Control of Structure and Stress transients on Seismicity and Deformation in Western Himalaya

Abstract:

One of the most longstanding unruptured segments along the Main Himalayan Thrust (MHT) resides at the westernmost end of the ‘central seismic gap’, in the Indian state of Uttarakhand. Surrounded by dense population centers, such as New Delhi to the south, Uttarakhand’s last major event was the 1803 Mw ~7.5-7.9 (Bilham, R., 2019) Garhwal-Himalaya earthquake; whereas the most recent strong events were the 1991 Mw 6.8 Uttarkashi and 1999 Mw 6.7 Chamoli earthquakes. Given that previous studies estimate the recurrence interval for great earthquakes along the Himalayan arc to be approximately between 250 and 500 years, there is a strong consensus that Uttarakhand is overdue for a large >M8 plate boundary-type earthquake. Here, we present a suite of findings from an earthquake catalog developed from a seismic network consisting of 50 broadband stations that blanketed the region from 2006 to 2008. Particularly, we detect and locate thousands of micro-to-moderate sized earthquakes with high-precision, and determine their source parameters, allowing us to identify active fault structures. We infer the bulk of seismicity to be occurring along, and in close proximity to, the physiographic transition (PT-2) from Lesser to High Himalaya. We also observe sparse seismicity in the up-dip locked portion of the MHT, as well as at greater depths below the décollement. Interestingly, a significant portion of seismicity is triggered by small transient stresses imparted by the passage of large teleseismic waves, highlighting areas of highly susceptible (critically stressed) faults. The conditions that initiate and sustain such swarm and aftershock-like sequences in this tectonic regime are explored here in detail. By discerning the geometry of faults and their state of stress we offer an updated description for how this segment of the MHT is deforming, distributing tectonic strain, and being primed to produce a destructive earthquake in Western Himalaya.