Seismicity and source parameters of moderate earthquakes in Sikkim Himalaya

In this study, we accurately relocate 360 earthquakes in the Sikkim Himalaya through the application of the double-difference algorithm to 4 years of data accrued from a eleven-station broadband seismic network. The analysis brings out two major clusters of seismicity—one located in between the main central thrust (MCT) and the main boundary thrust (MBT) and the other in the northwest region of Sikkim that is site to the devastating Mw6.9 earthquake of September 18, 2011. Keeping in view the limitations imposed by the Nyquist frequency of our data (10 Hz), we select 9 moderate size earthquakes (5.3 ≥ Ml ≥ 4) for the estimation of source parameters. Analysis of shear wave spectra of these earthquakes yields seismic moments in the range of 7.95 × 1021 dyne-cm to 6.31 × 1023 dyne-cm and corner frequencies in the range of 1.8–6.25 Hz. Smaller seismic moments obtained in Sikkim when compared with the rest of the Himalaya vindicates the lower seismicity levels in the region. Interestingly, it is observed that most of the events having larger seismic moment occur between MBT and MCT lending credence to our observation that this is the most active portion of Sikkim Himalaya. The estimates of stress drop and source radius range from 48 to 389 bar and 0.225 to 0.781 km, respectively. Stress drops do not seem to correlate with the scalar seismic moments affirming the view that stress drop is independent over a wide moment range. While the continental collision scenario can be invoked as a reason to explain a predominance of low stress drops in the Himalayan region, those with relatively higher stress drops in Sikkim Himalaya could be attributed to their affinity with strike-slip source mechanisms. Least square regression of the scalar seismic moment (M0) and local magnitude (Ml) results in a relation LogM0 = (1.56 ± 0.05)Ml + (8.55 ± 0.12) while that between moment magnitude (Mw) and local magnitude as Mw = (0.92 ± 0.04)Ml + (0.14 ± 0.06). These relations could serve as useful inputs for the assessment of earthquake hazard in this seismically active region of Himalaya.