Aftershocks of 26th january 2001 Bhuj earthquake and seismotectonics of the Kutch region (original) (raw)
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Physics of the Earth and Planetary Interiors, 2018
Seismotectonic features of the Ahar-Varzaghan complex fault system are reviewed by the mainshocks/aftershocks study of the 2012 Ahar-Varzaghan earthquake doublet (Mw=6.5 & Mw=6.3). The early aftershocks were accurately monitored by a dense local seismological network. The mainshocks hypocenters were relocated using a new velocity model for the area. The regional displacement waveforms of the mainshocks and large aftershocks were jointly inverted for their moment tensors and centroids. Precisely located aftershocks are mostly distributed in three distinct clusters. These E-W trending clusters are situated to the north of the surface rupture over a ~30 km long and 10 km wide zone, extending from the ground surface down to the depth of ~15 km. Based on our results, the first mainshock (M1) with Mw=6.5 nucleated 3 km east of the surface rupture at a depth of 10 km. The rupture evolved toward the west and shallower depths on an almost E-W striking rightlateral strike-slip vertical fault plane and produced ~12 km surface rupture. Its main energy was released at about 5 km west of the hypocenter at an average depth of 5 km. About 11 minutes later, the second mainshock (M2) with Mw=6.3 nucleated 5 km northwest of the M1 hypocenter and at a depth of 15 km. It occurred on an ENE-WSW striking, north-dipping (65° to 70°) fault plane with a dominant reverse mechanism and strike-slip component. The M2 related rupture 2 also expanded to the west and to shallower depths and released most of its energy ~5 km west of the respective hypocenter at an average depth of 11 km. The eastern aftershocks mainly show the right-lateral strike-slip mechanism on almost E-W trending fault, close to the location of the first main centroid (M1). The northern aftershocks mostly include reverse mechanisms with strikeslip component near the M2 centroid. A big gap of aftershock activity is observed close to the M1 centroid location, most likely associated with the area of maximum slip. We do not observe any N-S lineament of aftershocks neither near the M2 hypocenter and nor its centroid, thus making the suggestion of some previous investigators that M2 ruptured an N-S trending fault unlikely.
The 2001 Kutch (Bhuj) earthquake: Coseismic surface features and their significance
The Mw 7.7, 2001 Kutch (Bhuj) earthquake that occurred in the northwestern fringes of the Indian craton is the most damaging earthquake during the recent history. Although the main fault rupture did not reach the surface, the epicentral area is characterized by the development of secondary features, including flexures and folds that are related to compressional deformation, in a wide area of the Banni Plain. Based on the spatial distribution of these structures and their inferred mechanics, we propose that the earthquake originated on an imbricate blind thrust, located north of the Kutch mainland fault. Besides surface deformation, the earthquake also induced widespread liquefaction, leading to ground failure including lateral spreading. Although a large earthquake had occurred in the Rann of Kutch in 1819, preliminary assessment based on ancient monuments and temples in the region indicates that the source of the 2001 earthquake may not have experienced similar size events at least since 9th century A.D. Occurrences of this and the 1819 earthquake underscore the need for recognizing hidden faults in the Kutch-Saurashtra region and assessing their seismogenic potential.
Journal of Earth System Science, 2003
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 w 7.5) 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 (< 10 km) 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 aftershocks M ≥ 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.
Characterization of the causative fault system for the 2001 Bhuj earthquake of M w 7.7
Tectonophysics, 2004
Precise hypocenters (ERH < 0.5 km, ERZ < 1.0 km) of 600 aftershocks (M w 2.0 -5.3) delineate a east -west trending blind thrust dipping ( f 45j) towards south (named as North Wagad Fault, NWF), about 25 km north of Kachchh main land fault (KMF), as the causative fault for the 2001 Bhuj earthquake of M w 7.7. The aftershock zone involves a crustal volume of 60 Â 40 Â 35 km 3 , lying between KMF and NWF. The waveform inversion of 45 aftershocks of M 3.0 -5.3 suggest that the E -W trending south-dipping reverse faulting mainly characterizes the aftershock zone; however, some reverse faulting along NW -SE as well as NE -SW planes and some strike-slip faulting along NW -SE trending vertical plane are also noticed. The estimated P-axes point on an average towards N -S, while, T-axes orient in E -W agreeing well with the prevailing regional plate tectonic stress directions. The estimated velocity model from one-dimensional inversion of 8000 P and 5000 S travel times from 600 aftershocks deciphers a detailed crustal structure of the region. Upper 0 -6 km, on an average suggests a low velocity zone characterizing the Jurassic and younger sediments. The depth range 6 -42 km is characterized on an average by larger values of V p (6.31 to 6.98 km/s with an average of 6.71 km/s), V s (3.64 to 4.05 km/s with an average of 3.85 km/s) and V p /V s (1.69 to 1.81 with an average of 1.75). The high crustal velocities can be attributed to the existence of a high velocity mafic intrusive/rift pillow structure beneath the region probably emplaced during the rifting time. This, brittle, competent high velocity zone beneath the epicentral area in response to the compression due to the northward movement of the Indian plate could induce sufficient local stress perturbation for generating large intraplate earthquakes of M w z 7.5 in the lower crust. Further within the overall higher velocity depth range a layer characterized by large values of V p (6.98 km/s), V s (3.854 km/s) and V p /V s (1.81) is found in the depth range of 20.5 -30 km, which is inferred to be fractured and saturated (high crack density and probably fluid-filled). Presence of numerous fractures in this layer is confirmed by a high b-value (0.75 -1.1) in this layer. This layer might have facilitated the nucleation process of the 2001 Bhuj earthquake. The major rupture and 47% of the aftershocks have occurred within this layer. D
Tectonophysics, 2009
We have carried out a detailed analysis of seismicity in the vicinity of the 26 Jan 2001 Bhuj earthquake (Mw 7.7). From the depth sections of 24 parallel profiles, and from the b-value cross section, we claim the existence of a hidden fault which conjugates to the major rupture fault (i.e. North Wagad Fault) of the 2001 Bhuj earthquake. The proximity of the intersection of these faults to the focus suggests a close association Bhuj main shock generation. A circular pattern in the profiles also provides evidence for the existence of an intrusive, consistent with earlier findings from gravity-magnetic modeling and tomography studies. The location of the fault intersection within the intrusive support a model where both play a significant role in the earthquake generation. The intersection of the conjugate faults acts as a stress concentrator, while their presence within a big pluton possibly will facilitate the stress amplification. This mechanism might explain the occurrence of two Mw = 7.7 earthquakes in a relatively short time span of 182 years in the Kachchh rift. The b-value cross section displays high and low b-value patches along the two intersecting faults. This suggests a model of a faulted block that consists of two kinds of segment, the locked and the unlocked. Locked segments do not easily participate in creeping and therefore generate strong magnitude aftershocks (M N 3) while unlocked segments easily creep and result in only weak aftershocks (M b 3). The different fault segments with weak and strong magnitude gathers will result in high and low b-values, respectively.
Geophysical Journal International, 2015
The Ahar-Varzghan doublet earthquakes with magnitudes M W 6.5 and 6.4 occurred on 2012 August 11 in northwest Iran and were followed by many aftershocks. In this paper, we analyse ∼5 months of aftershocks of these events. The Ahar-Varzghan earthquakes occurred along complex faults and provide a new constraint on the earthquake hazard in northwest Iran. The general pattern of relocated aftershocks defines a complex seismic zone covering an area of approximately 25 × 10 km 2 . The Ahar-Varzghan aftershock sequence shows a secondary activity which started on November 7, approximately 3 months after the main shocks, with a significant increase in activity, regarding both number of events and their magnitude. This stage was characterized by a seismic zone that propagated to the west of the main shocks. The catalogue of aftershocks for the doublet earthquake has a magnitude completeness of M c 2.0. A below-average b-value for the Ahar-Varzghan sequence indicates a structural heterogeneity in the fault plane and the compressive stress state of the region. Relocated aftershocks occupy a broad zone clustering east-west with near-vertical dip which we interpret as the fault plane of the first of the doublet main shocks (M W 6.5). The dominant depth range of the aftershocks is from 3 to about 20 km, and the focal depths decrease toward the western part of the fault. The aftershock activity has its highest concentration in the eastern and middle parts of the active fault, and tapers off toward the western part of the active fault segment, indicating mainly a unilateral rupture toward west.
The 2003 December 26 Bam earthquake (Iran), Mw 6.6, aftershock sequence
Geophysical Journal International, 2005
From 2003 December 29 to 2004 January 30, a dense seismological network of 23 stations was installed in the epicentral area of the 2003 December 26 Bam earthquake to study the aftershock seismicity. We select the 331 earthquakes recorded at a minimum of 10 stations, with rms less than 0.1 s and uncertainties less than 1 km, to infer the precise geometry of the seismicity in the fault region. We also process the data with the Double Difference technique to confirm the results. The aftershock cluster is 25 km long, trends north–south, and is located 5 km west of the Bam-Baravat escarpment, exactly beneath the observed surface breaks. At depth, aftershocks are concentrated between 6 and 20 km, beneath the upper layer of relatively low velocity that experienced the maximum slip, and they dip slightly westward. The southernmost part of the aftershock cluster is narrow and defines the rupture zone that is likely the Bam-Baravat fault at depth. However, it is unlikely that it is connected at surface to the Bam-Baravat escarpment but more likely to the co-seismic ruptures south of Bam. On the contrary, the distribution of the northernmost aftershocks spread into a more complex pattern, which is consistent with a northward propagation of the rupture along the fault plane. The focal mechanisms are consistent with right-lateral strike-slip faulting on N–S trending faults, parallel to the Bam-Baravat escarpment.
Journal of Geophysical Research: Solid Earth, 2002
We analyze the rupture history of this earthquake, the largest thrust earthquake since 1977, primarily using broadband SH wave seismograms. We show that the earthquake occurred on a shallow dipping thrust fault (strike 109°, dip 9°, rake 72°) with a seismic moment of 2.7  10 21 N m. The rupture propagated bilaterally at an average speed of 9090% of the shear wave speed, on a fault extending 180 km west and 50 km east of the hypocenter, with very variable width ranging from 30 to 100 km at different locations along strike. The mean slip over a 230 km  100 km fault area was 4 m and the mean stress drop 1.9 MPa. The slip distribution is very nonuniform over the fault, with the largest slip of 9012 m being near the hypocentral depth ($10 km). The rupture is very complex, propagating first to the west and then, after a $15-s delay, to the east. We interpret this delay as being due to the existence of a inhomogeneous barrier just east of the hypocenter, which initially acted as a barrier to rupture propagation toward the east, but subsequently failed due to stress increase on it generated by the rupture to its west and with the rupture then continuing to propagate eastward. This barrier acted as a stress concentrator before the earthquake, giving rise to foreshocks and was also the initiator of rupture for this earthquake. The aftershock zone closely corresponds to the region in which rupture occurred, the area with greatest aftershock density lying entirely within the area of highest moment release.
Journal of Geophysical Research, 1980
Relocations of earthquakes, recorded by a local network of stations in Afghanistan and Tadjikistan in 1966 and 1967, indicate a narrow seismic zone (width • 30 kin) dipping steeply into the mantle to a depth of 300 km beneath the Pamir and Hindu Kush ranges. Very low seismicity was observed at depths less than about 70 kin, the approximate depth of the Moho. Clear gaps in activity exist also within the zone of intermediate depth seismicity. One gap, about 50 km wide near 37øN and at depths greater than lO0 kin, separates a steeply northward dipping zone to the southwest from a steeply southeastward dipping zone to the northeast. This gap probably marks either a tear in the downgoing slab or a gap between two oppositely dipping slabs. Fault plane solutions, determined by Soboleva for events between 1960 and 1967, generally show steeply plunging T axes approximately within the planar seismic zone. They therefore are grossly similar to those at island arcs where no deep earthquakes occur and presumably result from gravitational body forces acting on a relatively dense slab of lithosphere. At the same time there is a very large variation in the fault plane solutions, much larger than is common at island arcs. Introduction Although it does not have an island arc structure, the Pamir-Hindu Kush region is the source of very high intermediate depth seismicity. This region is one of the most active sources of earthquakes felt within the USSR, even though most of it lies outside of the USSR, in Afghanistan. Accordingly, Soviet seismologists have devoted considerable attention to its study. An extensive network of stations has been operated in Tadjikistan for 20 years by the Tadjik Institute of Seismo-Resistant Construction and Seismology (TISSS) of the Academy of Sciences of the Tadjik SSR and by the Institute of Physics of the Earth (IFZ) of the Academy of Sciences of the USSR (in Moscow). Moreover, in 1966 and 1967 a special network was installed in Afghanistan and along the Soviet-Afghan boundary by the IFZ to study the seis-1966 and 1967 allowed the most precise determinations of hypocenter that were possible at that time [Lukk and Nersesov, 1970]. These hypocenters defined an approximately planar zone that dips steeply into upper mantle and extends in an east-west direction for nearly 700 km. With careful analytical and graphical techniques, but without the aid of high-speed computers, Lukk and Nersesov [1970] simultaneously determined a velocity structure for the crust and upper mantle and located the earthquakes. In the present paper we extend their study and present relocations of these same events using a computer. The data obtained with this network were also used to infer a high-velocity zone surrounding Massachusetts 02139.
Pure and Applied Geophysics, 2006
We employed layered model joint hypocentral determination (JHD) with station corrections to improve location identification for the 26 January, 2001 Mw 7.7 Bhuj early and late aftershock sequence. We relocated 999 early aftershocks using the data from a close combined network (National Geophysical Research Institute, India and Center for Earthquake Research Institute, USA) of 8–18 digital seismographs during 12–28 February, 2001. Additionally, 350 late aftershocks were also relocated using the data from 4–10 digital seismographs/accelerographs during August 2002 to December 2004. These precisely relocated aftershocks (error in the epicentral location<30 meter, error in the focal depth estimation < 50 meter) delineate an east-west trending blind thrust (North Wagad Fault, NWF) dipping (~ 45°) southward, about 25 km north of Kachchh main land fault (KMF), as the causative fault for the 2001 Bhuj earthquake. The aftershock zone is confined to a 60-km long and 40-km wide region lying between the KMF to the south and NWF to the north, extending from 2 to 45 km depth. Estimated focal depths suggest that the aftershock zone became deeper with the passage of time. The P- and S-wave station corrections determined from the JHD technique indicate that the larger values (both +ve and -ve) characterize the central aftershock zone, which is surrounded by the zones of smaller values. The station corrections vary from −0.9 to +1.1 sec for the P waves and from −0.7 to +1.4 sec for the S waves. The b-value and p-value of the whole aftershock (2001–2004) sequences of Mw ≥ 3 are estimated to be 0.77 ± 0.02 and 0.99 ± 0.02, respectively. The p-value indicates a smaller value than the global median of 1.1, suggesting a relatively slow decay of aftershocks, whereas, the relatively lower b-value (less than the average b-value of 1.0 for stable continental region earthquakes of India) suggests a relatively higher probability for larger earthquakes in Kachchh in comparison to other stable continental regions of the Indian Peninsula. Further, based on the b-value, mainshock magnitude and maximum aftershock magnitude, the Bhuj aftershock sequence is categorized as the Mogi's type II sequence, indicating the region to be of intermediate level of stresses and heterogeneous rocks. It is inferred that the decrease in p-value and increase in aftershock zone, both spatially as well as depth over the passage of time, suggests that the decay of aftershocks perhaps could be controlled by visco-elastic creep in the lower crust.