Estimation of Crustal Thickness in Nepal Himalayas Using Local and Regional Earthquake Data

Abstract:

Variation of crustal thickness beneath Nepal Himalayas is estimated by tomographic inversion of regional earthquake data. The Nepal Himalayas is fairly well distributed with denser network and earthquakes. Some 10864 P- and 5293 S-arrival times from 821 selected events Mw > 4.0 recorded during 2004-2014 are used for this study; on average, almost 20 phases per event have been available. The tomographic results shed a new light on crustal thickness variation along and across the Nepal Himalayas. The crustal thickness varies between 40 and 80 km from foothills to high Himalayas, which is verified by synthetic modeling. The crustal thickness also widely varies along the strike of the Himalayas. The zones of higher and lower crustal thicknesses may be correlated with some hidden transverse structures in the foothills region, which are well reflected in gravity and magnetic maps. The estimated crustal thickness matches fairly well with the free air gravity anomaly; thinner crust corresponds to lower gravity anomaly and vice versa. Some correlation with the magnetic field anomaly is also observed. Higher magnetic anomaly corresponds to thicker crust. We propose that the more rigid segments of incoming Indian crust comprising of igneous and metamorphic rocks cause more compression in the Himalayan thrust zone and leads to stronger crustal thickening. Under thrusting of weaker crust / sediments, on the other hand, is associated with less shortening, and thus cause the thinner crust in the collision zone.