Abstract
Using detailed waveform analysis of Pn, sPn depth phases from the nearby seismic stations, attempts were made to resolve the focal depth of the 2011 Sikkim earthquake. Focal depth of 46.8 km thus determined was found closer to the GCMT source mechanism solution as compared to ISC solution (29.6 km). Re-examination of its source mechanism with the spatial/ depth distribution of its aftershocks sug- gested that both the nodal planes oriented WNW or ENE, initially ruptured but the orientation of meizoseismal area and larger concen- tration of its aftershocks parallel to the Tista lineament conformed WNW striking nodal plane relatively more active. The large value of the stress drop derived from S-wave spectra based on the data of Indian stations was attributed to its strike-slip mechanism and deeper focal depth. The stress drop of its foreshock was much lower than that the main shock. The b-value also showed a decrease during the last decade prior to the main earthquake.The recent Sikkim earthquake (Mw 6.9) has brought out the limitations of the microzoning of the Sikkim region attempted earlier.
Highlights
The earthquake of 18 September 2011 occurred in the northwestern Sikkim region of India close to the Nepal and Tibet border
The preliminary bulletins of this earthquake by the US Geological Survey (USGS) and India Meteorological Department (IMD) gave its focal depth as 19.7 km and 10 km respectively, which was revised to 46 km to 60 km based on GCMT solu
The 2011 Sikkim earthquake with its focal depth constrained to 46.8 km from the depth phases using the IMD stations in northeast India showed strike-slip faulting which was in agreement with the earlier studies
Summary
The earthquake of 18 September 2011 occurred in the northwestern Sikkim region of India close to the Nepal and Tibet border. According to the media reports, heavy damage to modern buildings occurred at Chungthan and Sorthang. Due to ambiguity in the assessment of their values based on the media reports or limited field surveys, the isoseismal patterns differed and higher intensities appear to have been estimated by Prajapati et al [2013]. The preliminary bulletins of this earthquake by the US Geological Survey (USGS) and India Meteorological Department (IMD) gave its focal depth as 19.7 km and 10 km respectively, which was revised to 46 km to 60 km based on GCMT solu-
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