Seismic imaging of the aftershock zone of the 2001 Mw 7.7 Bhuj earthquake, India (original) (raw)
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Bulletin of the Seismological Society of America, 2004
To comprehend the source processes of the 26 January 2001 Bhuj earthquake sequence of M W 7.7 and its influence on the seismic hazard of the Kachchh, we estimate various seismological parameters using reliable and accurate aftershock data. The estimated parameters led to several important findings including the delineation of an east-west-trending, south-dipping (Ϸ45Њ) fault (North Wagad fault [NWF]), which touches the surface about 25 km north of Kachchh Mainland fault (KMF). 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 10 to 45 km depth. Focal mechanism solutions of the mainshock and 25 significant aftershocks of M W Ն3.0 obtained from waveform inversion of broadband data and local earthquake moment tensor inversion suggest that the region between the KMF and NWF is mainly characterized by reverse faults with east-west trend and southerly dip, matching with the geological faults in the region. The tomographic inversion technique is used to invert 5516 P-wave travel times and 4061 S-P travel-time differences from 600 aftershocks recorded at 8-18 stations. Tomographic results suggest a regional high-velocity body (characterized by high V p [7.0-8.5 km/sec], high V s [4.0-4.8 km/sec], and low r [0.24-0.26]) with a head extending 60 km in north-south and 40 km in east-west at 10-40 km depths. This high-velocity anomaly is inferred to be a mafic pluton/rift pillow, which might have intruded during the rifting time (ϳ135 Ma). This crustal mafic pluton must be contributing significantly in accumulating large crustal stresses resulting in the generation of large earthquakes in this intraplate area. Another important result of our study is the detection of a low-velocity zone (low V p [6.5-7.0 km/sec], low V s [3.6-4.0 km/sec], large r [0.26-0.265]) within the mafic body at the hypocentral depth of the mainshock (ϳ18-25 km), which is inferred to be a fluid-filled (trapped aqueous fluid resulting from metamorphism) fractured rock mass. The analysis of depth distribution of b-values suggests a high b-value zone between 15 and 25 km depths, which further supports this contention. Hence, the presence of fluids at the hypocenter might have facilitated the occurrence of the 2001 Bhuj earthquake within the inferred mafic body in the lower crust.
Geophysical Research Letters, 2002
earthquake (Mw 7.6) is one of the most catastrophic Indian earthquakes. We have investigated the 3-D seismic velocity and Poisson's ratio structures of the Bhuj source area to understand the probable cause of triggering the earthquake. We used 1948 P and 1865 S-wave high-quality arrival times from 331 aftershocks recorded at a temporary seismic network. Significant variations up to 5% in velocity and 10% in Poisson's ratio are revealed in the aftershock area. The mainshock is located in a distinctive zone characterized by high-Vp, low-Vs and high Poisson's ratio (s) in the depth range of 20 to 30 km and extending 15 to 30 km laterally. This feature is very similar to that of the 1995 Kobe earthquake . The anomaly may be due to a fluid-filled, fractured rock matrix, which might have contributed to the initiation of the Bhuj earthquake.
Three-dimensional velocity imaging of the Kachchh seismic zone, Gujarat, India
Tectonophysics, 2008
To understand the causative mechanism of the continued occurrence of earthquakes in Kachchh, Gujarat for the last six years, we estimated high-resolution three-dimensional Vp, Vs and Vp/Vs structures in the aftershock zones of the 2001 Mw7.7 Bhuj and 2006 Mw5.6 Gedi earthquakes. We used 13,862 P-and 13,736 S-wave high-quality arrival times collected from the seismograms of 2303 aftershocks recorded at 5-18 three-component seismograph stations during 2001-06. Seismic images revealed a marked spatial variation in the velocities (from −20% to + 14% in Vp, from −12% to 13% in Vs, and from −12% to 12% increase in Vp/Vs) in the 0-34 km depth range beneath the Bhuj aftershock zone. Relatively more increase in Vp than Vs, resulting in an increase in Vp/Vs in the crust beneath the seismically active causative fault (North Wagad Fault, NWF) zone of 2001 Bhuj mainshock suggests a rigid, mafic crust beneath the region. They also delineate an increase of 8% in Vp and 14% in Vs, and a decrease of 4% in Vp/Vs in the almost vertical rupture zone of the 2006 Gedi earthquake extending up to 12 km depth. This high velocity body associated with the Gedi mainshock is inferred to be a gabbroic intrusive. The Banni region and the Wagad uplift are found to be associated with high velocity intrusive bodies (inferred to be mafic) extending from 5 to 35 km depth, which might have intruded during the rifting in early Jurassic (~160 Ma). Aftershock activity is mainly confined to the zones characterized by high Vp, high Vs and low Vp/Vs ratio, which might be representing the strong, competent and brittle parts of the fault zone/intrusive bodies that could accumulate large strain energy for generating aftershocks for more than six years. It is inferred that the crustal stress concentrations associated with the intrusive bodies are contributing significant perturbation to the crustal stress regime to generate the intraplate earthquakes in the Kachchh rift zone. A few patches of slow (Vp and Vs) and high Vp/Vs between 10 to 30 km depth have also been detected on the causative 45°south dipping north Wagad fault (NWF) for the 2001 mainshock, which may be attributed to the fluid filled fractured rock matrix. Interestingly, the 2001 Bhuj mainshock hypocenter is found to be associated with such a low velocity patch. Thus, it is inferred that the occurrence of 2001 Bhuj mainshock might have facilitated by the presence of fluids at hypocentral depth.
Journal of Asian Earth Sciences, 2020
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Variations of seismic velocities in the Kachchh rift zone, Gujarat, India, during 2001–2013
Tectonophysics, 2016
We herein study variations of seismic velocities in the main rupture zone (MRZ) of the Mw 7.7 2001 Bhuj earthquake for the time periods [2001-05, 2006-08, 2009-10 and 2011-13], by constructing dVp(%), dVs(%) and d(Vp/Vs)(%) tomograms using high-quality arrival times of 28,902 P-and 28,696 S-waves from 4644 precise JHD (joint hypocentral determination) relocations of local events. Differential tomograms for 2001-05 reveal a marked decrease in seismic velocities (low dVp, low dVs and high d(Vp/Vs)) in the MRZ (at 5-35 km depths) during 2001-10, which is attributed to an increase in crack/fracture density (higher pore fluid pressure) resulted from the intense fracturing that occurred during the mainshock and post-seismic periods. While we observe a slight recovery or increase in seismic velocities 2011-13, this could be related to the healing process (lower pore fluid pressure due to sealing of cracks) of the causative fault zone of the 2001 Bhuj mainshock. The temporal reduction in seismic velocities is observed to be higher at deeper levels (more fluid enrichment under nearlithostatic pressure) than that at shallower levels. Fluid source for low velocity zone (LVZ) at 0-10 km depths (with high d(Vp/Vs)) could be attributed to the presence of meteoric water or soft alluvium sediments with higher water content, while fluid source for LVZ at 10-35 km depths could be due to the presence of brine fluids (released from the metamorphic dewatering) and volatile CO 2 (emanating from the crystallization of carbonatite melts in the asthenosphere), in fractures and pores. We also imaged two prominent LVZs associated with the Katrol Hill fault zone and Island Belt fault zone, extending from shallow upper-crust to sub-crustal depth, which might be facilitating the deeper circulation of metamorphic fluids/volatile CO 2 , thereby, the generation of lower crustal earthquakes occurring in the Kachchh rift zone.
Geophysical Journal International, 2012
We provide a new hypothesis for the deep subsurface structures near the Bhuj 2001 earthquake region based on magnetotelluric (MT) investigations carried out close to the epicentre zone. 2-D inversion of broad-band MT data of two profiles of lengths 32 km (AA) and 52 km (BB) revealed a thick (∼3 km) highly conductive (1-4-m) surface layer of fluviomarine Mesozoic-Cenozoic sediments. The models delineate the hypocentre zone located at ∼20-25 km depth that manifests the high resistivity-conductivity transition zone. The accumulation of compressive stresses post-rifting along this weak zone has resulted in the reverse slip of Bhuj 2001 earthquake. The reverse fault (F 1) associated with the earthquake is believed to be an ancient normal fault formed during the rifting phase. Contrary to earlier suggested theories, we suggest that F 1 got initiated along the high resistivity-conductivity transition zone causing the Bhuj 2001 event. The geoelectric models revealed a laterally extending partially resistive zone at 20-30 km depth range showing a tendency to extend further deep. Model calculations using synthetic data also support this observation. Therefore, we hypothesize the presence of a basal detachment, marking the transition zone between the continental crust and the lithospheric upper mantle at ∼40 km depth, intersected by the F 1. The geoelectric models suggest that the crustal thinning caused the asthenospheric upwelling and/or serpentinization leading to the ascent of volatiles and melts. The subsurface geometry in Kachchh basin suggests the thickskinned deformation.
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
Seismic imaging of the Proterozoic Cuddapah basin, south India and regional geodynamics
Cuddapah basin has been believed to be one of the largest intra-cratonic Proterozoic sedimentary basins of India situated in the eastern part of the Dharwar craton of the south Indian shield, which is magmatically infested and contain thick column of sediments. Its overall sedimentary thickness as well as the nature of crustal evolution has remained enigmatic. Based on deep seismic sounding and other geological studies in the past, it was perceived that this basin may contain as much as 10-12 km thick sediments. The results of our present analysis derived up to a depth of ∼12 km reveals five layered upper crust associated with velocities (i) 4.50 km/s, (ii) 5.20-5.30 km/s, (iii) 5.50-5.80 km/s, (iv) 5.85-6.00 km/s, and (v) 6.40 km/s, out of which second and third layers correspond to upper and lower Cuddapah sediments. The results suggest the presence of only 4.0 km thick sediments in the deepest part of the basin below the Nallamalai fold belt, which has its implications in the developmental history of the basin. A thermal driving force was invoked by earlier workers to account for estimated 10-12 km thick sediments. However, the present estimate of only 4 km basement depth in the Cuddapah basin shows that the role of the thermal driving force may be marginal, particularly in the deeper eastern Cuddapah, as isostatic subsidence due to sedimentary accumulation alone is enough to explain the basin depth. Further, a basement sag of about 10 km would have logically needed lateral extension of the order of several hundreds of kilometers. However, as our present estimate of the sediment thickness gets reduced from earlier 10 km to only 4 km, the size of the basin (44,500 km 2 ) would be in conformity with isostatic subsidence due to sedimentary accumulation hypothesis. The structural features derived from present analysis like maximum depth observed near the thrust/suture on the basin margin from where it decreases away from it, its association of shallow marine sediments, the arcuate shape of the basin along with its areal dimension resembles foreland basin between continent-continent collisions.
Journal of Asian Earth …, 2012
a b s t r a c t During the 1st decade of the 21st century, the study area of Talala, Saurashtra of western India witnessed three damaging earthquakes of moderate magnitude, year 2007 [Mw 5.0; Mw 4.8] and in the year 2011 [Mw 5.1] that generated public panic in the region. The last damaging moderate earthquake of the 20th October 2011 in Talala region (21.09°N;70.45°E), located at about 200 km south to the devastating 2001 Bhuj (23.412°N, 70.232°E) mainshock (Mw 7.6), jolted the entire Saurashtra region of Gujarat.