Incision and Fault slip rates along Himalayan Frontal Thrust in Nahan Salient in Northwestern Himalayas: Implications for seismic hazard assessment.

Abstract

<p>Himalayas are seismically very active regions of the world due to ongoing continent-continent collision between India and Eurasia. The Himalayas are known to have hosted deadliest earthquakes in the past century and considering the exponential growth of population in megacities of Gangetic plains, a proper seismic hazard evaluation is very critical in this region. In this regard, the present and past slip rates along the Himalayan Frontal Thrust (HFT) are very important for understanding the convergence pattern and recurrence intervals of major earthquakes. Although geodetically derived short-term convergence rates are consistence with geologically derived long-term slip rates, this correlation is based on selected studies of uplifted Holocene terraces reporting geologically derived slip rates in Central and North-West Himalayas. There is no such reporting of Geological uplift rates from Nahan Salient in NW Himalayas. We have identified uplifted and truncated quaternary terraces along HFT in Nahan Salient Northwest Himalayas through cartosat-I stereo data. We mapped and dated the uplifted terraces in order to understand the long-term convergence rates over Holocene time period. The vertical incision rates are then calculated with the help of OSL ages and height of terraces. Assuming the vertical uplift is due to repeated past earthquakes along HFT dipping at 30°, vertical uplift rates are calculated to be 2.6 mm/yr, which equates to a fault slip rate of 5.16 mm/yr and a horizontal shortening rate of 3 mm/yr. Along with that last tectonic activity along HFT is also bracketed using age of uplifted terraces and unfaulted capping units from an exposed section of HFT fault plane along river section. The OSL ages suggest that the HFT was active between 3.8±0.4Ka and 0.706±0.15Ka. Assuming that no deformation has occurred along HFT after 0.706±0.15Ka a slip deficit of 3.6 m has been accumulated which is sufficient to generate a large earthquake in the Nahan Salient NW Himalayas.</p>

Holocene slip rates and their implications for seismic hazard along the Himalayan Frontal Thrust in western Himalayan Nahan salient

“…The last seismic event recorded in this area triggered slip along the Black Mango Tear Fault around 1 400 AD (Kumar et al, 2001). In a recent study based on undeformed fluvial terrace across the HFT in Nahan salient, Singh et al (2022) favour that the HFT is locked since at least the last 0.7 ± 0.1 ka and as a result, slip deficit is growing along the locked portion. Using the minimum timing of locking of HFT, we deduced a ~6.2–8.5 m slip deficit along the HFT in Nahan salient.…”

“…Deformation estimates using faulted and abandoned strath surfaces along the Khetpurali and Markanda river sections yield slip rates of 12.2 ± 0.8 mm/a and 10.4 ± 0.8 mm/a since mid-Holocene. If the HFT is locked for the last 600-700 a, as suggested by Singh et al (2022) and Kumar et al (2001), then a probable 6.2-8.5 m slip deficit exists on the HFT. This much slip deficit is capable enough to trigger at least one Mw ~7.7 seismic event in Nahan area.…”