Dynamically Triggered Events in a Low Seismically Active Region of Gujarat, Northwest India, during the 2012 M-w 8.6 Indian Ocean Earthquake

The mainland region of Gujarat, northwest India, is a less investigated region than other parts of India with a low seismicity rate. An M-w > 4.7 earthquake has not occurred in this region for 15 yr, and no M-w > 5.5 events since 1971. We analyze the local earthquake catalog and waveforms to examine dynamic triggering in the region by the 2012 M-w 8.6 Indian Ocean earthquake, which triggered widespread seismicity globally. Further detection of possibly missing microearthquakes is conducted by applying the matched filter technique to the waveforms. We identify six microearthquakes (similar to M-L 1.0-2.1) triggered during the surface and coda wave of the 2012 mainshock. Also, an earthquake of M-w 2.6 was likely triggered five hours after the mainshock near Bhavnagar city, because the record since 2006 would indicate such a magnitude event to have only a 0.8% chance of occurring independently any given day. Indeed, only 35 earthquakes with M-w >= 2.5 were recorded since 2006 within a 100 km radius of this city. The beta-statistics indicate an increase in seismicity and further confirm the triggering. The seismicity rate increased immediately after the 2012 mainshock and continued for three days, indicating a possible delayed triggering. The delayed triggering may be due to the crustal fluid, and/or subcritical crack growth model may be responsible for triggering. Our study suggests that dynamic triggering tends to occur near active faults that have ruptured in ancient times. Other recent earthquakes, for example, 2011 Tohoku-Oki, did not trigger seismicity despite significant peak dynamic stresses values. Investigation of dynamic triggering in regions experiencing infrequent earthquakes can be crucial in understanding the origin of such earthquakes, which can be achieved by grasping the ambient stresses and geodynamic mechanisms in a particular region. Thus, we evaluate character and behavior of high-amplitude surface waves to grasp better the undergoing processes and stress transfer in the intraplate mainland region.