• Basab Mukhopadhyay

      Articles written in Journal of Earth System Science

    • Seismicity pattern in north Sumatra–Great Nicobar region: In search of precursor for the 26 December 2004 earthquake

      Sujit Dasgupta Basab Mukhopadhyay Auditeya Bhattacharya

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      We analyse the seismicity pattern including 𝑏-value in the north Sumatra–Great Nicobar region from 1976 to 2004. The analysis suggests that there were a number of significant, intermediate and short-term precursors before the magnitude 7.6 earthquake of 2 November 2002. However, they were not found to be so prominent prior to the magnitude 9.0 earthquake of 26 December 2004 though downward migration of activity and a 50-day short-term quiescence was observed before the event. The various precursors identified include post-seismic and intermediate-term quiescence of 13 and 10 years respectively, between the 1976 (magnitude 6.3) and 2002 earthquakes with two years (1990–1991) of increase in background seismicity; renewed seismicity, downward migration of seismic activity and foreshocks in 2002, just before the mainshock. Spatial variation in 𝑏-value with time indicates precursory changes in the form of high 𝑏-value zone near the epicenter preceding the mainshocks of 2004 and 2002 and temporal rise in 𝑏-value in the epicentral area before the 2002 earthquake.

    • Cinder cone morphometry in relation to gravity anomaly zones in the Harrat al Birk and Asir foreland, SW Saudi Arabia

      Basab Mukhopadhyay Manoj Mukhopadhyay Eslam Elawadi Uday Kumar Ghosh Kaushik Pramanik

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      The Harrat al Birk (HAB) of the alkali basalts of the post-Miocene age erupted through a thick Precambrian crust along the Red Sea coast. The 200 cinder cones scatter within HAB and three distinct Bouguer anomaly zones are identified as the coastal gravity high (CGH), axial gravity low (AGL) and a gravity high further east Jabal As-Sawda gravity high (JSGH). Interpreted satellite imagery for 150 sizeable cinder cones in three gravity anomaly zones (134 (90%) in CGH, 10 (6%) in AGL and 6 (4%) in JSGH) are morphologically analysed. The juvenile cinder cones (age <1 Ma) are characterised by sub-vertical slope angles (<40$^{\circ}$) with a variable average geomorphic slope, 27$^{\circ}$–40$^{\circ}$ in JSGH, 25$^{\circ}$ in AGL and 2$^{\circ}$–15$^{\circ}$ in CGH. The oldest cones of JSGH display a high $H_{\rm{co}}$/$W_{\rm{co}}$ ratio with angular asymmetry at their base angles. The major feeder lineaments/faults are oriented NNW–SSE to NW–SE, and they supply volcanic material to cinder cones within a distance of 2 km. The results, when integrated with geological maps and gravity profiles, provide a probable root-plumbing system of the volcano edifice. HAB is the produce of the volcanic process from the source area in JSGH that hosts the major cinders, where the tertiary gabbroic dyke zone acts as a vertical magma sheet for the cinders.

    • An estimation of probable seismic hazard in the active deformation front of the Himalayan arc

      BASAB MUKHOPADHYAY

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      In this study, the strain rate (SR) in grid-nodes in and around Himalaya that was computed from 1252 GPS station data, is also used. The earthquake catalogue of Himalaya between 1225 and 2017 is compiled. The active tectonic front of the Himalaya is subdivided into 14 zones between MFT and MCT, where MHT is locked. For each zone, magnitude-completeness ($M_{c}$), ‘a’ and ‘b’ values, geodetic moment rate (Mgd), seismic moment rate (Msm), moment ratio (Mgd/Msm) are calculated. It is observed that the moment ratio is <1 in zone 1 (Kashmir), 3 (Kangra), 5 (Almora), 6 (Central-gap 1), 8 (Pokhra–Kathmandu), 9 (Everest), 11 (W Bhutan), 12 (E Bhutan), 13 (Arunachal) and 14 (Eastern Syntaxis). These zones show good agreement between geodetic and seismic moment rates, and are vulnerable to large earthquakes ($M_{w}$ 8.15–8.95) in the distant future. Presently, zone 2 (Kishtwar), 4 (Nahan), 7 (Central-Gap 2), and 10 (Sikkim) are vulnerable as they show moderate-low b-values, high moment ratio (>1), and long seismic hiatus for large earthquakes. Moreover, zones 2, 4, 10, 12, and 13 have un-ruptured patches of MHT and have the capacity to spawn $M_{w}$ 6.75–7.95 earthquake at any point of time. The rupture generated in these zones will transgress the zone boundaries and terminate in the sub-surface Precambrian ridge systems continuing from Peninsular India below Himalaya.

      $\bf{Highlights}$

      $\bullet$ The earthquake catalogue of Himalaya between 1225 and 2017 is compiled.

      $\bullet$ The active tectonic front of the Himalaya is subdivided into 14 zones (1–14) between MFT and MCT.

      $\bullet$ The moment ratio < 1 in 10 zones [1 (Kashmir), 3 (Kangra), 5 (Almora), 6 (Central-gap1), 8 (Pokhra–Kathmandu), 9 (Everest), 11 (W Bhutan), 12 (E Bhutan), 13 (Arunachal) and 14 (Eastern Syntaxis)], show good agreement between geodetic and seismic moment rates, and are vulnerable for large earthquake ($M_{w}$ 8.15–8.95) in distant future.

      $\bullet$ Presently, zones 2 (Kishtwar), 4 (Nahan), 7 (Central-Gap 2) and 10 (Sikkim) are vulnerable as they show moderate-low b-values, high moment-ratio (7gt;1) and long seismic hiatus. Moreover, zones 2, 4, 10, 12 and 13 have un-ruptured patches of Main Himalayan Thrust (MHT) and have capacity to spawn $M_{w}$ 6.75–7.95 earthquake any point of time.

      $\bullet$ The rupture generated in these zones will transgress the zone boundaries and terminate in the sub-surface Precambrian ridge systems continuing from Peninsular India below Himalaya.

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