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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.

Yadav R.B.S.,Indian National Center for Ocean Information Services | Bayrak Y.,Karadeniz Technical University | Tripathi J.N.,Allahabad University | Chopra S.,Ministry of Earth science | And 2 more authors.
Pure and Applied Geophysics | Year: 2012

The maximum likelihood estimation method is applied to study the geographical distribution of earthquake hazard parameters and seismicity in 28 seismogenic source zones of NW Himalaya and the adjoining regions. For this purpose, we have prepared a reliable, homogeneous and complete earthquake catalogue during the period 1500-2010. The technique used here allows the data to contain either historical or instrumental era or even a combination of the both. In this study, the earthquake hazard parameters, which include maximum regional magnitude (M max), mean seismic activity rate (λ), the parameter b (or β = b/log e) of Gutenberg-Richter (G-R) frequency-magnitude relationship, the return periods of earthquakes with a certain threshold magnitude along with their probabilities of occurrences have been calculated using only instrumental earthquake data during the period 1900-2010. The uncertainties in magnitude have been also taken into consideration during the calculation of hazard parameters. The earthquake hazard in the whole NW Himalaya region has been calculated in 28 seismogenic source zones delineated on the basis of seismicity level, tectonics and focal mechanism. The annual probability of exceedance of earthquake (activity rate) of certain magnitude is also calculated for all seismogenic source zones. The obtained earthquake hazard parameters were geographically distributed in all 28 seismogenic source zones to analyze the spatial variation of localized seismicity parameters. It is observed that seismic hazard level is high in Quetta-Kirthar-Sulaiman region in Pakistan, Hindukush-Pamir Himalaya region and Uttarkashi-Chamoli region in Himalayan Frontal Thrust belt. The source zones that are expected to have maximum regional magnitude (M max) of more than 8. 0 are Quetta, southern Pamir, Caucasus and Kashmir-Himanchal Pradesh which have experienced such magnitude of earthquakes in the past. It is observed that seismic hazard level varies spatially from one zone to another which suggests that the examined regions have high crustal heterogeneity and seismotectonic complexity. © 2011 Springer Basel AG.

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.

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 | 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.

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