Structural context of the 2015 pair of Nepal earthquakes (Mw 7.8 and Mw 7.3): an analysis based on slip distribution, aftershock growth, and static stress changes

Abstract

The Great Himalayan earthquakes are believed to originate on the Main Himalayan Thrust, and their ruptures lead to deformation along the Main Frontal Thrust (MFT). The rupture of the April 25, 2015 (Mw 7.8), earthquake was east-directed, with no part relayed to the MFT. The aftershock distribution, coseismic elevation change of similar to 1 m inferred from the InSAR image, and the spatial correspondence of the subtle surface deformations with PT2, a previously mapped out-of-sequence thrust, lead us to explore the role of structural heterogeneities in constraining the rupture progression. We used teleseismic moment inversion of P- and SH-waves, and Coulomb static stress changes to map the slip distribution, and growth of aftershock area, to understand their relation to the thrust systems. Most of the aftershocks were sourced outside the stress shadows (slip > 1.65 m) of the April 25 earthquake. The May 12 (Mw 7.3) earthquake that sourced on a contiguous patch coincides with regions of increased stress change and therefore is the first known post-instrumentation example of a late, distant, and large triggered aftershock associated with any large earthquake in the Nepal Himalaya. The present study relates the slip, aftershock productivity, and triggering of unbroken stress barriers, to potential out-of-sequence thrusts, and suggests the role of stress transfer in generating large/great earthquakes.