prediction of strong earthquake within the southwestern shelf of Sakhalin Island and its realization during the August 2, 2007, Nevelsk earthquake

Materials of the long- and short-term predictions of the destructive earthquake with the magnitude M _LH = 6.6 ± 0.6 within the southwestern shelf of Sakhalin Island are described. The long-term prediction was issued in December 2005 and was affirmed by the Russian Council of Experts on Earthquake Forecasting and Seismic Hazard Assessment in August 2006. The August 17(18), 2006, Gornozavodsk earthquake with a magnitude of M _w = 5.6 was the beginning of the realization of this prediction. Six days after its occurrence, the short-term prediction of a much more serious seismic event in the alarm region was prepared. One year later, the prediction of the August 2, 2007, Nevelsk earthquake with a magnitude of M _w = 6.2 (M _LH = 6.2) proved to be correct.     

The Ms = 7.0 Uureg Nuur earthquake of 15.05.1970 (Mongolian Altai): the aftershock process and current seismicity in the epicentral area

The aftershock process induced by the Ms = 7.0 Uureg Nuur earthquake, one of the largest events in the Altai, has been studied comprehensively. As an additional experiment, a temporary local network of seismic stations was deployed in 2006 in the epicentral area of the earthquake to gain more insights into the current tectonic activity. The aftershocks of the Uureg Nuur event were restricted to small faults in the interior of fault blocks rather than those being localized along border faults. Seismic activity across the directions of large faults has apparently been generated by a fault (in the Tsagaan Shuvuut Range) reactivated during the Uureg Nuur earthquake. The aftershock process, at its final phase, involved an adjacent crust block.     

Source parameters of the 2001 Mw 7.7 Bhuj earthquake,Gujarat, India,aftershock sequence

We present the estimated source parameters from SH-wave spectral modeling of selected 463 aftershocks (2002–06) of the 26 January 2001 Bhuj earthquake, the well-recorded largest continental intraplate earthquake. The estimated seismic moment (M_o), corner frequency (f_c), source radius (r) and stress drop (Δσ) for aftershocks of moment magnitude 1.7 to 5.6 range from 3.55×10¹¹ to 2.84×10¹⁷ N-m, 1.3 to 11.83 Hz, 107 to 1515 m and 0.13 to 26.7 MPa, respectively, while the errors in f_c and Δσ are found to be 1.1 Hz and 1.1 MPa, respectively. We also notice that the near surface attenuation factor (k) values vary from 0.02 to 0.03. Our estimates reveal that the stress drop values show more scatter (Mo^{0.5 to 1} is proportional to Δσ) toward the larger M_o values (≥10^14.5 N-m), while they show a more systematic nature (Mo³ is proportional to Δσ) for smaller M_o values (<10^14.5 N-m), which can be explained as a consequence of a nearly constant rupture radius for smaller aftershocks in the region. The large stress drops (= 10 MPa) associated with events on the north Wagad fault (at 15–30 km depth) and Gedi fault (at 3–15 km depth) can be attributed to the large stress developed at hypocentral depths as a result of high fluid pressure and the presence of mafic intrusive bodies beneath these two fault zones.     

On the aftershock sequence of a 4.6 mb earthquake of the Garhwal Himalaya

Locally recorded data for eighteen aftershocks of a magnitude(m_b) 4.6 earthquake occurring near Ukhimath in the Garhwal Himalaya were analysed. A master event technique was adopted to locate seventeen individual aftershock hypocentres relative to the hypocentre of the eighteenth aftershock chosen as the master event. The aftershock epicentres define an approximately 30 km² rupture zone commensurate with the magnitude of the earthquake. The distribution of epicentres within this zone and the limited amount of first motion data support the view that a group of parallel, sub-vertical, sinistral strike-slip faults oriented N46°, transverse to the regional NW-SE trend of the Garhwal Himalaya, was involved in this seismic episode. Since the estimated focal depth range for aftershocks of this sequence is 3–14 km, we infer that this transverse fault zone extends through the upper crustal layer to a depth of 14 km at least.     

Deformation precursors of the Ust-Barguzin earthquake of May 20, 2008

Doklady Earth Sciences -     

Spatial variation of aftershock activity across the rupture zone of the 17 August 1999 Izmit earthquake, Turkey

We examined the spatial variation in the aftershock activity from the 17 August 1999 Izmit, Turkey earthquake. We found that this aftershock sequence is non-uniform both in space and time, aspects that need to be taken into account in any further statistical analysis. Other aspects of this aftershock sequence are similar to other aftershock sequences, namely low b-values and a high degree of spatial variation. We have detected three zones of relatively high b-values, two of which coincide with asperities revealed by previous slip inversion studies. The third zone with an anomalous b-value is located beyond the fault rupture and indicates a weakened fractured zone in the Yalova-Tuzla area. This b-value analysis provided no evidence for any significant difference that may exist between the two sides of the mainshock fault plane.     

Prediction of the 6.6 Grevena-Kozani earthquake of May 13, 1995

Seismic Electric Signals (SES) were recorded by VAN-group on April 18–19, 1995, at Ioannina station; they resulted in an official prediction that was sent (two weeks before the earthquake occurrence) to the Greek authorities as well as to various International Institutes.The observation of these electrical variations was confirmed by Gruszow et al. (1996); however, they claim that these signals could be attributed to a (non determined) nearby artificial source with huge intensity (IL≈4 × 10⁴Am, for r ≈ 2 km. or 1.6 × 10⁵Am, for r ≈ 4 km). This claim is not valid, because, such an artificial source (cf. horizontal point current dipole) should have produced: (a) electrical field variations having amplitudes two orders of magnitude, larger than the observed ones; this is theoretically shown and experimentally verified and (b) magnetic field variations mainly on the horizontal field, while, in the present case, they have been recorded mainly on the vertical component.Furthermore, we show that the above SES obey the criteria, suggested by Varotsos and Lazaridou (1991), for discriminating SES from noise.     

The Dibra (Albania) earthquake of November 30, 1967

On November 30, 1967, a strong earthquake of magnitude M = 6.6 struck the Dibra region, eastern Albania, causing considerable loss of human life and grave material damage both in the territory of Albania and that of Yugoslavia.The object of this study is to describe the effects of this earthquake on landscape and buildings, as well as to define its macroseismic field. The study further deals with some features of the aftershocks of M 4.0 distributed in time and space, the aftershock activity and the focal-mechanism solution of the main event.From the study of the macroseismic field of this earthquake and its fault, which extends over 10 km in a 40° northeasterly direction, from the distribution of aftershocks in space and the focal-mechanism solution of this earthquake, the conclusion has been reached that this event is connected with the Vlora—Dibra seismogenic belt.The authors have mentioned the existence of this traverse belt as early as 1969 (Sulstarova and Koçiaj, 1969). The existence of this belt is also shown by the chronological and geographical distribution of some strong earthquakes in Albania in the period 1800–1967 (their macroseismic field and the position of their epicentres), and by the focal-mechanism solutions of some of these earthquakes. The Vlora—Elbasan—Dibra transverse seismogenic belt continues for several hundred kilometres northeast and southwest beyond the territory of Albania.     

2004年12月26日苏门答腊-安达曼大地震构造特征及地震海啸灾害

2004年12月26日在印度尼西亚苏门答腊岛西侧海域发生的地震是自1964 年阿拉斯加大地震以来最大的地震,震级达到9级或9级以上。它是由印度洋板块向缅甸微板块底下俯冲过程中的逆断层作用造成的。印度洋板块以每年6~7 cm的速率向北北东方向运动,与南亚板块发生斜向聚敛俯冲,此运动在该地区解耦为印度洋板块沿巽他海沟的正向俯冲及缅甸微板块东侧的右旋走向平移运动。主震破裂模型研究的结果表明,破裂是由南向北传播的,地震破裂带长达1 200余km,宽度约100 km,最大位移约为20 m,地震断层向上穿透海沟底面,估计约有10 m左右的错距。这次大地震的同震效应导致地球自转轴摆动、地球自转加速,日长缩短。据目前统计,地震引发的大海啸造成305 276人死亡,被此次海啸夺走生命的人数超过了有史以来历次大海啸灾难中死亡人数的总和。     

5.12汶川8级地震次生地质灾害的基本特征及其形成机制浅析

5.12汶川8级大地震沿龙门山断裂带形成长350多km,宽约50 km的地表破裂带,触发了1万多处崩塌、滑坡、泥石流(碎屑流)地质灾害,其中巨型灾害体87处、大型灾害体606处,形成了136个较大规模的堰塞湖。地震地质灾害的链生特征显著,形成地震-崩塌、地震-滑坡-碎屑流-堰塞湖-堰塞坝溃决-泥石流等典型地质灾害链。地震次生地质灾害具有分布范围广、数量多、种类全、密度大、强度高、致灾重的特点。在部分地区,崩塌、滑坡和碎屑流的分布面积占地震极重灾区面积的30%~58％,甚至高达80％。据初步统计,崩塌、滑坡和碎屑流共导致大约2万人死亡,其中北川县老县城滑坡导致1 600多人死亡。地震次生地质灾害主要沿断裂带、河谷和交通线分布。崩塌、滑坡的破裂源主要位于河流拐弯处靠近侵蚀岸一侧、山脊两侧及坡肩部位,这与上述部位对地震动峰值加速度的放大作用直接相关。地震次生地质灾害主要受地震动峰值加速度和地形控制,其次为岩性、斜坡结构、活动断裂、人类工程活动。许多大型崩塌、滑坡还具有高速远程的特征,部分崩塌、滑坡 碎屑流位移达数km,速度高达100~300 m/s,其运动轨迹复杂多变,常常导致多处人员伤亡,是高山峡谷地区地质灾害风险评估和减灾防灾必须面临的新课题。根据上述情况,文中对汶川地震次生地质灾害的基本特征、分布规律和主要影响因素进行了初步总结,并对地震滑坡的形成机制和运动模式进行了初步探讨。首次提出高山峡谷地区单一斜坡上呈阶梯状多级滑动的群发性地震滑坡的形成模式:强烈地震往往引起剧烈的地面震动,而高陡的山脊及其坡肩部位对地震波具有明显的放大作用,因此,上述部位往往是地震滑坡的高易发地段,当地震动峰值加速度超过不稳定性斜坡的临界峰值加速度时,斜坡失稳破坏形成一系列的群发性滑坡,从上到下往往形成阶梯状多级滑动的滑坡群,此种模式适用于残坡积层、风化层地震滑坡和主滑面较缓的地震基岩滑坡。最后,指出了今后应重点研究的科学问题,并对防灾减灾措施提出了一些建议。     

Investigation of earthquake recurrence networks: the cases of 2014 and 2015 aftershock sequences in Ionian Islands,Greece

Natural Hazards - The investigation of earthquake recurrence networks that were constructed from two aftershock sequences in Greece, is performed, aiming to detect whether the structure of networks...