The Seismotectonics and Seismogenesis of the Main Himalayan Thrust in Nepal and India
- Mendoza, Manuel Matthew
- Advisor(s): Ghosh, Abhijit
The Main Himalayan Thrust (MHT) is the cardinal fault (décollement) that accommodates most of the relative convergence between the Indian plate and Eurasian plate. This collision led to a series of complex faulting and folding events that have since created the Himalaya. Consequently, this region has prompted seismological studies to discern the physical properties of the MHT, and characterize deformation processes taking place above and below it. However, probing the Himalayan subsurface in high fidelity is challenging, requiring the deployment of large, dense seismic networks above various, and structurally distinctive segments of the MHT. This is necessary to capture earthquakes across a range of magnitudes (micro to great), locate their hypocenters accurately, and make reliable interpretations accordingly. In this dissertation, we use waveform data from two well-distributed seismic networks temporarily deployed in central Nepal and Uttarakhand, India, to investigate the behavior of seismicity, their influence on one another, and their relationship to Himalayan tectonics. After the 2015 Mw 7.8 Gorkha earthquake, we deployed the “NAMASTE” seismic network to capture the prolific sequence of aftershocks. From 2005-2008, the Uttarakhand seismic network blanketed a segment of the MHT that is considerably more seismically quiet due to the ongoing and heightened buildup of tectonic strain. From these data sets we develop high-quality earthquake catalogs using a combination of traditional and novel techniques. Focal mechanisms were also computed to determine the orientation and slip direction of structures delineated by their locations. At shallow depths, we find a duplex structure in both regions, and construct a refined geometric model of the MHT incorporating this structure. At greater depths beneath Uttarakhand, specifically, we provide evidence of earthquakes near the Moho that may be associated with flexural bending of the Indian lithosphere or localized processes. Lastly, we document the first observed instance of remotely triggered earthquakes in Uttarakhand, reaffirming the notion that faults here are critically stressed, and governed by both static and dynamic processes. We therefore stress the importance of using these observations to help constrain other geological and geophysical models that investigate deformation, the behavior of large earthquakes, and high seismic hazard along the Himalaya.