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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

Prakash R.,National Center for Seismology | Singh R.K.,National Center for Seismology
Geomatics, Natural Hazards and Risk | Year: 2016

Spatio-temporal variations of seismicity within 300 km of the main Nepal earthquake of 25 April 2015 showed seismic quiescence since 2007. Decadal changes in b-value using the Gutenberg–Richter relation showed a well-marked decrease during the period January 2005–April 2015 preceding the main earthquake. Stress drop of this earthquake in the inter plate region was found to be 3.4 MPa which is much lower than the intra plate Bhuj earthquake, 2001. The un-ruptured portion of the seismic gap in western Nepal lies between longitude 82.5°E and 84.5°E, whose 200 km length (if assumed to rupture entirely in one earthquake) coupled with locked zone of about 100 km from GPS data, may generate an earthquake of magnitude about 8 although no historical data for a major earthquake is as yet available. © 2016 The Author(s). Published by Taylor & Francis.

Kundu B.,National Institute of Technology Rourkela | Ghosh A.,University of California at Riverside | Mendoza M.,University of California at Riverside | Burgmann R.,University of California at Berkeley | And 2 more authors.
Geophysical Research Letters | Year: 2016

The 2012 East Indian Ocean earthquake (Mw 8.6), so far the largest intraoceanic plate strike-slip event ever recorded, modulated tectonic tremors in the Cascadia subduction zone. The rate of tremor activity near Vancouver Island increased by about 1.5 times from its background level during the passage of seismic waves of this earthquake. In most cases of dynamic modulation, large-amplitude and long-period surface waves stimulate tremors. However, in this case even the small stress change caused by body waves generated by the 2012 earthquake modulated tremor activity. The tremor modulation continued during the passage of the surface waves, subsequent to which the tremor activity returned to background rates. Similar tremor modulation is observed during the passage of the teleseismic waves from the Mw 8.2 event, which occurs about 2 h later near the Mw 8.6 event. We show that dynamic stresses from back-to-back large teleseismic events can strongly influence tremor sources. ©2016. American Geophysical Union. All Rights Reserved.

Kumar V.,National Center for Seismology | Kumar D.,Kurukshetra University | Chopra S.,Institute of Seismological Research
Journal of Asian Earth Sciences | Year: 2016

The scaling relation and self similarity of earthquake process have been investigated by estimating the source parameters of 34 moderate size earthquakes (mb 3.4–5.8) occurred in the NW Himalaya. The spectral analysis of body waves of 217 accelerograms recorded at 48 sites have been carried out using in the present analysis. The Brune's ω−2 model has been adopted for this purpose. The average ratio of the P-wave corner frequency, fc(P), to the S-wave corner frequency, fc(S), has been found to be 1.39 with fc(P) > fc(S) for 90% of the events analyzed here. This implies the shift in the corner frequency in agreement with many other similar studies done for different regions. The static stress drop values for all the events analyzed here lie in the range 10–100 bars average stress drop value of the order of 43 ± 19 bars for the region. This suggests the likely estimate of the dynamic stress drop, which is 2–3 times the static stress drop, is in the range of about 80–120 bars. This suggests the relatively high seismic hazard in the NW Himalaya as high frequency strong ground motions are governed by the stress drop. The estimated values of stress drop do not show significant variation with seismic moment for the range 5 × 1014–2 × 1017 N m. This observation along with the cube root scaling of corner frequencies suggests the self similarity of the moderate size earthquakes in the region. The scaling relation between seismic moment and corner frequency Mofc 3=3.47×1016Nm/s3 estimated in the present study can be utilized to estimate the source dimension given the seismic moment of the earthquake for the hazard assessment. The present study puts the constrains on the important parameters stress drop and source dimension required for the synthesis of strong ground motion from the future expected earthquakes in the region. Therefore, the present study is useful for the seismic hazard and risk related studies for NW Himalaya. © 2016 Elsevier Ltd

Hough S.E.,U.S. Geological Survey | Martin S.S.,Nanyang Technological University | Gahalaut V.,National Center for Seismology | Joshi A.,Indian Institute of Technology Roorkee | And 2 more authors.
Natural Hazards | Year: 2016

We use 21 strong motion recordings from Nepal and India for the 25 April 2015 moment magnitude (MW) 7.8 Gorkha, Nepal, earthquake together with the extensive macroseismic intensity data set presented by Martin et al. (Seism Res Lett 87:957–962, 2015) to analyse the distribution of ground motions at near-field and regional distances. We show that the data are consistent with the instrumental peak ground acceleration (PGA) versus macroseismic intensity relationship developed by Worden et al. (Bull Seism Soc Am 102:204–221, 2012), and use this relationship to estimate peak ground acceleration from intensities (PGAEMS). For nearest-fault distances (RRUP < 200 km), PGAEMS is consistent with the Atkinson and Boore (Bull Seism Soc Am 93:1703–1729, 2003) subduction zone ground motion prediction equation (GMPE). At greater distances (RRUP > 200 km), instrumental PGA values are consistent with this GMPE, while PGAEMS is systematically higher. We suggest the latter reflects a duration effect whereby effects of weak shaking are enhanced by long-duration and/or long-period ground motions from a large event at regional distances. We use PGAEMS values within 200 km to investigate the variability of high-frequency ground motions using the Atkinson and Boore (Bull Seism Soc Am 93:1703–1729, 2003) GMPE as a baseline. Across the near-field region, PGAEMS is higher by a factor of 2.0–2.5 towards the northern, down-dip edge of the rupture compared to the near-field region nearer to the southern, up-dip edge of the rupture. Inferred deamplification in the deepest part of the Kathmandu valley supports the conclusion that former lake-bed sediments experienced a pervasive nonlinear response during the mainshock (Dixit et al. in Seismol Res Lett 86(6):1533–1539, 2015; Rajaure et al. in Tectonophysics, 2016. Ground motions were significantly amplified in the southern Gangetic basin, but were relatively low in the northern basin. The overall distribution of ground motions and damage during the Gorkha earthquake thus reflects a combination of complex source, path, and site effects. We also present a macroseismic intensity data set and analysis of ground motions for the MW7.3 Dolakha aftershock on 12 May 2015, which we compare to the Gorkha mainshock and conclude was likely a high stress-drop event. © 2016 Springer Science+Business Media Dordrecht (outside the USA)

Prajapati S.K.,National Center for Seismology | Dadhich H.K.,National Center for Seismology | Chopra S.,Institute of Seismological Research
Journal of Asian Earth Sciences | Year: 2016

A devastating earthquake of Mw 7.8 struck central Nepal on 25th April, 2015 (6:11:25 UT) which resulted in more than ∼9000 deaths, and destroyed millions of houses. Standing buildings, roads and electrical installations worth 25-30. billions of dollars are reduced to rubbles. The earthquake was widely felt in the northern parts of India and moderate damage have been observed in the northern part of UP and Bihar region of India. Maximum intensity IX, according to the USGS report, was observed in the meizoseismal zone, surrounding the Kathmandu region. In the present study, we have compiled available information from the print, electronic media and various reports of damages and other effects caused by the event, and interpreted them to obtain Modified Mercalli Intensities (MMI) at over 175 locations spread over Nepal and surrounding Indian and Tibet region. We have also obtained a number of strong motion recordings from India and Nepal seismic network and developed an empirical relationship between the MMI and peak ground acceleration (PGA), peak ground velocity (PGV). We have used least square regression technique to derive the empirical relation between the MMI and ground motion parameters and compared them with the empirical relationships available for other regions of the world. Further, seismic intensity information available for historical earthquakes, which have occurred in the Nepal Himalaya along with the present intensity data has been utilized for developing an attenuation relationship for the studied region using two step regression analyses. The derived attenuation relationship is useful for assessing damage of a potential future large earthquake (earthquake scenario-based planning purposes) in the region. © 2016 Elsevier Ltd.

Sharma B.,National Center for Seismology | Chopra S.,National Center for Seismology | Chopra S.,Institute of Seismological Research | Kumar V.,National Center for Seismology
Natural Hazards | Year: 2016

Earthquakes are deadliest among all the natural disasters. The areas that have experienced great/large earthquakes in the past may experience big event in future. In this study, we have simulated Kangra earthquake (1905, Mw 7.8) and a hypothetical great earthquake (Mw 8.5) in the north-west Himalaya using Empirical Green’s Function (EGF) technique. Recordings of Dharamsala earthquake (1986, Mw 5.4) are used as Green function with a heterogeneous source model and an asperity. It has been observed that the towns of Kangra and Dharamsala can expect ground accelerations in excess of 1 g in case of a Mw 8.5 earthquake and could have experienced an acceleration close to 1 g during 1905 Kangra earthquake. The entire study region can expect acceleration in excess of 100 cm/s2 in case of Mw 7.8 and 200 cm/s2 in case of Mw 8.5. The sites located near the rupture initiation point can expect accelerations in excess of 1 g for the magnitudes simulated. For validation, the estimates of the PGA for Mw 7.8 simulation are compared with isoseismal studies carried out in the same region after the Kangra earthquake of 1905 by converting PGA values to intensities. It was found that the results are comparable. The target earthquakes (Mw 7.8 and Mw 8.5) are simulated at depth of 20 km and 30 km to examine the effect of PGA for different depths. The PGA values obtained in the present analysis gave us an idea about the level of accelerations experienced in the area during 1905 Kangra earthquake. Future construction in the area can be regulated, and built environ can be strengthened using PGA values obtained in the present analysis. © 2015, Springer Science+Business Media Dordrecht.

Sharma B.,National Center for Seismology | Chopra S.,National Center for Seismology | Chopra S.,Institute of Seismological Research | Chingtham P.,National Center for Seismology | Kumar V.,National Center for Seismology
Natural Hazards | Year: 2016

In the present work, acceleration response spectra are determined from earthquakes which have occurred in the NE region and the effect of local geology on its shape is studied. One hundred and ninety-five strong ground motion time histories from 45 earthquakes which have occurred in the NE region having a magnitude range of 3.5 ≤ Mw ≤ 6.9 and a distance range of 20–600 kms are used. It is observed that the shape of the normalized acceleration response spectra is influenced by the local site conditions and regional geology. The influence of magnitude and distance on the spectra is also studied. The present study is carried out for three categories of rocks: Pre-Cambrian, Tertiary and Quaternary. It is inferred that the acceleration response spectra in the current Indian code designed for the entire country are applicable for NE region as it is within the spectral limits prescribed in Indian code. The ground motion is amplified at higher frequencies for stations located on hard rock, while for stations located on alluvium sites, it is amplified at lower frequencies. The sites located on hard rock show lowest values of spectral acceleration than the sites located on alluvium sites. The results obtained in the present study are compared with the similar results obtained in the stable continent region like Gujarat. It is found that the dominating period of response spectrum of similar rock types is found to be at higher side for NE region as compared to Gujarat region. This may be attributed towards the tectonic complexity of the NE region than the stable continent region like Gujarat. © 2016 Springer Science+Business Media Dordrecht

Gahalaut V.K.,National Center for Seismology | Kundu B.,National Institute of Technology Rourkela
Geomatics, Natural Hazards and Risk | Year: 2016

Earthquakes in the Indo-Burmese wedge occur due to India-Sunda plate motion. These earthquakes generally occur at depth between 25 and 150 km and define an eastward gently dipping seismicity trend surface that coincides with the Indian slab. Although this feature mimics the subduction zone, the relative motion of Indian plate predominantly towards north, earthquake focal mechanisms suggest that these earthquakes are of intra-slab type which occur on steep plane within the Indian plate. The relative motion between the India and Sunda plates is accommodated at the Churachandpur-Mao fault (CMF) and Sagaing Fault. The 4 January 2016 Manipur earthquake (M 6.7) is one such earthquake which occurred 20 km west of the CMF at ∼60 km depth. Fortunately, this earthquake occurred in a very sparse population region with very traditional wooden frame houses and hence, the damage caused by the earthquake in the source region was very minimal. However, in the neighbouring Imphal valley, it caused some damage to the buildings and loss of eight lives. The damage in Imphal valley due to this and historical earthquakes in the region emphasizes the role of local site effect in the Imphal valley. © 2016 Informa UK Limited, trading as Taylor & Francis Group

Kundu B.,National Institute of Technology Rourkela | Vissa N.K.,National Institute of Technology Rourkela | Gahalaut V.K.,CSIR - Central Electrochemical Research Institute | Gahalaut V.K.,National Center for Seismology
Geophysical Research Letters | Year: 2015

Groundwater usage in the Indo-Gangetic plains exceeds replenishment of aquifers, leading to substantial reduction in the mass. Such anthropogenic crustal unloading may promote long-term fault slip or may modulate seismic activity in the adjoining Himalayan region. Our simulation using Gravity Recovery and Climate Experiment data and hydrological models of such a process indicates that the thrust earthquakes on the Main Himalayan Thrust (MHT), including the recent 25 April 2015 Mw 7.8 Gorkha, Nepal earthquake, are probably influenced by the anthropogenic groundwater unloading process in the Gangetic plains. The groundwater withdrawal leading to crustal unloading in the Gangetic plains causes a significant component of horizontal compression which adds to the secular interseimic compression at the seismogenic depth (5-20 km) on the MHT beneath the Himalayan arc and at hypocentral depth of the 2015 Gorkha, Nepal earthquake. This effect enhances the Coulomb stress on the locked zone of MHT. © 2015. American Geophysical Union. All Rights Reserved.

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