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Abstract

We present high-resolution tomographic image of Gorkha earthquake source region based on traveltime inversion of local and regional earthquake data. Our results are well resolved up to ~90 km depth. Our high-resolution tomographic image shows heterogenous crustal architecture with substantial variations along-strike as well as across-strike of Nepal Himalayas with patches of high/low-velocity zones. Our tomographic model exhibits a close correlation with varying rupture process and its relationship to tectonic setting accompanying the main shock inferring role of crustal heterogeneities in controlling source rupture dynamics. We infer that seismogenesis is governed by crustal architecture, heterogeneities, and geometry of MHT. Low-velocity anomalies in shallower part may correspond to alignments of faults/tectonics features representing weak zones of seismogenic crust and high-velocity anomalies may represent hard and rigid blocks. This suggests that earthquakes normally initiate at interface/boundary of high- and low-velocity anomalies separating two anomalous zones of structural heterogeneities. High-velocity blocks have relatively higher mechanical strength and hence act as stress concentrators. We observed that main shock of Mw 7.8 nucleated in mid-crustal ramp zone of MHT in a zone of lower crustal thickness. As rupture propagated eastward along down-dip portion of MHT it reached the higher crustal thickness and higher seismic velocity where Mw 7.3 event triggered. We infer that this may have blocked the rupture propagations towards east-southeastward and hence controls spatial limits of rupture. We infer that first earthquake led to an increase in strain in more rigid body that ruptured when it was strained beyond its elastic limit.