P. Baidya - Academia.edu (original) (raw)

Papers by P. Baidya

Journal of Geodynamics, 2018

Surface waves generated by earthquakes that occurred near Sumatra, Andaman-Nicobar Island chain a... more Surface waves generated by earthquakes that occurred near Sumatra, Andaman-Nicobar Island chain and Sunda arc are used to estimate crustal and upper mantle S wave velocity structure of Bay of Bengal. Records of these seismic events at various stations located along the eastern coast of India and a few stations in the north eastern part of India are selected for such analysis. These stations lie within regional distance of the selected earthquakes. The selected events are shallow focused with magnitude greater than 5.5. Data of 65, 37, 36, 53 and 36 events recorded at Shillong, Bokaro, Visakhapatnam, Chennai and Trivandrum stations respectively are used for this purpose. The ray paths from the earthquake source to the recording stations cover different parts of the Bay of Bengal. Multiple Filtering Technique (MFT) is applied to compute the group velocities of surface waves from the available data. The dispersion curves thus obtained for this data set are within the period range of 15 to 120 s. Joint inversion of Rayleigh and Love wave group velocity is carried out to obtain the subsurface information in terms of variation of S wave velocity with depth. The estimated S wave velocity at a given depth and layer thickness can be considered to be an average value for the entire path covered by the corresponding ray paths. However, we observe variation in the value of S wave velocity and layer thickness from data recorded at different stations, indicating lateral variation in these two parameters. Thick deposition of sediments is observed along the paths followed by surface waves to Shillong and Bokaro stations. Sediment thickness keeps on decreasing as the surface wave paths move further south. Based on velocity variation the sedimentary layer is further divided in to three parts; on top lay unconsolidated sediment, underlain by consolidated sediment. Below this lies a layer which we consider as meta-sediments. The thickness and velocity of these layers decrease from north to south. The crustal material has higher velocity at the southern part compared to that at the northern part of Bay of Bengal indicating that it changes from more oceanic type in the southern part of the Bay to more continental type to its north. Both Moho and lithosphereasthenosphere boundary (LAB) dips gently towards north. Thicknesses of both lithosphere and asthenosphere also increase in the same direction. The mantle structure shows complex variation from south to north indicating possible effect of repeated changes in type of tectonic activity in the Bay of Bengal.

Natural Hazards, 2018

Some regions of Indian subcontinent are highly earthquake prone area, and some are not. However, ... more Some regions of Indian subcontinent are highly earthquake prone area, and some are not. However, among them northeast region of India is most geologically complex one, which are found to be highly earthquake hazard prone region. This region has experienced several disastrous earthquakes like Assam earthquake of 1950 for 8.1 and Shillong earthquake of 1897 for 8.7 magnitude in the past. Several methods are used for seismic hazard measurement. Among these methods the site response analysis is found to be most valuable part of the seismic hazard assessment. In this work we have used Nakamura technique (H/V site response spectral ratio) for estimating the site amplification of seismic ground motion. Fundamental frequencies estimated are in the range 0.6-10 Hz for the 15 broadband stations of India Meteorological Department distributed over the study region. ZIRO station and its surrounding area are found to have high liquefaction indices (kg) value of 250. The horizontal-to-vertical ratio curve variation of higher amplitude with lower frequency helped to find thick soil zones in Aizawl and Agartala. Similar analysis also gave flat response at some sites near Jorhat and Dibrugarh having thin soil. The H/V rotate analysis helped us to see the variation of fundamental frequency with azimuth. Based on the observations, frequency and amplitude counter map of the northeast region represented. Further considering the site amplification due to very high fundamental frequency for Shillong, Guwahati, Imphal and Itanagar city can be considered under high seismic hazard scanner in the northeast India region.

Quaternary International, 2017

Characteristics of dispersion curves of Rayleigh and Love waves of 2005 Kashmir earthquake and it... more Characteristics of dispersion curves of Rayleigh and Love waves of 2005 Kashmir earthquake and its aftershocks are utilized to investigate the sub-surface structural variations within the northern margin of the Indian continent (i.e. Indo-Gangetic Plains) and the frontal part of the northwestern Himalaya. Surface waves of these earthquakes recorded in the Broad Band Seismometers (BBS) installed at Ajmer, Delhi and Shimla were used for this investigation. Estimated group velocities along three paths from earthquake source region to these stations are in the range of 2.17 km/s to 3.94 km/s which are sufficient to investigate the crustal and uppermost mantle structures. Paths covering northern margin of the Indian continent has very low group velocities for periods less than 10-s that indicate presence of thick sediments. High differences are observed between Love and Rayleigh wave group velocities for the paths crossing Himalayan wedge that can be ascribed to the effect of anisotropy due to India-Eurasia collision. The inversion of dispersion curves was performed to investigate sub-surface shear wave velocity structure and variation of anisotropy with depth for crust and uppermost mantle. A~6km thick sediments divided into two layers (unconsolidated and consolidated) are seen in the Indo Gangetic Plain deposited above underthrusting Indian plate. The Conrad and Moho depths increase from Ajmer, the southernmost station in the IGP to Shimla, the station lying in the Himalayan wedge. This shows the dipping geometry of the Indian plate. The average Moho depth along these paths from earthquake source to Ajmer station is 42.5 km, to Delhi station is 44 km and to Shimla station is at 46 km. Average values of Conrad depth for these ray paths are 18.5 km, 19.5 km and 26.5 km respectively. The S wave velocity along Ajmer path is higher than that along the other two paths. This may be due to lateral heterogeneity of the crust and mantle for the study area. Frontal western Himalaya is anisotropic in nature, whereas the IGP region is not. Anisotropy effect in the western Himalaya is most pronounced in the lower crustal material of the underthrusted Indian plate. It is quite large in the uppermost mantle below the frontal Himalayan region. We infer that anisotropy in the lower crust and uppermost mantle could be due to development of preferably oriented folds, faults and fractures and possible reorientation of crystals as a consequence of ongoing deformation caused by continent-continent collision.

Tectonophysics, 2016

UTC) was the strongest earthquake to strike Manipur since 1988. Using data from Indian stations, ... more UTC) was the strongest earthquake to strike Manipur since 1988. Using data from Indian stations, we constrain the hypocentral depth of the mainshock at 59 ± 3.8 km and determine a strike-slip mechanism with a moderate reverse component on a steeply dipping plane. Though coseismic offsets from GPS measurements from four nearby sites were inadequate to provide further constraints on the focal mechanism, they were consistent with the magnitude and hypocentral depth of the earthquake. The epicentre of the mainshock was located 15-km west of the Churachandpur Mao Fault (CMF) but it was unrelated to this structure and was instead a typical intra-slab earthquake within the Indian plate. A strong motion instrument at the Loktak Power Station (LOK), 56-km from the epicentre, recorded a peak ground acceleration (PGA) of 0.027g while a PGA of 0.103g was recorded at Shillong (SHL) at an epicentral distance of 111-km. We also present macroseismic observations from 461 locations in northeastern India and the adjacent areas for this earthquake. The highest intensities (~7 EMS) were observed in the Manipur Valley and in the hills to the west while shaking was perceptible as far as Delhi and Jaipur. Lastly, we present a catalogue of 333 felt earthquakes in Manipur from 1588 ± 1 CE to 1955 derived from the royal chronicle of the kings of Manipur known as the Cheitharon Kumpapa, discuss important historical earthquakes in the region, and also estimate intensity magnitudes for the 1852 (M I 6.5 ± 0.8), 1869 (M I 7.1 ± 0.7), 1880 (M I 6.3 ± 0.7) and 2016 (M I 6.8 ± 0.8) earthquakes.

Proceedings of the Indian National Science Academy, 2014

The process of dealing with earthquake disasters essentially involves three most important and in... more The process of dealing with earthquake disasters essentially involves three most important and interdependent components-(i) comprehensive understanding of the earthquake generation processes and the interior of the earth, (ii) disaster mitigation and preventive measures, and (iii) work through the ultimate goal of earthquake prediction. The basic and primary requirement towards addressing all these tasks is-high quality seismological data which is homogeneous and complete in time and space. India Meteorological Department (IMD), under the Earth System Science Organization (ESSO), Ministry of Earth Sciences (MoES), is the nodal agency of Government of India for monitoring earthquake activity in and around the country. IMD maintains the national seismological network consisting of a total of 82 observatories spread over the length and breadth of the country. The paper aims at discussing different methods/approaches adopted by IMD and other m a j o r a g e n c i e s i n t h e c o u n t r y f o r g e n e r a t i o n o f v a r i o u s t y p e s o f e a r t h q u a k e d a t a p r o d u c t s i n s t a n d a r d f o r m a t s , t h e a n a l y s e s and archival tools and policy guidelines for supply and sharing amongst the user agencies. The paper also deals with the types of seismic instrumentation/networks in operation, network growth through historical times, data completion aspects, present level(s) of earthquake detection and location, future requirements and plans of upgradation. The policy guidelines being followed for seismological data sharing and supply have also been highlighted.

Journal of Geophysical Research B: Solid Earth, 2013

Ever since its breakup from the Gondwanaland~140 Myr ago, the Indian plate was ravaged by four ho... more Ever since its breakup from the Gondwanaland~140 Myr ago, the Indian plate was ravaged by four hot spots. Although the surface manifestations of such deep processes are evident in terms of large igneous provinces like the Deccan and the fast drift of the Indian plate, the modifications to the deep structure remain to be grasped. In this study, we investigate the mantle transition zone (TZ) structure beneath the Indian shield region using 14,000 teleseismic receiver functions from 77 broadband stations sited on diverse geologic terrains. The arrival times of the P-to-s (Ps) conversions from the 410 km discontinuity at most cratonic stations appear to be delayed by~2 s in comparison with the times observed for other Precambrian shield regions like Africa, Australia, and Canada. Such delays in the conversions from the 410 km discontinuity below the Indian shield suggest low shear wave speeds in the lithospheric and sub-lithospheric mantles due to higher temperatures, together with a thinner high velocity lid that contrasts with a thicker one found beneath most Archean cratons. A thin transition zone beneath most of the cratonic stations lends support to the enhanced temperatures within the TZ itself. Also, a further delay of the TZ discontinuities is observed for stations on the southern granulite terrain, which was under the influence of the Marion plume that is responsible for the separation of Madagascar from India. Although the data do not conclusively show evidence for a 520 km discontinuity, an LVL atop the 410 cannot be ruled out beneath certain geological provinces of the Indian shield.

Seismological Research Letters, 2005

Current …, 1997

A moderate earthquake occurred in the morning of 22 May 1997 close to Jabalpur town in Madhya Pra... more A moderate earthquake occurred in the morning of 22 May 1997 close to Jabalpur town in Madhya Pradesh. The earthquake records of seismological observatories maintained by India Meteorological Department were analysed to obtain source parameters. Of these, ten observatories in ...

Natural Hazards, 2012

In this paper, we report that the ratio of broadband energy (0.01-2 Hz) to highfrequency energy (... more In this paper, we report that the ratio of broadband energy (0.01-2 Hz) to highfrequency energy (0.3-2 Hz), E r , estimated from regional seismograms of India, might be a useful parameter in estimating tsunami potential of earthquakes in the Sumatra-Andaman region. E r is expected to be sensitive to the depth as well as to the source characteristics of an earthquake. Since a shallow and slow earthquake has a greater tsunamigenic potential, E r may be a useful diagnostic parameter. We base our analysis on broadband seismograms of the great earthquakes of Sumatra-Andaman (2004, M w

Tectonophysics, 2013

Abstract P and S receiver functions (PRFs and SRFs, respectively) for 21 broad-band seismograph s... more Abstract P and S receiver functions (PRFs and SRFs, respectively) for 21 broad-band seismograph stations of the India Meteorological Department (IMD) illuminate lithosphere and the underlying mantle of some previously poorly sampled regions of the Indian sub-continent. Our analysis demonstrates that the Archean and Early Proterozoic lithospheric keel of the Indian shield has been reworked by younger processes. We find very low S-wave velocities in the uppermost mantle (from 4.0 to 4.3 km/s) to the north of the Deccan Volcanic Province (Kutch region and Aravalli Craton) (1), in the south (Southern Granulite Terrain and Sri Lanka) (2) and in the north-east (Gangetic Plane, Bengal Basin and Singhbuhm Craton) (3). The anomalies 1 and 2 may extend into the transition zone. Early arrivals of the S410p seismic phase are indicative of anomalously high Vp/Vs ratio (~ 1.9) in the upper mantle of the low-velocity regions, whereas late arrivals in the western Himalaya, Ladakh and western Tibet are consistent with the previously found indications of anomalously low Vp/Vs ratio. A transition from the high-S-velocity mantle lid to a layer of slightly lower velocity is seen in part of the data but a straightforward interpretation of this transition as the lithosphere–asthenosphere boundary is problematic. A mafic S velocity in the upper crust and a pronounced low-S-velocity layer in the lower crust beneath the eruptive center is practically the only specific feature in the lithosphere that may be linked to the Deccan Traps. A separation in depths between the 410-km and 660-km discontinuities varies laterally in a range from 240 to 270 km. The largest uplift of the 410-km discontinuity (up to 390 km) is observed beneath the foothills of the Himalaya where it is caused by cooling of the transition zone by the ongoing continental collision.

We present a preliminary source study of the Muzaffarabad earthquake of 8 October 2005 (Mw 7.6) a... more We present a preliminary source study of the Muzaffarabad earthquake of 8 October 2005 (Mw 7.6) and the far-field ground motions that it generated. Our analysis is based on regional broadband seismograms recorded at stations operated by the India Meteorological Department (IMD) which are situated to the south of the epicentre, and at non-IMD stations which are located to the north. We find that the source spectrum of the earthquake is reasonably consistent with ω 2 -source model with a seismic moment, M 0 , of 2.94 x 10 20 N-m and a corner frequency, fc, of 0.051 Hz (Brune stress drop of 9.5 MPa). The radiated seismic energy, E R , estimated from the empirical Green's function (EGF) technique is 2.70 × 10 16 J. This yields a normalized radiated energy, E R /M 0 , of 9.1 × 10 -5 , and an apparent stress, τ a , of 2.7 MPa. The rupture area of 100 × 15 km2 (estimated from slip distribution mapped from the inversion of teleseismic body waves) gives a static stress drop of about 11.3 MPa. From these source parameters we estimate a radiation efficiency of 0.49, implying a 'brittle' rupture typical of interplate events. Stochastic method requires a stress drop of -10 MPa to explain the observed peak ground motions (A max and V max ) recorded at regional distances, and predicts A max and V max exceeding 1 g and 100 cm/s, respectively at hard sites in the epicentral region. The source parameters and far-field ground motions of the Muzaffarabad and Bhuj earthquakes are quite similar even though the tectonic environment and the depth of their occurrence are distinct.

Geomatics, Natural Hazards and Risk, 2015

Journal of Geophysical Research B: Solid Earth, 2013

ABSTRACT Ever since its breakup from the Gondwanaland ~140 Myr ago, the Indian plate was ravaged ... more ABSTRACT Ever since its breakup from the Gondwanaland ~140 Myr ago, the Indian plate was ravaged by four hot spots. Although the surface manifestations of such deep processes are evident in terms of large igneous provinces like the Deccan and the fast drift of the Indian plate, the modifications to the deep structure remain to be grasped. In this study, we investigate the mantle transition zone (TZ) structure beneath the Indian shield region using ~14,000 teleseismic receiver functions from 77 broadband stations sited on diverse geologic terrains. The arrival times of the P-to-s (Ps) conversions from the 410 km discontinuity at most cratonic stations appear to be delayed by ~2 s in comparison with the times observed for other Precambrian shield regions like Africa, Australia, and Canada. Such delays in the conversions from the 410 km discontinuity below the Indian shield suggest low shear wave speeds in the lithospheric and sub-lithospheric mantles due to higher temperatures, together with a thinner high velocity lid that contrasts with a thicker one found beneath most Archean cratons. A thin transition zone beneath most of the cratonic stations lends support to the enhanced temperatures within the TZ itself. Also, a further delay of the TZ discontinuities is observed for stations on the southern granulite terrain, which was under the influence of the Marion plume that is responsible for the separation of Madagascar from India. Although the data do not conclusively show evidence for a 520 km discontinuity, an LVL atop the 410 cannot be ruled out beneath certain geological provinces of the Indian shield.

Russian Journal of Earth Sciences, 2008

Seismological Research Letters, 2005

... Singh, SK, BK Bansal, SN Bhattacharya, JF Pacheco, RS Dattatrayam, M. Ordaz, G. Suresh, Kamal... more ... Singh, SK, BK Bansal, SN Bhattacharya, JF Pacheco, RS Dattatrayam, M. Ordaz, G. Suresh, Kamal, and SE Hough (2003). ... Speed and size of the Sumatra earthquake, Nature 434,581 -582.[Medline]. Tolstoy, Maya and DelWayne R. Bohnenstiehl (2005). ...

Pure and Applied Geophysics, 2006

Q C -estimates of Kachchh Basin in western India have been obtained in a high frequency range fro... more Q C -estimates of Kachchh Basin in western India have been obtained in a high frequency range from 1.5 to 24.0 Hz using the aftershock data of Bhuj earthquake of January 26, 2001 recorded within an epicentral distance of 80 km. The decay of coda waves of 30 sec window from 186 seismograms has been analysed in four lapse time windows, adopting the single backscattering model. The study shows that Q c is a function of frequency and increases as frequency increases. The frequency dependent Q c relations obtained for four lapse-time windows are: Q c =82 f 1.17 (20-50 sec), Q c =106 f 1.11 (30-60 sec), Q c =126f 1.03 (40-70 sec) and Q c =122f 1.02 (50-80 sec). These empirical relations represent the average attenuation properties of a zone covering the surface area of about 11,000, 20,000, 28,000 and 38,000 square km and a depth extent of about 60, 80, 95, 110 km, respectively. With increasing window length, the degree of frequency dependence, n, decreases marginally from 1.17 to 1.02, whereas Q 0 increases significantly from 82 to 122. At lower frequencies up to 6 Hz, Q c )1 of Kachchh Basin is in agreement with other regions of the world, whereas at higher frequencies from 12 to 24 Hz it is found to be low. Figure 1 Map showing epicenters of aftershocks of Bhuj earthquake used in the Q c analysis, tectonic features in the Kachchh Basin and the locations of recording stations.

Natural Hazards, 2012

In this paper, we report that the ratio of broadband energy (0.01-2 Hz) to highfrequency energy (... more In this paper, we report that the ratio of broadband energy (0.01-2 Hz) to highfrequency energy (0.3-2 Hz), E r , estimated from regional seismograms of India, might be a useful parameter in estimating tsunami potential of earthquakes in the Sumatra-Andaman region. E r is expected to be sensitive to the depth as well as to the source characteristics of an earthquake. Since a shallow and slow earthquake has a greater tsunamigenic potential, E r may be a useful diagnostic parameter. We base our analysis on broadband seismograms of the great earthquakes of Sumatra-Andaman (2004, M w * 9.2) and Nias (2005, M w 8.6), 41 of their aftershocks, and the earthquakes of north Sumatra (2010, M w 7.8) and Nicobar (2010, M w 7.4) recorded at VISK, a station located on the east coast of India. In the analysis, we also included the two recent, great strike-slip earthquakes of north Sumatra (2012, M w 8.6, 8.2) recorded at VISK and three south Sumatra earthquakes (2007, M w 8.5; 2007, M w 7.9; 2010, M w 7.8) recorded at PALK, a station in Sri Lanka. We find that E r is a function of depth; shallower earthquakes have higher E r values than the deeper ones. Thus, E r may be indicative of tsunamigenic potential of an earthquake. As M w and E r increase so does the tsunami potential. In addition to the parameter E r , the radiated seismic energy, E s , may be estimated from the regional seismograms in India using empirical Green's function technique. The technique yields reliable E s for the great Sumatra and Nias earthquakes. E r and E s computed from VISK data, along with M w and focal mechanism, may be useful in

Proceedings of the Indian National Science Academy, 2014

The process of dealing with earthquake disasters essentially involves three most important and in... more The process of dealing with earthquake disasters essentially involves three most important and inter-dependent components -(i) comprehensive understanding of the earthquake generation processes and the interior of the earth, (ii) disaster mitigation and preventive measures, and (iii) work through the ultimate goal of earthquake prediction. The basic and primary requirement towards addressing all these tasks is -high quality seismological data which is homogeneous and complete in time and space. India Meteorological Department (IMD), under the Earth System Science Organization (ESSO), Ministry of Earth Sciences (MoES), is the nodal agency of Government of India for monitoring earthquake activity in and around the country. IMD maintains the national seismological network consisting of a total of 82 observatories spread over the length and breadth of the country. The paper aims at discussing different methods/approaches adopted by IMD and other major agencies in the country for generation of various types of earthquake data products in standard formats, the analyses and archival tools and policy guidelines for supply and sharing amongst the user agencies. The paper also deals with the types of seismic instrumentation/networks in operation, network growth through historical times, data completion aspects, present level(s) of earthquake detection and location, future requirements and plans of upgradation. The policy guidelines being followed for seismological data sharing and supply have also been highlighted.

Journal of Geodynamics, 2018

Surface waves generated by earthquakes that occurred near Sumatra, Andaman-Nicobar Island chain a... more Surface waves generated by earthquakes that occurred near Sumatra, Andaman-Nicobar Island chain and Sunda arc are used to estimate crustal and upper mantle S wave velocity structure of Bay of Bengal. Records of these seismic events at various stations located along the eastern coast of India and a few stations in the north eastern part of India are selected for such analysis. These stations lie within regional distance of the selected earthquakes. The selected events are shallow focused with magnitude greater than 5.5. Data of 65, 37, 36, 53 and 36 events recorded at Shillong, Bokaro, Visakhapatnam, Chennai and Trivandrum stations respectively are used for this purpose. The ray paths from the earthquake source to the recording stations cover different parts of the Bay of Bengal. Multiple Filtering Technique (MFT) is applied to compute the group velocities of surface waves from the available data. The dispersion curves thus obtained for this data set are within the period range of 15 to 120 s. Joint inversion of Rayleigh and Love wave group velocity is carried out to obtain the subsurface information in terms of variation of S wave velocity with depth. The estimated S wave velocity at a given depth and layer thickness can be considered to be an average value for the entire path covered by the corresponding ray paths. However, we observe variation in the value of S wave velocity and layer thickness from data recorded at different stations, indicating lateral variation in these two parameters. Thick deposition of sediments is observed along the paths followed by surface waves to Shillong and Bokaro stations. Sediment thickness keeps on decreasing as the surface wave paths move further south. Based on velocity variation the sedimentary layer is further divided in to three parts; on top lay unconsolidated sediment, underlain by consolidated sediment. Below this lies a layer which we consider as meta-sediments. The thickness and velocity of these layers decrease from north to south. The crustal material has higher velocity at the southern part compared to that at the northern part of Bay of Bengal indicating that it changes from more oceanic type in the southern part of the Bay to more continental type to its north. Both Moho and lithosphereasthenosphere boundary (LAB) dips gently towards north. Thicknesses of both lithosphere and asthenosphere also increase in the same direction. The mantle structure shows complex variation from south to north indicating possible effect of repeated changes in type of tectonic activity in the Bay of Bengal.

Natural Hazards, 2018

Some regions of Indian subcontinent are highly earthquake prone area, and some are not. However, ... more Some regions of Indian subcontinent are highly earthquake prone area, and some are not. However, among them northeast region of India is most geologically complex one, which are found to be highly earthquake hazard prone region. This region has experienced several disastrous earthquakes like Assam earthquake of 1950 for 8.1 and Shillong earthquake of 1897 for 8.7 magnitude in the past. Several methods are used for seismic hazard measurement. Among these methods the site response analysis is found to be most valuable part of the seismic hazard assessment. In this work we have used Nakamura technique (H/V site response spectral ratio) for estimating the site amplification of seismic ground motion. Fundamental frequencies estimated are in the range 0.6-10 Hz for the 15 broadband stations of India Meteorological Department distributed over the study region. ZIRO station and its surrounding area are found to have high liquefaction indices (kg) value of 250. The horizontal-to-vertical ratio curve variation of higher amplitude with lower frequency helped to find thick soil zones in Aizawl and Agartala. Similar analysis also gave flat response at some sites near Jorhat and Dibrugarh having thin soil. The H/V rotate analysis helped us to see the variation of fundamental frequency with azimuth. Based on the observations, frequency and amplitude counter map of the northeast region represented. Further considering the site amplification due to very high fundamental frequency for Shillong, Guwahati, Imphal and Itanagar city can be considered under high seismic hazard scanner in the northeast India region.

Quaternary International, 2017

Characteristics of dispersion curves of Rayleigh and Love waves of 2005 Kashmir earthquake and it... more Characteristics of dispersion curves of Rayleigh and Love waves of 2005 Kashmir earthquake and its aftershocks are utilized to investigate the sub-surface structural variations within the northern margin of the Indian continent (i.e. Indo-Gangetic Plains) and the frontal part of the northwestern Himalaya. Surface waves of these earthquakes recorded in the Broad Band Seismometers (BBS) installed at Ajmer, Delhi and Shimla were used for this investigation. Estimated group velocities along three paths from earthquake source region to these stations are in the range of 2.17 km/s to 3.94 km/s which are sufficient to investigate the crustal and uppermost mantle structures. Paths covering northern margin of the Indian continent has very low group velocities for periods less than 10-s that indicate presence of thick sediments. High differences are observed between Love and Rayleigh wave group velocities for the paths crossing Himalayan wedge that can be ascribed to the effect of anisotropy due to India-Eurasia collision. The inversion of dispersion curves was performed to investigate sub-surface shear wave velocity structure and variation of anisotropy with depth for crust and uppermost mantle. A~6km thick sediments divided into two layers (unconsolidated and consolidated) are seen in the Indo Gangetic Plain deposited above underthrusting Indian plate. The Conrad and Moho depths increase from Ajmer, the southernmost station in the IGP to Shimla, the station lying in the Himalayan wedge. This shows the dipping geometry of the Indian plate. The average Moho depth along these paths from earthquake source to Ajmer station is 42.5 km, to Delhi station is 44 km and to Shimla station is at 46 km. Average values of Conrad depth for these ray paths are 18.5 km, 19.5 km and 26.5 km respectively. The S wave velocity along Ajmer path is higher than that along the other two paths. This may be due to lateral heterogeneity of the crust and mantle for the study area. Frontal western Himalaya is anisotropic in nature, whereas the IGP region is not. Anisotropy effect in the western Himalaya is most pronounced in the lower crustal material of the underthrusted Indian plate. It is quite large in the uppermost mantle below the frontal Himalayan region. We infer that anisotropy in the lower crust and uppermost mantle could be due to development of preferably oriented folds, faults and fractures and possible reorientation of crystals as a consequence of ongoing deformation caused by continent-continent collision.

Tectonophysics, 2016

UTC) was the strongest earthquake to strike Manipur since 1988. Using data from Indian stations, ... more UTC) was the strongest earthquake to strike Manipur since 1988. Using data from Indian stations, we constrain the hypocentral depth of the mainshock at 59 ± 3.8 km and determine a strike-slip mechanism with a moderate reverse component on a steeply dipping plane. Though coseismic offsets from GPS measurements from four nearby sites were inadequate to provide further constraints on the focal mechanism, they were consistent with the magnitude and hypocentral depth of the earthquake. The epicentre of the mainshock was located 15-km west of the Churachandpur Mao Fault (CMF) but it was unrelated to this structure and was instead a typical intra-slab earthquake within the Indian plate. A strong motion instrument at the Loktak Power Station (LOK), 56-km from the epicentre, recorded a peak ground acceleration (PGA) of 0.027g while a PGA of 0.103g was recorded at Shillong (SHL) at an epicentral distance of 111-km. We also present macroseismic observations from 461 locations in northeastern India and the adjacent areas for this earthquake. The highest intensities (~7 EMS) were observed in the Manipur Valley and in the hills to the west while shaking was perceptible as far as Delhi and Jaipur. Lastly, we present a catalogue of 333 felt earthquakes in Manipur from 1588 ± 1 CE to 1955 derived from the royal chronicle of the kings of Manipur known as the Cheitharon Kumpapa, discuss important historical earthquakes in the region, and also estimate intensity magnitudes for the 1852 (M I 6.5 ± 0.8), 1869 (M I 7.1 ± 0.7), 1880 (M I 6.3 ± 0.7) and 2016 (M I 6.8 ± 0.8) earthquakes.

Proceedings of the Indian National Science Academy, 2014

The process of dealing with earthquake disasters essentially involves three most important and in... more The process of dealing with earthquake disasters essentially involves three most important and interdependent components-(i) comprehensive understanding of the earthquake generation processes and the interior of the earth, (ii) disaster mitigation and preventive measures, and (iii) work through the ultimate goal of earthquake prediction. The basic and primary requirement towards addressing all these tasks is-high quality seismological data which is homogeneous and complete in time and space. India Meteorological Department (IMD), under the Earth System Science Organization (ESSO), Ministry of Earth Sciences (MoES), is the nodal agency of Government of India for monitoring earthquake activity in and around the country. IMD maintains the national seismological network consisting of a total of 82 observatories spread over the length and breadth of the country. The paper aims at discussing different methods/approaches adopted by IMD and other m a j o r a g e n c i e s i n t h e c o u n t r y f o r g e n e r a t i o n o f v a r i o u s t y p e s o f e a r t h q u a k e d a t a p r o d u c t s i n s t a n d a r d f o r m a t s , t h e a n a l y s e s and archival tools and policy guidelines for supply and sharing amongst the user agencies. The paper also deals with the types of seismic instrumentation/networks in operation, network growth through historical times, data completion aspects, present level(s) of earthquake detection and location, future requirements and plans of upgradation. The policy guidelines being followed for seismological data sharing and supply have also been highlighted.

Journal of Geophysical Research B: Solid Earth, 2013

Ever since its breakup from the Gondwanaland~140 Myr ago, the Indian plate was ravaged by four ho... more Ever since its breakup from the Gondwanaland~140 Myr ago, the Indian plate was ravaged by four hot spots. Although the surface manifestations of such deep processes are evident in terms of large igneous provinces like the Deccan and the fast drift of the Indian plate, the modifications to the deep structure remain to be grasped. In this study, we investigate the mantle transition zone (TZ) structure beneath the Indian shield region using 14,000 teleseismic receiver functions from 77 broadband stations sited on diverse geologic terrains. The arrival times of the P-to-s (Ps) conversions from the 410 km discontinuity at most cratonic stations appear to be delayed by~2 s in comparison with the times observed for other Precambrian shield regions like Africa, Australia, and Canada. Such delays in the conversions from the 410 km discontinuity below the Indian shield suggest low shear wave speeds in the lithospheric and sub-lithospheric mantles due to higher temperatures, together with a thinner high velocity lid that contrasts with a thicker one found beneath most Archean cratons. A thin transition zone beneath most of the cratonic stations lends support to the enhanced temperatures within the TZ itself. Also, a further delay of the TZ discontinuities is observed for stations on the southern granulite terrain, which was under the influence of the Marion plume that is responsible for the separation of Madagascar from India. Although the data do not conclusively show evidence for a 520 km discontinuity, an LVL atop the 410 cannot be ruled out beneath certain geological provinces of the Indian shield.

Seismological Research Letters, 2005

Current …, 1997

A moderate earthquake occurred in the morning of 22 May 1997 close to Jabalpur town in Madhya Pra... more A moderate earthquake occurred in the morning of 22 May 1997 close to Jabalpur town in Madhya Pradesh. The earthquake records of seismological observatories maintained by India Meteorological Department were analysed to obtain source parameters. Of these, ten observatories in ...

Natural Hazards, 2012

In this paper, we report that the ratio of broadband energy (0.01-2 Hz) to highfrequency energy (... more In this paper, we report that the ratio of broadband energy (0.01-2 Hz) to highfrequency energy (0.3-2 Hz), E r , estimated from regional seismograms of India, might be a useful parameter in estimating tsunami potential of earthquakes in the Sumatra-Andaman region. E r is expected to be sensitive to the depth as well as to the source characteristics of an earthquake. Since a shallow and slow earthquake has a greater tsunamigenic potential, E r may be a useful diagnostic parameter. We base our analysis on broadband seismograms of the great earthquakes of Sumatra-Andaman (2004, M w

Tectonophysics, 2013

Abstract P and S receiver functions (PRFs and SRFs, respectively) for 21 broad-band seismograph s... more Abstract P and S receiver functions (PRFs and SRFs, respectively) for 21 broad-band seismograph stations of the India Meteorological Department (IMD) illuminate lithosphere and the underlying mantle of some previously poorly sampled regions of the Indian sub-continent. Our analysis demonstrates that the Archean and Early Proterozoic lithospheric keel of the Indian shield has been reworked by younger processes. We find very low S-wave velocities in the uppermost mantle (from 4.0 to 4.3 km/s) to the north of the Deccan Volcanic Province (Kutch region and Aravalli Craton) (1), in the south (Southern Granulite Terrain and Sri Lanka) (2) and in the north-east (Gangetic Plane, Bengal Basin and Singhbuhm Craton) (3). The anomalies 1 and 2 may extend into the transition zone. Early arrivals of the S410p seismic phase are indicative of anomalously high Vp/Vs ratio (~ 1.9) in the upper mantle of the low-velocity regions, whereas late arrivals in the western Himalaya, Ladakh and western Tibet are consistent with the previously found indications of anomalously low Vp/Vs ratio. A transition from the high-S-velocity mantle lid to a layer of slightly lower velocity is seen in part of the data but a straightforward interpretation of this transition as the lithosphere–asthenosphere boundary is problematic. A mafic S velocity in the upper crust and a pronounced low-S-velocity layer in the lower crust beneath the eruptive center is practically the only specific feature in the lithosphere that may be linked to the Deccan Traps. A separation in depths between the 410-km and 660-km discontinuities varies laterally in a range from 240 to 270 km. The largest uplift of the 410-km discontinuity (up to 390 km) is observed beneath the foothills of the Himalaya where it is caused by cooling of the transition zone by the ongoing continental collision.

We present a preliminary source study of the Muzaffarabad earthquake of 8 October 2005 (Mw 7.6) a... more We present a preliminary source study of the Muzaffarabad earthquake of 8 October 2005 (Mw 7.6) and the far-field ground motions that it generated. Our analysis is based on regional broadband seismograms recorded at stations operated by the India Meteorological Department (IMD) which are situated to the south of the epicentre, and at non-IMD stations which are located to the north. We find that the source spectrum of the earthquake is reasonably consistent with ω 2 -source model with a seismic moment, M 0 , of 2.94 x 10 20 N-m and a corner frequency, fc, of 0.051 Hz (Brune stress drop of 9.5 MPa). The radiated seismic energy, E R , estimated from the empirical Green's function (EGF) technique is 2.70 × 10 16 J. This yields a normalized radiated energy, E R /M 0 , of 9.1 × 10 -5 , and an apparent stress, τ a , of 2.7 MPa. The rupture area of 100 × 15 km2 (estimated from slip distribution mapped from the inversion of teleseismic body waves) gives a static stress drop of about 11.3 MPa. From these source parameters we estimate a radiation efficiency of 0.49, implying a 'brittle' rupture typical of interplate events. Stochastic method requires a stress drop of -10 MPa to explain the observed peak ground motions (A max and V max ) recorded at regional distances, and predicts A max and V max exceeding 1 g and 100 cm/s, respectively at hard sites in the epicentral region. The source parameters and far-field ground motions of the Muzaffarabad and Bhuj earthquakes are quite similar even though the tectonic environment and the depth of their occurrence are distinct.

Geomatics, Natural Hazards and Risk, 2015

Journal of Geophysical Research B: Solid Earth, 2013

ABSTRACT Ever since its breakup from the Gondwanaland ~140 Myr ago, the Indian plate was ravaged ... more ABSTRACT Ever since its breakup from the Gondwanaland ~140 Myr ago, the Indian plate was ravaged by four hot spots. Although the surface manifestations of such deep processes are evident in terms of large igneous provinces like the Deccan and the fast drift of the Indian plate, the modifications to the deep structure remain to be grasped. In this study, we investigate the mantle transition zone (TZ) structure beneath the Indian shield region using ~14,000 teleseismic receiver functions from 77 broadband stations sited on diverse geologic terrains. The arrival times of the P-to-s (Ps) conversions from the 410 km discontinuity at most cratonic stations appear to be delayed by ~2 s in comparison with the times observed for other Precambrian shield regions like Africa, Australia, and Canada. Such delays in the conversions from the 410 km discontinuity below the Indian shield suggest low shear wave speeds in the lithospheric and sub-lithospheric mantles due to higher temperatures, together with a thinner high velocity lid that contrasts with a thicker one found beneath most Archean cratons. A thin transition zone beneath most of the cratonic stations lends support to the enhanced temperatures within the TZ itself. Also, a further delay of the TZ discontinuities is observed for stations on the southern granulite terrain, which was under the influence of the Marion plume that is responsible for the separation of Madagascar from India. Although the data do not conclusively show evidence for a 520 km discontinuity, an LVL atop the 410 cannot be ruled out beneath certain geological provinces of the Indian shield.

Russian Journal of Earth Sciences, 2008

Seismological Research Letters, 2005

... Singh, SK, BK Bansal, SN Bhattacharya, JF Pacheco, RS Dattatrayam, M. Ordaz, G. Suresh, Kamal... more ... Singh, SK, BK Bansal, SN Bhattacharya, JF Pacheco, RS Dattatrayam, M. Ordaz, G. Suresh, Kamal, and SE Hough (2003). ... Speed and size of the Sumatra earthquake, Nature 434,581 -582.[Medline]. Tolstoy, Maya and DelWayne R. Bohnenstiehl (2005). ...

Pure and Applied Geophysics, 2006

Q C -estimates of Kachchh Basin in western India have been obtained in a high frequency range fro... more Q C -estimates of Kachchh Basin in western India have been obtained in a high frequency range from 1.5 to 24.0 Hz using the aftershock data of Bhuj earthquake of January 26, 2001 recorded within an epicentral distance of 80 km. The decay of coda waves of 30 sec window from 186 seismograms has been analysed in four lapse time windows, adopting the single backscattering model. The study shows that Q c is a function of frequency and increases as frequency increases. The frequency dependent Q c relations obtained for four lapse-time windows are: Q c =82 f 1.17 (20-50 sec), Q c =106 f 1.11 (30-60 sec), Q c =126f 1.03 (40-70 sec) and Q c =122f 1.02 (50-80 sec). These empirical relations represent the average attenuation properties of a zone covering the surface area of about 11,000, 20,000, 28,000 and 38,000 square km and a depth extent of about 60, 80, 95, 110 km, respectively. With increasing window length, the degree of frequency dependence, n, decreases marginally from 1.17 to 1.02, whereas Q 0 increases significantly from 82 to 122. At lower frequencies up to 6 Hz, Q c )1 of Kachchh Basin is in agreement with other regions of the world, whereas at higher frequencies from 12 to 24 Hz it is found to be low. Figure 1 Map showing epicenters of aftershocks of Bhuj earthquake used in the Q c analysis, tectonic features in the Kachchh Basin and the locations of recording stations.

Natural Hazards, 2012

In this paper, we report that the ratio of broadband energy (0.01-2 Hz) to highfrequency energy (... more In this paper, we report that the ratio of broadband energy (0.01-2 Hz) to highfrequency energy (0.3-2 Hz), E r , estimated from regional seismograms of India, might be a useful parameter in estimating tsunami potential of earthquakes in the Sumatra-Andaman region. E r is expected to be sensitive to the depth as well as to the source characteristics of an earthquake. Since a shallow and slow earthquake has a greater tsunamigenic potential, E r may be a useful diagnostic parameter. We base our analysis on broadband seismograms of the great earthquakes of Sumatra-Andaman (2004, M w * 9.2) and Nias (2005, M w 8.6), 41 of their aftershocks, and the earthquakes of north Sumatra (2010, M w 7.8) and Nicobar (2010, M w 7.4) recorded at VISK, a station located on the east coast of India. In the analysis, we also included the two recent, great strike-slip earthquakes of north Sumatra (2012, M w 8.6, 8.2) recorded at VISK and three south Sumatra earthquakes (2007, M w 8.5; 2007, M w 7.9; 2010, M w 7.8) recorded at PALK, a station in Sri Lanka. We find that E r is a function of depth; shallower earthquakes have higher E r values than the deeper ones. Thus, E r may be indicative of tsunamigenic potential of an earthquake. As M w and E r increase so does the tsunami potential. In addition to the parameter E r , the radiated seismic energy, E s , may be estimated from the regional seismograms in India using empirical Green's function technique. The technique yields reliable E s for the great Sumatra and Nias earthquakes. E r and E s computed from VISK data, along with M w and focal mechanism, may be useful in

Proceedings of the Indian National Science Academy, 2014

The process of dealing with earthquake disasters essentially involves three most important and in... more The process of dealing with earthquake disasters essentially involves three most important and inter-dependent components -(i) comprehensive understanding of the earthquake generation processes and the interior of the earth, (ii) disaster mitigation and preventive measures, and (iii) work through the ultimate goal of earthquake prediction. The basic and primary requirement towards addressing all these tasks is -high quality seismological data which is homogeneous and complete in time and space. India Meteorological Department (IMD), under the Earth System Science Organization (ESSO), Ministry of Earth Sciences (MoES), is the nodal agency of Government of India for monitoring earthquake activity in and around the country. IMD maintains the national seismological network consisting of a total of 82 observatories spread over the length and breadth of the country. The paper aims at discussing different methods/approaches adopted by IMD and other major agencies in the country for generation of various types of earthquake data products in standard formats, the analyses and archival tools and policy guidelines for supply and sharing amongst the user agencies. The paper also deals with the types of seismic instrumentation/networks in operation, network growth through historical times, data completion aspects, present level(s) of earthquake detection and location, future requirements and plans of upgradation. The policy guidelines being followed for seismological data sharing and supply have also been highlighted.