Institute of Seismological Research ISR

Ghandinagar, India

Institute of Seismological Research ISR

Ghandinagar, India
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Dixit M.,Institute of Seismological Research ISR | Singh A.P.,Institute of Seismological Research ISR | Mishra O.P.,National Center for Seismology
Journal of Seismology | Year: 2017

In the present study, fundamental Rayleigh waves with varying period from 10 to 80 s are used to obtain group velocity maps in the northwest Deccan Volcanic Province of India. About 350 paths are obtained using 53 earthquakes (4.8 ≤ M ≥ 7.9) recorded by the SeisNetG (Seismic Network of Gujarat). Individual dispersion curves of group velocity of Rayleigh wave for each source-station path are estimated using multiple filter technique. These curves are used to determine lateral distribution of Rayleigh wave group velocity by tomographic inversion method. Our estimated Rayleigh group velocity at varying depths showed conspicuous corroboration with three tectonic blocks [Kachchh Rift Basin (KRB), Saurashtra Horst (SH), and Mainland Gujarat (MG)] in the region. The seismically active KRB with a thicker crust is characterized as a low velocity zone at a period varying from 10 to 30 s as indicative of mantle downwarping or sagging of the mantle beneath the KRB, while the SH and MG are found to be associated with higher group velocities, indicating the existence of the reduced crustal thickness. The trend of higher group velocity was found prevailed adjacent to the Narmada and Cambay rift basins that also correspond to the reduced crust, suggesting the processes of mantle upwarping or uplifting due to mantle upwelling. The low velocities at periods longer than 40 s beneath the KRB indicate thicker lithosphere. The known Moho depth correlates well with the observed velocities at a period of about 30 s in the Gujarat region. Our estimates of relatively lower group velocities at periods varying from 70 to 80 s may correspond to the asthenospheric flow beneath the region. It is interesting to image higher group velocity for the thinner crust beneath the Arabian Sea adjacent to the west coast of Gujarat at the period of 40 s that may correspond to the upwarped or upwelled mantle beneath the Arabian Sea. Our results have better resolution estimated by a radius of equivalent circular averaging area for each period. © 2017 Springer Science+Business Media Dordrecht

Singh A.P.,Institute of Seismological Research ISR | Mishra O.P.,Geological Survey of India | Mishra O.P.,Disaster Management Center | Rastogi B.K.,Institute of Seismological Research ISR | Kumar D.,Kurukshetra University
Natural Hazards | Year: 2011

Several pieces of studies on the January 26, 2001, Bhuj earthquake (Mw 7.6) revealed that the mainshock was triggered on the hidden unmapped fault in the western part of Indian stable continental region that caused a huge loss in the entire Kachchh rift basin of Gujarat, India. Occurrences of infrequent earthquakes of Mw 7.6 due to existence of hidden and unmapped faults on the surface have become one of the key issues for geoscientific research, which need to be addressed for evolving plausible earthquake hazard mitigation model. In this study, we have carried out a detailed autopsy of the 2001 Bhuj earthquake source zone by applying three-dimensional (3-D) local earthquake tomography (LET) method to a completely new data set consisting of 576 local earthquakes recorded between November 2006 and April 2009 by a seismic network consisting of 22 numbers of three-component broadband digital seismograph stations. In the present study, a total of 7560 arrival times of P-wave (3820) and S-wave (3740) recorded at least 4 seismograph stations were inverted to assimilate 3-D P-wave velocity (Vp), S-wave velocity (Vs), and Poisson's ratio (σ) structures beneath the 2001 Bhuj earthquake source zone for reliable interpretation of the imaged anomalies and its bearing on earthquake hazard of the region. The source zone is located near the triple junction formed by juxtapositions of three Indian, Arabian, and Iranian tectonic plates that might have facilitated the process of brittle failure at a depth of 25 km beneath the KRB, Gujarat, which caused a gigantic loss to both property and persons of the region. There may be several hidden seismogenic faults around the epicentral zone of the 2001 Bhuj earthquake in the area, which are detectable using 3-D tomography to minimize earthquake hazard for a region. We infer that the use of detailed 3-D seismic tomography may offer potential information on hidden and unmapped faults beneath the plate interior to unravel the genesis of such big damaging earthquakes. This study may help in evolving a comprehensive earthquake risk mitigation model for regions of analogous geotectonic settings, elsewhere in the world. © 2011 Springer Science+Business Media B.V.

Singh A.P.,Institute of Seismological Research ISR | Mishra O.P.,ESSO Ministry of Earth science MoES
Tectonophysics | Year: 2015

In order to understand the processes involved in the genesis of monsoon induced micro to moderate earthquakes after heavy rainfall during the Indian summer monsoon period beneath the 2011 Talala, Saurashtra earthquake (Mw 5.1) source zone, we assimilated 3-D microstructures of the sub-surface rock materials using a data set recorded by the Seismic Network of Gujarat (SeisNetG), India. Crack attributes in terms of crack density (ε), the saturation rate (ξ) and porosity parameter (ψ) were determined from the estimated 3-D sub-surface velocities (Vp, Vs) and Poisson's ratio (σ) structures of the area at varying depths. We distinctly imaged high-ε, high-ξ and low-ψ anomalies at shallow depths, extending up to 9-15. km. We infer that the existence of sub-surface fractured rock matrix connected to the surface from the source zone may have contributed to the changes in differential strain deep down to the crust due to the infiltration of rainwater, which in turn induced micro to moderate earthquake sequence beneath Talala source zone. Infiltration of rainwater during the Indian summer monsoon might have hastened the failure of the rock by perturbing the crustal volume strain of the causative source rock matrix associated with the changes in the seismic moment release beneath the surface. Analyses of crack attributes suggest that the fractured volume of the rock matrix with high porosity and lowered seismic strength beneath the source zone might have considerable influence on the style of fault displacements due to seismo-hydraulic fluid flows. Localized zone of micro-cracks diagnosed within the causative rock matrix connected to the water table and their association with shallow crustal faults might have acted as a conduit for infiltrating the precipitation down to the shallow crustal layers following the fault suction mechanism of pore pressure diffusion, triggering the monsoon induced earthquake sequence beneath the source zone. © 2015 Elsevier B.V.

Singh A.P.,Institute of Seismological Research ISR | Mishra O.P.,Disaster Management Center | Kumar D.,Kurukshetra University | Kumar S.,Institute of Seismological Research ISR | Yadav R.B.S.,Indian National Center for Ocean Information Services
Journal of Earth System Science | Year: 2012

We analyzed 3365 relocated aftershocks with magnitude of completeness (Mc) ≥1.7 that occurred in the Kachchh Rift Basin (KRB) between August 2006 and December 2010. The analysis of the new aftershock catalogue has led to improved understanding of the subsurface structure and of the aftershock behaviour. We characterized aftershock behaviour in terms of a-value, b-value, spatial fractal dimension (D s), and slip ratio (ratio of the slip that occurred on the primary fault and that of the total slip). The estimated b-value is 1.05, which indicates that the earthquake occurred due to active tectonics in the region. The three dimensional b-value mapping shows that a high b-value region is sandwiched around the 2001 Bhuj mainshock hypocenter at depths of 20-25 km between two low b-value zones above and below this depth range. The D s-value was estimated from the double-logarithmic plot of the correlation integral and distance between hypocenters, and is found to be 2.64 ± 0.01, which indicates random spatial distribution beneath the source zone in a two-dimensional plane associated with fluid-filled fractures. A slip ratio of about 0.23 reveals that more slip occurred on secondary fault systems in and around the 2001 Bhuj earhquake (Mw 7.6) source zone in KRB. © Indian Academy of Sciences.

Singh A.P.,Institute of Seismological Research ISR | Annam N.,Institute of Seismological Research ISR | Kumar S.,Institute of Seismological Research ISR
Natural Hazards | Year: 2014

The Kachchh region is the second most seismically active region in India after the Himalaya. One of the disastrous Indian earthquakes of the millennium was the Bhuj earthquake of January 26, 2001, which caused about 14,000 casualties and huge property damage. The main reason for such devastation is due to lack of earthquake awareness and poor construction practices. Hence, an increase in the knowledge and awareness, based on improved seismic hazard assessment, is required to mitigate damage due to an earthquake. Natural predominant ground frequencies have been investigated in the Kachchh region of western India using ambient vibrations. The horizontal-to-vertical spectral ratio technique has been applied to estimate the predominant frequency at 126 sites. The ambient vibration measurements were conducted for about 1 h at each site in the continuous mode recording at 100 samples/s. We have validated the estimated predominant frequency with earthquake data recorded at six broadband stations in the region. It has been observed that geological time period has a significant effect on predominant frequency of the ground. The estimated predominant frequencies vary from 0.24 to 2.25 Hz for the Quaternary, 0.41-2.34 Hz for the Tertiary, 0.32-4.91 Hz for the Cretaceous, and 0.39-8.0 Hz for the Jurassic/Mesozoic. In the Deccan trap, it varies from 1.30 to 3.80 Hz. We found distinct variation of predominant frequencies of sites associated with hard rock and soft soil. The predominant frequencies were related to the thickness of the sediments, which are deduced by other geophysical and geological methods in the region. Our results suggest that frequencies of the region reveals the site characteristics that can be considered for studying the seismic risks to evolve a plan for disaster risk mitigation for the region. © 2014 Springer Science+Business Media Dordrecht.

Mahesh P.,Institute of Seismological Research ISR | Gupta S.,CSIR - Central Electrochemical Research Institute
Tectonophysics | Year: 2016

The Talala region in Saurashtra, Western India is one of the seismically active intraplate regions on the Earth. In recent past, this region has been site of moderate magnitude earthquakes as well as swarm-type earthquake activities. To understand the processes of earthquake generation in this intraplate setting, we constrained the earthquake distribution pattern along with the crustal seismic P-wave velocity (Vp) and Vp/. Vs variations, using local earthquakes data. We inverted 2470 P- and 2230 S-wave arrival times from 550 earthquakes which were recorded over 11 seismic stations during 2007 to 2012. The earthquakes distribution shows that the seismicity is following ~. NNE-SSW trend, extending for a distance of ~. 25. km and up to 15. km in depth. The seismic tomographic images show that the swarm-type earthquake activities at shallower depths are mostly in the zone of lower Vp and lower Vp/. Vs. Whereas, the moderate magnitude earthquakes are occurring in a ~. NW trending zone of higher Vp and higher Vp/. Vs, possibly indicating a zone of crystallized mafic magma, which was transported from deeper Earth. This zone represents a pronounced heterogeneity and provides locale for stress accumulation in this region. After 2001 Bhuj earthquake (Mw 7.7), due to stress perturbation the ~. NNE-SSW trending fault got activated and caused bigger earthquakes in this region. Moreover, the crystallized mafic magma is possibly feeding fluids at shallower depths for causing the swarm-type earthquake activities in this region. © 2016 Elsevier B.V.

Yadav R.B.S.,Institute of Seismological Research ISR | Yadav R.B.S.,Jawaharlal Nehru Technological University | Shanker D.,Indian Institute of Technology Roorkee | Chopra S.,Institute of Seismological Research ISR | Singh A.P.,Institute of Seismological Research ISR
Natural Hazards | Year: 2010

A regional time and magnitude predictable model has been applied to estimate the recurrence intervals for large earthquakes in the vicinity of 8 October 2005 Kashmir Himalaya earthquake (25°-40°N and 65°-85°E), which includes India, Pakistan, Afghanistan, Hindukush, Pamirs, Mangolia and Tien-Shan. This region has been divided into 17 seismogenic sources on the basis of certain seismotectonics and geomorphological criteria. A complete earthquake catalogue (historical and instrumental) of magnitude Ms ≥ 5.5 during the period 1853-2005 has been used in the analysis. According to this model, the magnitude of preceding earthquake governs the time of occurrence and magnitude of future mainshock in the sequence. The interevent time between successive mainshocks with magnitude equal to or greater than a minimum magnitude threshold were considered and used for long-term earthquake prediction in each of seismogenic sources. The interevent times and magnitudes of mainshocks have been used to determine the following predictive relations: logTt = 0.05 Mmin + 0.09 Mp - 0.01 log M0 + 01.14; and Mf = 0.21 Mmin - 0.01 Mp + 0.03 log M0 + 7.21 where, Tt is the interevent time of successive mainshocks, Mmin is minimum magnitude threshold considered, Mp is magnitude of preceding mainshock, Mf is magnitude of following mainshock and M0 is the seismic moment released per year in each seismogenic source. It was found that the magnitude of following mainshock (Mf) does not depend on the interevent time (Tt), which indicates the ability to predict the time of occurrence of future mainshock. A negative correlation between magnitude of following mainshock (Mf) and preceding mainshock (Mp) indicates that the larger earthquake is followed by smaller one and vice versa. The above equations have been used for the seismic hazard assessment in the considered region. Based on the model applicability in the studied region and taking into account the occurrence time and magnitude of last mainshock in each seismogenic source, the time-dependent conditional probabilities (PC) for the occurrence of next shallow large mainshocks (Ms ≥ 6.5), during next 20 years as well as the expected magnitudes have been estimated. © 2010 Springer Science+Business Media B.V.

Chopra S.,Institute of Seismological Research ISR | Kumar D.,Kurukshetra University | RastogiBal B.K.,Institute of Seismological Research ISR
Pure and Applied Geophysics | Year: 2010

The strong ground motions for the 2001 Bhuj (M w 7.6) India earthquake have been estimated on hard rock and B/C boundary (NEHRP) levels using a recently modified version of stochastic finite fault modeling based on dynamic corner frequency (MOTAZEDIAN and ATKINSON in Bull Seismol Soc Am 95, 995-1010 2005). Incorporation of dynamic corner frequency removes the limitations of earlier stochastic methods. Simulations were carried out at 13 sites in Gujarat where structural response recorder (SRR) recordings are available. In addition, accelerograms were simulated at the B/C boundary at a large number of points distributed on a grid. The corresponding response spectra have also been estimated. The values of peak ground accelerations and spectral accelerations at three periods (0.4, 0.75 and 1.25 s) are presented in the form of contour maps. The maximum value of peak ground acceleration (PGA) in the center of meizoseismal zone is 550 cm/s 2. The response spectral acceleration in same zone is 900 cm/s 2 (T = 0.4 s), 600 cm/s 2 (T = 0.75 s) and 300 cm/s 2 (T = 1.25 s). The innermost PGA contour is on the fault plane. A comparison of the PGA values obtained at 13 sites in this study with those obtained in earlier studies on the same sites, but employing different methods, show that the present PGA values are comparable at most of the sites. The rate of decay of PGA values is fast at short distances as compared to that at longer distances. The PGA values obtained here put some constraints on the expected values from a similar earthquake in the region. A synthetic intensity map has been prepared from the estimated values of PGA using an empirical relation. A comparison with the reported intensity map of the earthquake shows the synthetic MMI values, as expected, are lower by 1 unit compared to reported intensity map. The contour map of PGA along with the contour maps of spectral acceleration at various periods permit the assessment of damage potential to various categories of houses and other structures. Such information will be quite important in planning of mitigation and disaster management programs in the region. © 2010 Birkhäuser / Springer Basel AG.

Joshi A.,Indian Institute of Technology Roorkee | Mohan K.,Institute of Seismological Research ISR
Natural Hazards | Year: 2010

A method of seismic zonation based on the deterministic modeling of rupture planes is presented. Finite rupture planes along identified lineaments are modeled in the Uttarakhand Himalaya based on the semi empirical technique of Midorikawa (Tectonophysics 218:287-295, 1993). The expected peak ground acceleration thus estimated from this technique is divided into different zones similar to zones proposed by the Bureau of Indian standard, BIS (Indian standards code of practice for earthquake-resistant design of structures, 2002). The proposed technique has been applied to Kumaon Himalaya area and the surrounding region for earthquakes of magnitude M >6.0. Approximately 56000km2 study area is classified into the highest hazard zone V with peak accelerations of more than 400cm/s2. This zone V includes the cities of the Dharchula, Almora, Nainital, Haridwar, Okhimath, Uttarkashi, Pithorahargh, Lohaghat, Munsiari, Rudraprayag, and Karnprayag. The Sobla and Gopeshwar regions belong to zone IV, where peak ground accelerations of the order from 250 to 400cm/s2 can be expected. The prepared map shows that epicenters of many past earthquakes in this region lie in zone V, and hence indicating the utility of developed map in defining various seismic zones. © Springer Science+Business Media B.V. 2009.

Singh A.P.,Institute of Seismological Research ISR | Roy I.G.,Spaceage Geoconsulting | Kumar S.,Institute of Seismological Research ISR | Kayal J.R.,Institute of Seismological Research ISR
Natural Hazards | Year: 2013

Seismic source characteristics in the Kachchh rift basin and Saurashtra horst tectonic blocks in the stable continental region (SCR) of western peninsular India are studied using the earthquake catalog data for the period 2006-2011 recorded by a 52-station broadband seismic network known as Gujarat State Network (GSNet) running by Institute of Seismological Research (ISR), Gujarat. These data are mainly the aftershock sequences of three mainshocks, the 2001 Bhuj earthquake (Mw 7.7) in the Kachchh rift basin, and the 2007 and 2011 Talala earthquakes (Mw ≥ 5.0) in the Saurashtra horst. Two important seismological parameters, the frequency-magnitude relation (b-value) and the fractal correlation dimension (Dc) of the hypocenters, are estimated. The b-value and the Dc maps indicate a difference in seismic characteristics of these two tectonic regions. The average b-value in Kachchh region is 1.2 ± 0.05 and that in the Saurashtra region 0.7 ± 0.04. The average Dc in Kachchh is 2.64 ± 0.01 and in Saurashtra 2.46 ± 0.01. The hypocenters in Kachchh rift basin cluster at a depth range 20-35 km and that in Saurashtra at 5-10 km. The b-value and Dc cross sections image the seismogenic structures that shed new light on seismotectonics of these two tectonic regions. The mainshock sources at depth are identified as lower b-value or stressed zones at the fault end. Crustal heterogeneities are well reflected in the maps as well as in the cross sections. We also find a positive correlation between b- and Dc-values in both the tectonic regions. © 2013 Springer Science+Business Media Dordrecht.

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