Chandrani Singh - Academia.edu (original) (raw)

Papers by Chandrani Singh

Research paper thumbnail of Frequency dependent attenuation and relative site response of western Tibet

We have formulated frequency dependent Lg and Pg attenuation tomographic models to investigate th... more We have formulated frequency dependent Lg and Pg attenuation tomographic models to investigate thecrustal Q values and its tectonic implications beneath western Tibet. The frequency dependentbehaviour of both Lg and Pg are studied for the frequency bands of 0.2-0.6, 0.6-1.0 and 1.0-1.4 Hz atcentral frequencies of 0.4, 0.8, and 1.2 Hz, respectively, implementing both Two-Station Method(TSM) and Reverse Two-Station Method (RTSM). The amplitudes of both the waves are fundamentallysensitive to the crustal structures and are controlled by both scattering and intrinsic attenuation. Thefrequency dependent characteristics of QLg and QPg are consistent in nature for the region. Moderate tohigh Q values evident in the Lhasa terrane could supplement the trace of underthrusting Indianlithosphere beneath the region. The average Q values for both Lg and Pg increase with increasingfrequency. The frequency dependent parameter η shows quite high values, for both the waves usingTSM and RTSM, which ma...

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Research paper thumbnail of Seismicity and active tectonics:  New insights from Sikkim Himalaya

The Himalayas, which formed as a result of the impactful collision of the Indian plate with Euras... more The Himalayas, which formed as a result of the impactful collision of the Indian plate with Eurasian plate, is a tectonically complex and seismically active region. It has been a hotspot for many great earthquakes in the past. The continued collision coupled with the complex structural features has led to the persistent seismic activity of the region. The progressive collision led to the formation of distinct tectonic units bounded by thrust faults. The northeastern state of Sikkim in India, which is sandwiched between Nepal and Bhutan in the Himalayas, has been prone to frequent great earthquakes. The deployment of a dense seismic network consisting of 27 broadband seismometers, across Sikkim Himalayas and the northern part of West Bengal, since April 2019 has enabled us to monitor the seismic activity in the study region.Here, we present a study which aims at understanding the seismotectonic activity of the study region using local earthquakes (epicentral distance < 200km) reco...

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Research paper thumbnail of 2-D Sn attenuation tomography of Arunachal Himalaya

&amp;amp;lt;p&amp;amp;gt;In this study, we have presented the first high-resolution 2-D S... more &amp;amp;lt;p&amp;amp;gt;In this study, we have presented the first high-resolution 2-D Sn attenuation tomography image of&amp;amp;lt;br&amp;amp;gt;Arunachal Himalaya to enlighten the lithospheric structure of the area. The region is one of the&amp;amp;lt;br&amp;amp;gt;active segments of the Himalaya and least understood because of the inaccessibility and difficult&amp;amp;lt;br&amp;amp;gt;working conditions. 37 regional earthquakes within the epicentral distance of 250 - 1650 km were&amp;amp;lt;br&amp;amp;gt;recorded by 29 broadband seismic stations operated in Arunachal Himalaya covering the majority&amp;amp;lt;br&amp;amp;gt;portions of the eastern Himalaya are used in the present study.&amp;amp;lt;br&amp;amp;gt;Sn is the uppermost mantle refracted phase travelled with a velocity of 4.3 - 4.7 km/s. It&amp;amp;lt;br&amp;amp;gt;is highly sensitive to the velocity gradient and attenuation in the uppermost mantle. We have&amp;amp;lt;br&amp;amp;gt;categorised the propagation efficiencies of Sn as efficient, inefficient and blocked based on a&amp;amp;lt;br&amp;amp;gt;visual inspection. The inefficient and blocked Sn phases are observed mainly in the western side&amp;amp;lt;br&amp;amp;gt;of our study region. Further, we have obtained the Sn Q tomography model to examine lateral&amp;amp;lt;br&amp;amp;gt;variations in attenuation characteristics, employing the Two Station Method (TSM) using 567 station&amp;amp;lt;br&amp;amp;gt;pairs as input data. The central Arunachal Himalaya exhibits a low Q value (&amp;amp;amp;#8804; 50) whereas&amp;amp;lt;br&amp;amp;gt;Tawang and the western part of Arunachal Himalaya show a high value of Q &amp;amp;amp;#8804; 300. The obtained&amp;amp;lt;br&amp;amp;gt;results are well correlated with the tectonic fabric of the area.&amp;amp;lt;/p&amp;amp;gt;

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Research paper thumbnail of Direct S-wave derived seismic anisotropy beneath Eastern Ghats Mobile Belt and adjacent Archean cratons: Signature of collision and rifting

EGU General Assembly Conference Abstracts, Apr 1, 2019

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Research paper thumbnail of Lg Attenuation Characteristic of Gujarat Region (Western India)

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Research paper thumbnail of Crustal structure and tectonics of Bangladesh: New constraints from inversion of receiver functions

Tectonophysics, 2016

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Research paper thumbnail of A Novel Approach for Earthquake Early Warning System Design using Deep Learning Techniques

Earthquake signals are non-stationary in nature and thus in real-time, it is difficult to identif... more Earthquake signals are non-stationary in nature and thus in real-time, it is difficult to identify and classify events based on classical approaches like peak ground displacement, peak ground velocity. Even the popular algorithm of STA/LTA requires extensive research to determine basic thresholding parameters so as to trigger an alarm. Also, many times due to human error or other unavoidable natural factors such as thunder strikes or landslides, the algorithm may end up raising a false alarm. This work focuses on detecting earthquakes by converting seismograph recorded data into corresponding audio signals for better perception and then uses popular Speech Recognition techniques of Filter bank coefficients and Mel Frequency Cepstral Coefficients (MFCC) to extract the features. These features were then used to train a Convolutional Neural Network(CNN) and a Long Short Term Memory (LSTM) network. The proposed method can overcome the above-mentioned problems and help in detecting earth...

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Research paper thumbnail of An investigation of regional variations of coda wave attenuation in western Tibet

Journal of Seismology, 2020

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Research paper thumbnail of Lateral variation of crustal attenuation properties from southern Tibet to eastern Nepal Himalaya

Geophysical Journal International, 2019

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Research paper thumbnail of Singh review India supplement 2015

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Research paper thumbnail of Erratum to Seismic Attenuation Characteristics along the Hi-CLIMB Profile in Tibet from Lg Q Inversion

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Research paper thumbnail of Discerning Structure and Seismic Hazards in the Sikkim Himalayas

Eos

A dense seismic network in operation since 2019 will provide new insights into the tectonics of s... more A dense seismic network in operation since 2019 will provide new insights into the tectonics of seismically active Himalayan regions.

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Research paper thumbnail of Mantle deformation patterns beneath southern Tibet using splitting of direct-S waves

This study presents a total of 12008 shear wave splitting measurements obtained using the referen... more This study presents a total of 12008 shear wave splitting measurements obtained using the reference station technique applied to direct S-waves from 106 earthquakes recorded at 143 seismic stations of the Hi-CLIMB seismic network. The results reveal significant anisotropy in regions of southern Tibet where null or negligible anisotropy has been hitherto reported from SK(K)S measurements. While the individual fast polarization direction (FPD) at each station are found to be consistent, the splitting time delays (TDs) exhibit deviations particularly at stations located south of the Indus–Tsangpo Suture Zone. The fast polarization directions (FPDs) are oriented (a) NE–SW to E–W to the south of the Indus–Tsangpo Suture Zone (b) NE–SW to ENE–SSW between Bangong–Nujiang Suture Zone and the Indus–Tsangpo Suture Zone (ITSZ) and (c) E–W to the extreme north of the profile. The splitting time delays (dt) vary between 0.45 and 1.3 s south of the ITSZ (&lt;30N latitude), while they range from 0.9 to 1.4 s north of it. The overall trends are similar to SKS/SKKS results. However, the differences may be due to the not so near vertical paths of direct S waves which may sample the anisotropy in a different way in comparison to SKS waves, or insufficient number of SKS observations. The significant anisotropy (0.8 s) observed beneath Hi-malaya reveals a complex deformation pattern in the region and can be best explained by the combined effects of deformation related to shear at the base of the lithosphere and subduction related flows with possible contributions from the crust. Additional measurements obtained using direct S-waves provide new constraints in regions with complex anisotropy.

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Research paper thumbnail of Evolution of b-values before large earthquakes of mb ≥6.0 in Andaman region

We have considered six earthquakes of mb ≥6.0, with focal depths within 45 km, in Andaman region ... more We have considered six earthquakes of mb ≥6.0, with focal depths within 45 km, in Andaman region during 2000 to 2012 to examine the spatial variations of b-value at epicentre within a one-year period prior to the occurrence of the earthquakes. We have found a correlation between the low b for the one year time interval and the occurrence of large events. The epicentral b-values of six earthquakes are lower than 1.0 and five out of six earthquakes show very low b-value at the epicentre. Our study may indicate that b-value can be employed as a possible precursor for the forecasting of a major earthquake.

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Research paper thumbnail of Drained and undrained responses for Koyna-Warna earthquakes from 1993 to 1994 following impoundment of the Warna reservoir in India

The Koyna region along the west coast of India in Maharashtra, is a unique case of reservoir trig... more The Koyna region along the west coast of India in Maharashtra, is a unique case of reservoir triggered seismicity, where seismicity has been reported since 1963 after the initial impoundment of the Shivajisagar reservoir behind Koyna dam in 1961. The region was further destabilized following the filling of the Warna reservoir, situated 25 km south of Koyna, during the late 1980s. Increase in pore pressure by drained and undrained effects is known to weaken the substratum below a water reservoir, facilitating the onset of seismicity. Although their relative influences may vary with time and space, it is difficult to separate their individual contributions. The present study, using the well-located earthquakes of M ≥ 5 for the period 1993-94, is a preliminary attempt to identify these effects after impoundment of the Warna reservoir. It could provide a good opportunity for further modelling the strength changes due to reservoir impoundment.

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Research paper thumbnail of Seismic anisotropy and mantle deformation beneath Eastern Ghats Mobile Belt using direct-S waves

Precambrian Research

Abstract Seismic anisotropy observations provide a potential tool to constrain the signatures of ... more Abstract Seismic anisotropy observations provide a potential tool to constrain the signatures of various continental-scale deformations that have shaped the lithospheric structure of various mobile belts and ancient cratons across the world. They shed light on various aspects of evolutionary tectonics in any region. The current study primarily aims at examining the direct S-wave derived seismic anisotropy beneath the Eastern Ghats Mobile Belt (EGMB) and the adjacent Archean Singhbhum and Bastar Cratons. The granulite Terrane of EGMB represents the complex deformational history and is a crucial link in the reconstruction of Rodinia and Gondwana supercontinents. The Terrane underwent collision and rifting in Mesoproterozoic, followed by thermal overprint of the mid-Neoproterozoic to early Phanerozoic orogeny modifying its lithospheric structure. We have implemented the Reference Station Technique and obtained 854 well-constrained individual splitting measurements. The observations are based on 185 earthquake events (with Mw ⩾ 5.5, and epicentral distance ranging 30 ° to 90 ° ) recorded at 27 seismic stations deployed in the study area. The large splitting delay times (1.2–2.3 s) suggest that the lithospheric mantle is highly anisotropic. The NNE-SSW oriented Fast Polarization Directions (FPDs) observed at the Singhbhum Craton can be devoted to the evolutionary tectonic regime of the craton. The strained minerals at the deeper depths, result in FPD patterns sub-parallel to the Kerajung Fault Zone ( ~ 70 ° ) in the Angul Domain. The FPDs are predominantly oriented along the Absolute Plate Motion (APM) direction of the Indian plate ( ~ 39 ° ) for the Bastar Craton. However, the varying FPD patterns in the Eastern Ghats Boundary Shear Zone (Khariar Domain) signify the influence of the lithospheric deformation along with APM. The imprints of the continental building orogenies, that have modified the lithospheric structure of the study area over the Mesoproterozoic, mid-Neoproterozoic, and early Phanerozoic periods, are preserved as frozen anisotropic signatures. Our observations highlight these signatures. The study also investigates seismic anisotropic patterns in the hitherto uncharted regions close to Chilka Lake using the core-mantle refracted (SKS, SKKS, and PKS) and direct S phases. The markedly distinct E-W (average ~ 95 ° ) oriented fast wave azimuths, against the ~ 47 ° average FPD of the adjacent western Phulbani Domain, distinguishes it as a separate block amongst the collage of domains forming the Eastern Ghats Province.

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Research paper thumbnail of Depth-wise attenuation mechanism of seismic waves in the Andaman region

Soil Dynamics and Earthquake Engineering

Abstract This study evaluates the depth-dependent seismic wave attenuation characteristics in the... more Abstract This study evaluates the depth-dependent seismic wave attenuation characteristics in the Andaman region which is a part of the Andaman-Nicobar Subduction zone (ANSZ), and possible causes. We have analyzed seismic waveforms of 1402 earthquakes recorded at seven broadband seismic stations located over the Andaman Island, to estimate the direct S-wave (Qs), coda wave (Qc), intrinsic (Qi) and scattering (Qsc) attenuation parameters. The coda normalization method is used to calculate Qs. Additionally, the variation of Qc with frequency and coda window length is studied using the single back-scattering method. The separation of Qi and Qsc is achieved using the Wennerberg’s method. Results show a strong frequency dependence of Qc and Qs. At 1 Hz, the values of Qc are 136, 137, 137 and 146 for coda window length of 30, 40, 50 and 60 s respectively, with the corresponding frequency-dependent coefficient (n) of 0.82, 0.83, 0.85 and 0.82. At 1 Hz the observed Qi values show a close proximity to Qc values, which indicates a strong dominance of intrinsic attenuation. Also, it is observed that Q0i shows low value and remains almost the same between 115 and 132 km depth, with a slight increase in between 132 − 140 km. The observed results imply high intrinsic attenuation, possibly due to partial melting or fluid-filled rocks at these depths. Whereas Q0sc shows a different trend. It gradually increases up to a depth of 132 km, with a slight decrease beyond that. This indicates a decrease in heterogeneity up to 132 km and a slight increase in the same below that. Further, the estimates of pressure (P) and temperature (T) conditions of the subducting slab with depth show an increase in temperature from 996° to 1015°C and an increase in pressure from 33.8 to 41.2 kbar, as the depth increases from 115 to 140 km. The phase relationship for the basaltic rocks with 5 − 6 wt% bound H2O in eclogite facies reveals that the P − T condition observed in this study lies above the solidus temperature. This promotes the formation of interstitial liquid (5 − 10%) at these depths, probably causing high intrinsic attenuation.

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Research paper thumbnail of Sn attenuation tomography of southeastern Tibet: new constraints on lithospheric mantle deformation

Geophysical Journal International

Summary We have formulated a two-dimensional Sn attenuation tomographic model to investigate the ... more Summary We have formulated a two-dimensional Sn attenuation tomographic model to investigate the uppermost mantle shear wave Q and its tectonic implications beneath southeastern Tibet near Namche Barwa. To achieve our objective, we first compute inter-station Q values using the Two Station Method (TSM) analysis on 618 station pairs obtained from 26 regional earthquakes (Mw ≥5.5) with epicentral distances ranging from 5○ to 15○ recorded at 47 seismic stations belonging to the Namche Barwa network (XE network, 2003−2004). Furthermore, the QSn tomographic model is generated by utilizing these inter-station Q values. QSn values are varying from 101 to 490 in the region. The tomography image reveals high attenuation (≤200 Q values) in the central region. Regions of low attenuation (>200 Q values) are observed in the southern part and in some small regions beneath the northern side of the study area. Consecutive high-low-high QSn values have been observed in the south part of the Lhasa...

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Research paper thumbnail of Seismic attenuation structure across the Karakoram fault in western Tibet

Geophysical Journal International

SUMMARY 2-D attenuation maps are produced for the crust of western Tibet using local earthquakes ... more SUMMARY 2-D attenuation maps are produced for the crust of western Tibet using local earthquakes which are recorded by an array of 31 broad-band stations operated from 2007 July to 2011 May. Relative contribution of scattering ($Q_{sc}^{-1}$) and intrinsic ($Q_{i}^{-1}$) attenuation have been calculated using Multiple Lapse Time Window Analysis under the assumption of uniform distribution of multiple isotropic scattering and intrinsic absorption in a medium for five different frequency bands centred at 1.5, 3, 6, 12 and 18 Hz, respectively. All the events are selected on the basis of high signal-to-noise ratio having hypocentral distance within 200 km from the respective stations. The obtained Q−1 values show a strong frequency dependent nature which can be correlated to the degree of tectonic complexity and the heterogeneities present in the medium. The intrinsic absorption is found to be the dominant mechanism at all the frequency ranges for all stations except few (WT03, WT07 and...

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Research paper thumbnail of Source Spectral studies using Lg wave in western Tibet

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Research paper thumbnail of Frequency dependent attenuation and relative site response of western Tibet

We have formulated frequency dependent Lg and Pg attenuation tomographic models to investigate th... more We have formulated frequency dependent Lg and Pg attenuation tomographic models to investigate thecrustal Q values and its tectonic implications beneath western Tibet. The frequency dependentbehaviour of both Lg and Pg are studied for the frequency bands of 0.2-0.6, 0.6-1.0 and 1.0-1.4 Hz atcentral frequencies of 0.4, 0.8, and 1.2 Hz, respectively, implementing both Two-Station Method(TSM) and Reverse Two-Station Method (RTSM). The amplitudes of both the waves are fundamentallysensitive to the crustal structures and are controlled by both scattering and intrinsic attenuation. Thefrequency dependent characteristics of QLg and QPg are consistent in nature for the region. Moderate tohigh Q values evident in the Lhasa terrane could supplement the trace of underthrusting Indianlithosphere beneath the region. The average Q values for both Lg and Pg increase with increasingfrequency. The frequency dependent parameter η shows quite high values, for both the waves usingTSM and RTSM, which ma...

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Research paper thumbnail of Seismicity and active tectonics:  New insights from Sikkim Himalaya

The Himalayas, which formed as a result of the impactful collision of the Indian plate with Euras... more The Himalayas, which formed as a result of the impactful collision of the Indian plate with Eurasian plate, is a tectonically complex and seismically active region. It has been a hotspot for many great earthquakes in the past. The continued collision coupled with the complex structural features has led to the persistent seismic activity of the region. The progressive collision led to the formation of distinct tectonic units bounded by thrust faults. The northeastern state of Sikkim in India, which is sandwiched between Nepal and Bhutan in the Himalayas, has been prone to frequent great earthquakes. The deployment of a dense seismic network consisting of 27 broadband seismometers, across Sikkim Himalayas and the northern part of West Bengal, since April 2019 has enabled us to monitor the seismic activity in the study region.Here, we present a study which aims at understanding the seismotectonic activity of the study region using local earthquakes (epicentral distance < 200km) reco...

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Research paper thumbnail of 2-D Sn attenuation tomography of Arunachal Himalaya

&amp;amp;lt;p&amp;amp;gt;In this study, we have presented the first high-resolution 2-D S... more &amp;amp;lt;p&amp;amp;gt;In this study, we have presented the first high-resolution 2-D Sn attenuation tomography image of&amp;amp;lt;br&amp;amp;gt;Arunachal Himalaya to enlighten the lithospheric structure of the area. The region is one of the&amp;amp;lt;br&amp;amp;gt;active segments of the Himalaya and least understood because of the inaccessibility and difficult&amp;amp;lt;br&amp;amp;gt;working conditions. 37 regional earthquakes within the epicentral distance of 250 - 1650 km were&amp;amp;lt;br&amp;amp;gt;recorded by 29 broadband seismic stations operated in Arunachal Himalaya covering the majority&amp;amp;lt;br&amp;amp;gt;portions of the eastern Himalaya are used in the present study.&amp;amp;lt;br&amp;amp;gt;Sn is the uppermost mantle refracted phase travelled with a velocity of 4.3 - 4.7 km/s. It&amp;amp;lt;br&amp;amp;gt;is highly sensitive to the velocity gradient and attenuation in the uppermost mantle. We have&amp;amp;lt;br&amp;amp;gt;categorised the propagation efficiencies of Sn as efficient, inefficient and blocked based on a&amp;amp;lt;br&amp;amp;gt;visual inspection. The inefficient and blocked Sn phases are observed mainly in the western side&amp;amp;lt;br&amp;amp;gt;of our study region. Further, we have obtained the Sn Q tomography model to examine lateral&amp;amp;lt;br&amp;amp;gt;variations in attenuation characteristics, employing the Two Station Method (TSM) using 567 station&amp;amp;lt;br&amp;amp;gt;pairs as input data. The central Arunachal Himalaya exhibits a low Q value (&amp;amp;amp;#8804; 50) whereas&amp;amp;lt;br&amp;amp;gt;Tawang and the western part of Arunachal Himalaya show a high value of Q &amp;amp;amp;#8804; 300. The obtained&amp;amp;lt;br&amp;amp;gt;results are well correlated with the tectonic fabric of the area.&amp;amp;lt;/p&amp;amp;gt;

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Research paper thumbnail of Direct S-wave derived seismic anisotropy beneath Eastern Ghats Mobile Belt and adjacent Archean cratons: Signature of collision and rifting

EGU General Assembly Conference Abstracts, Apr 1, 2019

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Research paper thumbnail of Lg Attenuation Characteristic of Gujarat Region (Western India)

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Research paper thumbnail of Crustal structure and tectonics of Bangladesh: New constraints from inversion of receiver functions

Tectonophysics, 2016

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Research paper thumbnail of A Novel Approach for Earthquake Early Warning System Design using Deep Learning Techniques

Earthquake signals are non-stationary in nature and thus in real-time, it is difficult to identif... more Earthquake signals are non-stationary in nature and thus in real-time, it is difficult to identify and classify events based on classical approaches like peak ground displacement, peak ground velocity. Even the popular algorithm of STA/LTA requires extensive research to determine basic thresholding parameters so as to trigger an alarm. Also, many times due to human error or other unavoidable natural factors such as thunder strikes or landslides, the algorithm may end up raising a false alarm. This work focuses on detecting earthquakes by converting seismograph recorded data into corresponding audio signals for better perception and then uses popular Speech Recognition techniques of Filter bank coefficients and Mel Frequency Cepstral Coefficients (MFCC) to extract the features. These features were then used to train a Convolutional Neural Network(CNN) and a Long Short Term Memory (LSTM) network. The proposed method can overcome the above-mentioned problems and help in detecting earth...

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Research paper thumbnail of An investigation of regional variations of coda wave attenuation in western Tibet

Journal of Seismology, 2020

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Research paper thumbnail of Lateral variation of crustal attenuation properties from southern Tibet to eastern Nepal Himalaya

Geophysical Journal International, 2019

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Research paper thumbnail of Singh review India supplement 2015

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Research paper thumbnail of Erratum to Seismic Attenuation Characteristics along the Hi-CLIMB Profile in Tibet from Lg Q Inversion

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Research paper thumbnail of Discerning Structure and Seismic Hazards in the Sikkim Himalayas

Eos

A dense seismic network in operation since 2019 will provide new insights into the tectonics of s... more A dense seismic network in operation since 2019 will provide new insights into the tectonics of seismically active Himalayan regions.

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Research paper thumbnail of Mantle deformation patterns beneath southern Tibet using splitting of direct-S waves

This study presents a total of 12008 shear wave splitting measurements obtained using the referen... more This study presents a total of 12008 shear wave splitting measurements obtained using the reference station technique applied to direct S-waves from 106 earthquakes recorded at 143 seismic stations of the Hi-CLIMB seismic network. The results reveal significant anisotropy in regions of southern Tibet where null or negligible anisotropy has been hitherto reported from SK(K)S measurements. While the individual fast polarization direction (FPD) at each station are found to be consistent, the splitting time delays (TDs) exhibit deviations particularly at stations located south of the Indus–Tsangpo Suture Zone. The fast polarization directions (FPDs) are oriented (a) NE–SW to E–W to the south of the Indus–Tsangpo Suture Zone (b) NE–SW to ENE–SSW between Bangong–Nujiang Suture Zone and the Indus–Tsangpo Suture Zone (ITSZ) and (c) E–W to the extreme north of the profile. The splitting time delays (dt) vary between 0.45 and 1.3 s south of the ITSZ (&lt;30N latitude), while they range from 0.9 to 1.4 s north of it. The overall trends are similar to SKS/SKKS results. However, the differences may be due to the not so near vertical paths of direct S waves which may sample the anisotropy in a different way in comparison to SKS waves, or insufficient number of SKS observations. The significant anisotropy (0.8 s) observed beneath Hi-malaya reveals a complex deformation pattern in the region and can be best explained by the combined effects of deformation related to shear at the base of the lithosphere and subduction related flows with possible contributions from the crust. Additional measurements obtained using direct S-waves provide new constraints in regions with complex anisotropy.

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Research paper thumbnail of Evolution of b-values before large earthquakes of mb ≥6.0 in Andaman region

We have considered six earthquakes of mb ≥6.0, with focal depths within 45 km, in Andaman region ... more We have considered six earthquakes of mb ≥6.0, with focal depths within 45 km, in Andaman region during 2000 to 2012 to examine the spatial variations of b-value at epicentre within a one-year period prior to the occurrence of the earthquakes. We have found a correlation between the low b for the one year time interval and the occurrence of large events. The epicentral b-values of six earthquakes are lower than 1.0 and five out of six earthquakes show very low b-value at the epicentre. Our study may indicate that b-value can be employed as a possible precursor for the forecasting of a major earthquake.

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Research paper thumbnail of Drained and undrained responses for Koyna-Warna earthquakes from 1993 to 1994 following impoundment of the Warna reservoir in India

The Koyna region along the west coast of India in Maharashtra, is a unique case of reservoir trig... more The Koyna region along the west coast of India in Maharashtra, is a unique case of reservoir triggered seismicity, where seismicity has been reported since 1963 after the initial impoundment of the Shivajisagar reservoir behind Koyna dam in 1961. The region was further destabilized following the filling of the Warna reservoir, situated 25 km south of Koyna, during the late 1980s. Increase in pore pressure by drained and undrained effects is known to weaken the substratum below a water reservoir, facilitating the onset of seismicity. Although their relative influences may vary with time and space, it is difficult to separate their individual contributions. The present study, using the well-located earthquakes of M ≥ 5 for the period 1993-94, is a preliminary attempt to identify these effects after impoundment of the Warna reservoir. It could provide a good opportunity for further modelling the strength changes due to reservoir impoundment.

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Research paper thumbnail of Seismic anisotropy and mantle deformation beneath Eastern Ghats Mobile Belt using direct-S waves

Precambrian Research

Abstract Seismic anisotropy observations provide a potential tool to constrain the signatures of ... more Abstract Seismic anisotropy observations provide a potential tool to constrain the signatures of various continental-scale deformations that have shaped the lithospheric structure of various mobile belts and ancient cratons across the world. They shed light on various aspects of evolutionary tectonics in any region. The current study primarily aims at examining the direct S-wave derived seismic anisotropy beneath the Eastern Ghats Mobile Belt (EGMB) and the adjacent Archean Singhbhum and Bastar Cratons. The granulite Terrane of EGMB represents the complex deformational history and is a crucial link in the reconstruction of Rodinia and Gondwana supercontinents. The Terrane underwent collision and rifting in Mesoproterozoic, followed by thermal overprint of the mid-Neoproterozoic to early Phanerozoic orogeny modifying its lithospheric structure. We have implemented the Reference Station Technique and obtained 854 well-constrained individual splitting measurements. The observations are based on 185 earthquake events (with Mw ⩾ 5.5, and epicentral distance ranging 30 ° to 90 ° ) recorded at 27 seismic stations deployed in the study area. The large splitting delay times (1.2–2.3 s) suggest that the lithospheric mantle is highly anisotropic. The NNE-SSW oriented Fast Polarization Directions (FPDs) observed at the Singhbhum Craton can be devoted to the evolutionary tectonic regime of the craton. The strained minerals at the deeper depths, result in FPD patterns sub-parallel to the Kerajung Fault Zone ( ~ 70 ° ) in the Angul Domain. The FPDs are predominantly oriented along the Absolute Plate Motion (APM) direction of the Indian plate ( ~ 39 ° ) for the Bastar Craton. However, the varying FPD patterns in the Eastern Ghats Boundary Shear Zone (Khariar Domain) signify the influence of the lithospheric deformation along with APM. The imprints of the continental building orogenies, that have modified the lithospheric structure of the study area over the Mesoproterozoic, mid-Neoproterozoic, and early Phanerozoic periods, are preserved as frozen anisotropic signatures. Our observations highlight these signatures. The study also investigates seismic anisotropic patterns in the hitherto uncharted regions close to Chilka Lake using the core-mantle refracted (SKS, SKKS, and PKS) and direct S phases. The markedly distinct E-W (average ~ 95 ° ) oriented fast wave azimuths, against the ~ 47 ° average FPD of the adjacent western Phulbani Domain, distinguishes it as a separate block amongst the collage of domains forming the Eastern Ghats Province.

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Research paper thumbnail of Depth-wise attenuation mechanism of seismic waves in the Andaman region

Soil Dynamics and Earthquake Engineering

Abstract This study evaluates the depth-dependent seismic wave attenuation characteristics in the... more Abstract This study evaluates the depth-dependent seismic wave attenuation characteristics in the Andaman region which is a part of the Andaman-Nicobar Subduction zone (ANSZ), and possible causes. We have analyzed seismic waveforms of 1402 earthquakes recorded at seven broadband seismic stations located over the Andaman Island, to estimate the direct S-wave (Qs), coda wave (Qc), intrinsic (Qi) and scattering (Qsc) attenuation parameters. The coda normalization method is used to calculate Qs. Additionally, the variation of Qc with frequency and coda window length is studied using the single back-scattering method. The separation of Qi and Qsc is achieved using the Wennerberg’s method. Results show a strong frequency dependence of Qc and Qs. At 1 Hz, the values of Qc are 136, 137, 137 and 146 for coda window length of 30, 40, 50 and 60 s respectively, with the corresponding frequency-dependent coefficient (n) of 0.82, 0.83, 0.85 and 0.82. At 1 Hz the observed Qi values show a close proximity to Qc values, which indicates a strong dominance of intrinsic attenuation. Also, it is observed that Q0i shows low value and remains almost the same between 115 and 132 km depth, with a slight increase in between 132 − 140 km. The observed results imply high intrinsic attenuation, possibly due to partial melting or fluid-filled rocks at these depths. Whereas Q0sc shows a different trend. It gradually increases up to a depth of 132 km, with a slight decrease beyond that. This indicates a decrease in heterogeneity up to 132 km and a slight increase in the same below that. Further, the estimates of pressure (P) and temperature (T) conditions of the subducting slab with depth show an increase in temperature from 996° to 1015°C and an increase in pressure from 33.8 to 41.2 kbar, as the depth increases from 115 to 140 km. The phase relationship for the basaltic rocks with 5 − 6 wt% bound H2O in eclogite facies reveals that the P − T condition observed in this study lies above the solidus temperature. This promotes the formation of interstitial liquid (5 − 10%) at these depths, probably causing high intrinsic attenuation.

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Research paper thumbnail of Sn attenuation tomography of southeastern Tibet: new constraints on lithospheric mantle deformation

Geophysical Journal International

Summary We have formulated a two-dimensional Sn attenuation tomographic model to investigate the ... more Summary We have formulated a two-dimensional Sn attenuation tomographic model to investigate the uppermost mantle shear wave Q and its tectonic implications beneath southeastern Tibet near Namche Barwa. To achieve our objective, we first compute inter-station Q values using the Two Station Method (TSM) analysis on 618 station pairs obtained from 26 regional earthquakes (Mw ≥5.5) with epicentral distances ranging from 5○ to 15○ recorded at 47 seismic stations belonging to the Namche Barwa network (XE network, 2003−2004). Furthermore, the QSn tomographic model is generated by utilizing these inter-station Q values. QSn values are varying from 101 to 490 in the region. The tomography image reveals high attenuation (≤200 Q values) in the central region. Regions of low attenuation (>200 Q values) are observed in the southern part and in some small regions beneath the northern side of the study area. Consecutive high-low-high QSn values have been observed in the south part of the Lhasa...

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Research paper thumbnail of Seismic attenuation structure across the Karakoram fault in western Tibet

Geophysical Journal International

SUMMARY 2-D attenuation maps are produced for the crust of western Tibet using local earthquakes ... more SUMMARY 2-D attenuation maps are produced for the crust of western Tibet using local earthquakes which are recorded by an array of 31 broad-band stations operated from 2007 July to 2011 May. Relative contribution of scattering ($Q_{sc}^{-1}$) and intrinsic ($Q_{i}^{-1}$) attenuation have been calculated using Multiple Lapse Time Window Analysis under the assumption of uniform distribution of multiple isotropic scattering and intrinsic absorption in a medium for five different frequency bands centred at 1.5, 3, 6, 12 and 18 Hz, respectively. All the events are selected on the basis of high signal-to-noise ratio having hypocentral distance within 200 km from the respective stations. The obtained Q−1 values show a strong frequency dependent nature which can be correlated to the degree of tectonic complexity and the heterogeneities present in the medium. The intrinsic absorption is found to be the dominant mechanism at all the frequency ranges for all stations except few (WT03, WT07 and...

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Research paper thumbnail of Source Spectral studies using Lg wave in western Tibet

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