Relocation of aftershocks, focal mechanisms and stress inversion: Implications toward the seismo-tectonics of the causative fault zone of Mw7.6 2001 Bhuj earthquake (India)

The HYPODD relocation of 1172 aftershocks, recorded on 8–17 three-component digital seismographs, delineate a distinct south dipping E–W trending aftershock zone extending up to 35 km depth, which involves a crustal volume of 40 km × 60 km × 35 km. The relocated focal depths delineate the presence of three fault segments and variation in the brittle–ductile transition depths amongst the individual faults as the earthquake foci in the both western and eastern ends are confined up to 28 km depth whilst in the central aftershock zone they are limited up to 35 km depth. The FPFIT focal mechanism solutions of 444 aftershocks (using 8–12 first motions) suggest that the focal mechanisms ranged between pure reverse and pure strike slip except some pure dip slip solutions. Stress inversion performed using the P and T axes of the selected focal mechanisms reveals an N181°E oriented maximum principal stress with a very shallow dip (= 14°). The stress inversions of different depth bins of the P and T axes of selected aftershocks suggest a heterogeneous stress regime at 0–30 km depth range with a dominant consistent N–S orientation of the P-axes over the aftershock zone, which could be attributed to the existence of varied nature and orientation of fractures and faults as revealed by the relocated aftershocks.     

芦山Mw6.6地震序列的震源机制及震源区应力场

为研究2013年4月20日芦山M_w6.6地震的发震构造及孕震机理, 基于4月20日—6月1日地震序列中114次M≥3.0余震震源机制解, 深入分析了余震震源机制及震源区应力场的时空分布特征, 获得的主要认识如下: (1)芦山M≥3.0余震以逆冲型为主, 走滑型次之, 正断型最少, 震源机制P轴方位一致性较好, 以近NWW-SEE为优势方向, 倾角分布在0~30°, 表明余震活动主要受龙门山断裂所在的区域应力场控制; (2)芦山余震区压应力S1方位存在明显的局部空间分区差异, 以主震震中为界, 余震区南边S1方向总体呈NWW方向, 而余震区北边S1方向表现出由NW经EW向NE的逆时针旋转, 可能反映了余震区北边发震断层错动以逆冲为主兼有一定的走滑分量; (3)压应力S1方位随时间的变化不明显, 呈近NWW方向, 但其倾角逐渐变水平, 应力张量方差逐渐变大, 震源机制错动类型始终以逆冲为主, 随时间变的相对紊乱, 反映了震源区应力场随时间的调整变化特性; (4)深度剖面结果显示压应力方位与发震断层走向的夹角在80°~120°, 即近乎垂直, 震源断层面向NW倾斜, 芦山余震活动受控于近垂直发震断裂的挤压作用, 属于典型的逆冲断层.      

Aftershock activity of Bhuj earthquake of january 26th, 2001

Following a large-sized Bhuj earthquake (M _s = 7.6) of January 26th, 2001, a small aperture 4-station temporary local network was deployed, in the epicentral area, for a period of about three weeks and resulted in the recording of more than 1800 aftershocks (-0.07 ≤M _L <5.0). Preliminary locations of epicenters of 297 aftershocks (2.0 ≤M _L <5.0) have brought out a dense cluster of aftershock activity, the center of which falls 20 km NW of Bhachau. Epicentral locations of after-shocks encompass a surface area of about 50 × 40 km² that seems to indicate the surface projection of the rupture area associated with the earthquake. The distribution of aftershock activity above magnitude 3, shows that aftershocks are nonuniformly distributed and are aligned in the north, northwest and northeast directions. The epicenter of the mainshock falls on the southern edge of the delineated zone of aftershock activity and the maximum clustering of activity occurs in close proximity of the mainshock. Well-constrained focal depths of 122 aftershocks show that 89% of the aftershocks occurred at depths ranging between 6 and 25 km and only 7% and 4% aftershocks occur at depths less than 5 and more than 25 km respectively. The Gutenberg-Richter (GR) relationship, logN = 4.52 - 0.89M_L, is fitted to the aftershock data (1.0<-M _L<5.0) and theb-value of 0.89 has been estimated for the aftershock activity.     

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.     

Study of the epicentral trends and depth sections for aftershocks of the 26th january 2001, Bhuj earthquake in Western India

The Geological Survey of India (GSI) established a twelve-station temporary microearthquake (MEQ) network to monitor the aftershocks in the epicenter area of the Bhuj earthquake (M _w7.5) of 26th January 2001. The main shock occurred in the Kutch rift basin with the epicenter to the north of Bhachao village, at an estimated depth of 25 km (IMD). About 3000 aftershocks (M _d ≥ 1.0), were recorded by the GSI network over a monitoring period of about two and half months from 29th January 2001 to 15th April 2001. About 800 aftershocks (M _d ≥ 2.0) are located in this study. The epicenters are clustered in an area 60 km × 30 km, between 23.3‡N and 23.6‡N and 70‡E and 70.6‡E. The main shock epicenter is also located within this zone. Two major aftershock trends are observed; one in the NE direction and other in the NW direction. Out of these two trends, the NE trend was more pronounced with depth. The major NE-SW trend is parallel to the Anjar-Rapar lineament. The other trend along NW-SE is parallel to the Bhachao lineament. The aftershocks at a shallower depth (<10km) are aligned only along the NW-SE direction. The depth slice at 10 km to 20 km shows both the NE-SW trend and the NW-SE trend. At greater depth (20 km–38 km) the NE-SW trend becomes more predominant. This observation suggests that the major rupture of the main shock took place at a depth level more than 20 km; it propagated along the NE-SW direction, and a conjugate rupture followed the NW-SE direction. A N-S depth section of the aftershocks shows that some aftershocks are clustered at shallower depth ≤ 10 km, but intense activity is observed at 15–38 km depth. There is almost an aseismic layer at 10–15 km depth. The activity is sparse below 38 km. The estimated depth of the main shock at 25 km is consistent with the cluster of maximum number of the aftershocks at 20–38 km. A NW-SE depth section of the aftershocks, perpendicular to the major NE-SW trend, indicates a SE dipping plane and a NE-SW depth section across the NW-SE trend shows a SW dipping plane. The epicentral map of the stronger aftershocksM ≥ 4.0 shows a prominent NE trend. Stronger aftershocks have followed the major rupture trend of the main shock. The depth section of these stronger aftershocks reveals that it occurred in the depth range of 20 to 38 km, and corroborates with a south dipping seismogenic plane.     

The 1984 Abruzzo earthquake (Italy): an example of seismogenic process controlled by interaction between differently oriented synkinematic faults

The evolution of the seismogenic process associated with the M_s 5.8 Sangro Valley earthquake of May 1984 (Abruzzo, central Italy) is closely controlled by the Quaternary extensional tectonic pattern of the area. This pattern is characterised by normal faults mainly NNW striking, whose length is controlled by pre-existing Mio–Pliocene N100±10° left-lateral strike-slip fault zones. These are partly re-activated as right-lateral normal-oblique faults under the Quaternary extensional regime and behave as transfer faults.Integration of re-located aftershocks, focal mechanisms and structural features are used to explain the divergence between the alignment of aftershocks (WSW–ENE) and the direction of seismogenic fault planes defined by the focal mechanisms (NNW–SSE) of the main shock and of the largest aftershock (M_s=5.3).The faults that appear to be involved in the seismogenic process are the NNW–SSE Barrea fault and the E–W M. Greco fault. There is field evidence of finite Quaternary deformation indicating that the normal Barrea fault re-activates the M. Greco fault as right-lateral transfer fault. No surface faulting was observed during the seismic sequence. The apparently incongruent divergence between aftershocks and nodal planes may be explained by interpreting the M. Greco fault as a barrier to the propagation of earthquake rupturing. The rupture would have nucleated on the Barrea fault, migrating along-strike towards NNW. The sharp variation in direction from the Barrea to the M. Greco fault segments would have represented a structural complexity sufficient to halt the rupture and subsequent concentration of post-seismic deformation as aftershocks around the line of intersection between the two fault planes.Fault complexities, similar to those observed in the Sangro Valley, are common features of the seismic zone of the Apennines. We suggest that the zones of interaction between NW–SE and NNW–SSE Plio-Quaternary faults and nearly E–W transfer faults, extending for several kilometres in the same way as M. Greco does, might act as barriers to the along-strike propagation of rupture processes during normal faulting earthquakes. This might have strong implications on seismic hazard, especially for the extent of the maximum magnitude expected on active faults during single rupture episodes.     

The 1988 Tennant Creek,northern territory,earthquakes: A synthesis

Three large earthquakes with surface‐wave magnitudes 6.3–6.7 on 22 January 1988 were associated with 32 km of surface faulting on two main scarps 30 km southwest of Tennant Creek in the Northern Territory. These events provide an excellent opportunity to study the mechanics of midplate earthquakes because of the abundance of geological and geophysical data in the area, the proximity of the Warramunga seismic array and the ease of access to the fault zone. The 1988 earthquakes were located in the North Australian Craton in an area that had no history of moderate or large earthquakes before 1986. Additionally, no smaller earthquakes from the fault zone were identified at the Warramunga array, which is situated only 30 km from the nearest scarp, between the 1965 installation of the array and 1986. The main shocks were preceded by a swarm of moderatesized (magnitude 4–5) earthquakes in January 1987 and many smaller aftershocks throughout 1987. Careful relocation of all teleseismically recorded earthquakes from the fault zone shows that the 1987 activity was concentrated in an area only 6 km across in the gap between the two main fault scarps. The main shocks also nucleated in the centre of the fault zone near the 1987 activity. Field observations of scarp morphology indicate that the scarp is divided into three segments, each showing primarily reverse faulting. However, whereas the western and eastern segments show movement of the southern block over the northern, the central scarp segment shows the opposite, with the northern block thrust over the southern block.

Analysis of the first arrival times at Warramunga suggests that the three main shocks were associated with the western, central and eastern scarp segments, respectively. The locations of aftershocks determined using data from temporary seismograph arrays in the epicentral area define three inclined zones of activity that are interpreted as fault planes. In the western and eastern portions of the aftershock zone, these concentrations of activity dip to the south at 45° and 35°, respectively, but in the central section the aftershock zone dips to the north at 55°. Focal mechanisms derived from modelling broadband teleseismic data show thrust and oblique thrust faulting for the three main shocks. The first event ruptured unilaterally up and to the northwest on the westernmost fault segment, while the third main shock ruptured horizontally to the southeast. Modelling of repeat levelling data from the epicentral area requires at least three distinct fault planes, with the eastern and western planes dipping to the south and the central plane dipping to the north. The combination of scarp morphology, aftershock distribution and elevation data makes a strong case for rupture of fault planes in conjugate orientation during the 22 January 1988 Tennant Creek earthquakes. More than 20000 aftershocks have been recorded at Warramunga and activity continues to the present‐day with occasional shocks felt in the town of Tennant Creek and some recent off‐fault aftershocks located directly under the Warramunga seismic array. Stratigraphic relationships exposed in trenches excavated across the scarps suggest that during the Quaternary, a large earthquake ruptured the surface along one segment of the 1988 scarps.     

Rupture mechanism of the 1993 Killari earthquake, India: constraints from aftershocks and static stress change

The Killari earthquake of September 29, 1993 (Mw=6.2) in peninsular India triggered several aftershocks that were recorded by a network of 21 stations. We computed the change in regional static stress caused by coseismic slip on the earthquake rupture and correlated it with the aftershocks with a view to constrain some of the rupture parameters of this earthquake. We evaluated the six available estimates of fault plane solutions for this earthquake and concluded that reverse slip on a 42° dipping, N112° trending fault, which extends up to the surface from a depth of 7 km, produces maximum correlation between the increased static stress and aftershock distribution. Our analysis suggests that the majority of coseismic slip occurred on the part of the rupture that lies in the depth range of 3–6.5 km.     

芦山地震发震构造及其与汶川地震关系讨论

芦山地震发生在巴彦喀拉块体与华南块体之间龙门山推覆构造带南段。野外考察表明,芦山地震在震中区没有形成具有构造地质意义的地震地表破裂带,仅在各山前陡坡地带出现平行于山麓陡坡的张性地裂缝、山地基岩崩塌、滑坡等边坡震动失稳现象和震动引起的砂土液化现象。重新定位的芦山地震余震分布、震源机制解和地表构造地质等分析表明,芦山地震的发震断层为一条现今尚未出露地表、其上断点仍埋藏在地下9 km以下地壳中的一条盲逆断层,走向212°,倾向NW,倾角38°±2°,上断点以上至地表的构造变形符合断层扩展背斜模型。根据汶川地震和芦山地震的余震空间分布、地震破裂过程、深浅构造关系等差异反映出它们是分别发生在龙门山推覆构造带中段和南段的两次独立地震破裂事件。     

Seismicity, deformation and seismic hazard in the western rift of Corinth: New insights from the Corinth Rift Laboratory (CRL)

This paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of Corinth, Greece. The Corinth Rift Laboratory (CRL) is set up near the western end of the rift, where instrumental seismicity and strain rate is highest. The seismicity is clustered between 5 and 10 km, defining an active layer, gently dipping north, on which the main normal faults, mostly dipping north, are rooting. It may be interpreted as a detachment zone, possibly related to the Phyllade thrust nappe. Young, active normal faults connecting the Aigion to the Psathopyrgos faults seem to control the spatial distribution of the microseismicity. This seismic activity is interpreted as a seismic creep from GPS measurements, which shows evidence for fast continuous slip on the deepest part on the detachment zone. Offshore, either the shallowest part of the faults is creeping, or the strain is relaxed in the shallow sediments, as inferred from the large NS strain gradient reported by GPS. The predicted subsidence of the central part of the rift is well fitted by the new continuous GPS measurements. The location of shallow earthquakes (between 5 and 3.5 km in depth) recorded on the on-shore Helike and Aigion faults are compatible with 50° and 60° mean dip angles, respectively. The offshore faults also show indirect evidence for high dip angles. This strongly differs from the low dip values reported for active faults more to the east of the rift, suggesting a significant structural or rheological change, possibly related to the hypothetical presence of the Phyllade nappe. Large seismic swarms, lasting weeks to months, seem to activate recent synrift as well as pre-rift faults. Most of the faults of the investigated area are in their latest part of cycle, so that the probability of at least one moderate to large earthquake (M = 6 to 6.7) is very high within a few decades. Furthermore, the region west to Aigion is likely to be in an accelerated state of extension, possibly 2 to 3 times its mean interseismic value. High resolution strain measurement, with a borehole dilatometer and long base hydrostatic tiltmeters, started end of 2002. A transient strain has been recorded by the dilatometer, lasting one hour, coincident with a local magnitude 3.7 earthquake. It is most probably associated with a slow slip event of magnitude around 5 ± 0.5. The pore pressure data from the 1 km deep AIG10 borehole, crossing the Aigion fault at depth, shows a 1 MPa overpressure and a large sensitivity to crustal strain changes.     

The Bishop's Castle (UK) Earthquake of 2 April 1990

A large earthquake, by British standards, occurred near Bishop's Castle in the Welsh Borders on 2 April 1990 at 13:46 GMT. This magnitude 5.1 ML event was felt over a wide area of Britain, from Ayrshire in the north to Cornwall in the south, Kent in the east and Dublin in the west. The epicentre was near the village of Clun, 7 km SSW of Bishop's Castle. Damage was minor and limited to the epicentral area, north to Wrexham and in particular Shrewsbury, which suffered most. Results from a macroseismic survey by BGS revealed that the maximum intensity in the epicentral area was 6 MSK. The mainshock had a focal depth of 14.3±4.7 km; however, better located aftershocks further constrained the mid-crustal seismicity to 15±0.2 km in the best cases. The marked lack of aftershocks contrasts with some previous similar magnitude events for intraplate earthquakes in Britain and throughout the world and may represent a large stress drop due to almost total relief of strain energy by the mainshock. The aftershock epicentral distribution shows a preference for an approximately N-S orientation which is consistent with one of the focal planes of the mainshock focal mechanism and suggests that this is the fault plane. Movement on this plane was predominantly strike-slip with a component of thrust and was consistent with a maximum compressive stress axis orientated NW-SE. The NE striking Welsh Borderland Fault System dominates the epicentral area; however, there is no surface fault which can clearly be related to the seismicity.     

Evidence for surface rupture associated with the Mw 6.3 L'Aquila earthquake sequence of April 2009 (central Italy)

An earthquake of Mw = 6.3 struck L'Aquila town (central Italy) on 6 April 2009 rupturing an ∼18-km-long SW-dipping normal fault. The aftershock area extended for a length of more than 35 km and included major aftershocks on 7 and 9 April and thousands of minor events. Surface faulting occurred along the SW-dipping Paganica fault with a continuous extent of ∼2.5 km. Ruptures consist of open cracks and vertical dislocations or warps (0.1m maximum throw) with an orientation of N130°–140°. Small triggered slip and shaking effects also took place along nearby synthetic and antithetic normal faults. The observed limited extent and small surface displacement of the Paganica ruptures with respect to the height of the fault scarps and vertical throws of palaeo-earthquakes along faults in the area put the faulting associated with the L'Aquila earthquake in perspective with respect to the maximum expected magnitude and the regional seismic hazard.     

高密度电法在和田隐伏断层探测中的应用

西昆仑山前一系列的新生代背斜的翼部往往伴生向南斜倾的逆断裂,这些断裂隐伏于早更新世或中更新世洪积砂砾石层,构成了和田隐伏断裂带。该断裂带由2条陡坎状断层组成,宽约10.6km,深度20~300m,落差110~270m。根据卫星影像和地质工作成果,和田隐伏断裂的一支从和田市南部穿过,但南郊飞机场附近没有露头,为探查断层在该区域的位置及深度,沿垂直于推测的断层走向布设2条测线,采用温纳（WN）测量系统对其进行探测。探测结果表明：2条测线皆发现了逆断层,均上切错断了上覆晚更新世地层,其中测线1存在2处断层,一处断层倾向南,倾角50°左右,另一处断层倾向北,倾角约55°;测线2存在一处断层,倾向北,倾角近80°。通过比对高密度电法资料和地质出露点剖面,推断和田断层在第四系以来有过两次明显的活动,导致晚更新世砂砾石土层被断错。     

Aftershock Propagation Characteristics During the First Three Hours Following the 26 December 2004 Sumatra-Andaman Earthquake

Within three hours of the mainshock rupture of the 26 December 2004 Sumatra-Andaman earthquake, 45 aftershocks occurred that are distributed all along the mega-thrust fault plane and also along the West Andaman fault. Seven of these aftershocks struck sequentially and unilaterally from the mainshock in the south towards north within 2h 9m 50.76s indicating an overall rate of aftershock propagation to the tune of 167 meters/sec. Seismic moment calculated from fault parameters gives a value of 1.2 × 10³⁰ dyne cm. Three separate fault segments are identified from distribution of aftershocks with propagation rates 330, 250 and 85 meters/sec in the southern, central and northern segments. These 7 unilaterally propagating shocks along the mega-thrust are probably not aftershocks of the mainshock rather these are sequentially triggered shocks each rupturing a small segment of the fault. Location of the mainshock and several aftershocks are guided by several lithospheric hinge faults identified previously.     

额尔古纳地块中生代火山岩盆地基底构造特征:来自灵泉盆地的启示

灵泉盆地布格重力异常特征表明灵泉盆地两侧重力低异常区主要为侏罗-白垩系断陷区,中部主要为基底隆起区.对布格重力异常进行向上延拓处理,结果发现深部地质体具有"东、西深,中间浅"分布特征;剩余重力异常也说明中部局部重力高主要反映基底隆起,东部和西部局部重力低主要反映侏罗-白垩系断陷.灵泉盆地基底断裂早期以北东向为主,晚期发育北西向断裂,区内还有早期近南北向和东西向断裂存在.将灵泉盆地构造单元划分为西部断陷区、中部隆起区和东部断陷区之后发现,灵泉盆地实际上是个相对隆起而不是断陷盆地,这是盆地发生构造反转作用的结果,额尔古纳地块上的其他中生代火山岩盆地普遍具有这种模式,额尔古纳地块中生代盆地基底总体上具有"南深北浅"的特点.      

The Mw = 6.0, 7 September 1999 Athens Earthquake

On 7 September 1999 at 11:56 GMT a destructive earthquake (M_w = 6.0) occurred close to Athens (Greece). The rupture process is examined using data from the Cornet local permanent network, as well as teleseismic recordings. Data recorded by a temporary seismological network were analyzed to study the aftershock sequence. The mainshock was relocated at 38.105°N, 23.565°E, about 20 km northwest of Athens. Four foreshocks were also relocated close to the mainshock. The modeling of teleseismic P and SH waves provides a well-constrained focal mechanism of the mainshock (strike = 105°, dip = 55° and rake = -80°) at a depth of 8 km and a seismic moment M₀ = 1.010²⁵ dyn·cm. The obtained fault plane solution represents normal faulting indicating an almost north-south extension. More than 3500 aftershocks were located, 1813 of which present RMS < 0.1 s and ERH, ERZ < 1.0 km. Two main clusters were distinguished, while the depth distribution is concentrated between 2 and 11 km. Over 1000 fault plane solutions of aftershocks were constrained, the majority of which also correspond to N–S extension. No surface breaks were observed but the fault plane solution of the mainshock is in agreement with the tectonics of the area and with the focal mechanisms obtained by aftershocks. The hypocenter of the mainshock is located on the deep western edge of the fault plane. The relocated epicenter coincides with the fringe that represents the highest deformation observed on the differential interferometric image. The calculated source duration is 5 sec, while the estimated dimensions of the fault are 15 km length and 10 km width. The source process is characterized by unilateral eastward rupture propagation, towards the city of Athens. An evident stop phase observed in the recordings of the Cornet local stations is interpreted as a barrier caused by the Aegaleo Mountain.     

Blind reverse faulting system associated with the Mont Chenoua-Tipaza earthquake of 29 October 1989 (north-central Algeria)

A shallow and damaging earthquake struck the region of Tipaza located on the coast, 70 km to the west of Algiers (north-central Algeria). The main shock was felt as far as 200 km from the epicentral area and particularly in the urbanized zone of Algiers. The main shock epicentre determined by CRAAG is on Mt Chenoua, close to the coastline. Coseismic surface breaks with 4.0 km of fault length and 7.0 cm of vertical displacement appeared on the southern side of Mt Chenoua. They consist of cracks and fissures on vertical bedding planes that belong to a Neogene flexure. The aftershock activity was recorded for several weeks and comprised more than 100 seismic events per day, with 1 < M < 5. Aftershocks are distributed in a ENE-WSW to NE-SW zone extending offshore. A NW-SE cross-section indicates that seismic events affect the crust to a maximum depth of 20 km, showing a complex fault-plane geometry dipping to the NW. For the main shock, the focal mechanism solution obtained from the readings of first motion polarities of seismograms yields an ENE-WSW reverse fault dipping to the NNW, which is in good agreement with both field observations and the aftershock distribution at depth.
The occurrence of the Mt Chenoua-Tipaza moderate-sized event in a previously identified active zone improves the seismotectonic characterization of this part of the Tellan Atlas mountains. This thrust and fold geological domain also shows active folds capable of producing larger earthquakes. Since only a small portion of the Sahel anticline ridge has been reactivated, a serious seismic hazard must be urgently recognized in the Algiers region.     

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.     

Aftershock Propagation Characteristics During the First Three Hours Following the 26 December 2004 Sumatra-Andaman Earthquake

Within three hours of the mainshock rupture of the 26 December 2004 Sumatra-Andaman earthquake, 45 aftershocks occurred that are distributed all along the mega-thrust fault plane and also along the West Andaman fault. Seven of these aftershocks struck sequentially and unilaterally from the mainshock in the south towards north within 2h 9m 50.76s indicating an overall rate of aftershock propagation to the tune of 167 meters/sec. Seismic moment calculated from fault parameters gives a value of 1.2 × 10³⁰ dyne cm. Three separate fault segments are identified from distribution of aftershocks with propagation rates 330, 250 and 85 meters/sec in the southern, central and northern segments. These 7 unilaterally propagating shocks along the mega-thrust are probably not aftershocks of the mainshock rather these are sequentially triggered shocks each rupturing a small segment of the fault. Location of the mainshock and several aftershocks are guided by several lithospheric hinge faults identified previously.     

Estimates of source parameters of <Emphasis Type="Italic">M</Emphasis>4.9 Kharsali earthquake using waveform modelling

This paper presents the computation of time series of the 22 July 2007 M 4.9 Kharsali earthquake. It occurred close to the Main Central Thrust (MCT) where seismic gap exists. The main shock and 17 aftershocks were located by closely spaced eleven seismograph stations in a network that involved VSAT based real-time seismic monitoring. The largest aftershock of M 3.5 and other aftershocks occurred within a small volume of 4 × 4 km horizontal extent and between depths of 10 and 14 km. The values of seismic moment (M _∘) determined using P-wave spectra and Brune’s model based on f ² spectral shape ranges from 10¹⁸ to 10²³ dyne-cm. The initial aftershocks occurred at greater depth compared to the later aftershocks. The time series of ground motion have been computed for recording sites using geometric ray theory and Green’s function approach. The method for computing time series consists in integrating the far-field contributions of Green’s function for a number of distributed point source. The generated waveforms have been compared with the observed ones. It has been inferred that the Kharsali earthquake occurred due to a northerly dipping low angle thrust fault at a depth of 14 km taking strike N279°E, dip 14° and rake 117°. There are two regions on the fault surface which have larger slip amplitudes (asperities) and the rupture which has been considered as circular in nature initiated from the asperity at a greater depth shifting gradually upwards. The two asperities cover only 10% of the total area of the causative fault plane. However, detailed seismic imaging of these two asperities can be corroborated with structural heterogeneities associated with causative fault to understand how seismogenesis is influenced by strong or weak structural barriers in the region.