The 2015 Mw 7.8 Gorkha Earthquake: Constraining the Geometry of the Main Himalayan Thrust from Space Geodesy

Abstract:

The 2015 Mw 7.8 Gorkha earthquake ruptured a significant portion of the Main Himalayan Thrust (MHT), the main active structure responsible for the building of the Himalayas and for most destructive earthquakes in the past. Since this structure does not slip during the interseismic period, only indirect evidences allowed to infer its geometry at depth, a crucial point for numerous questions ranging from the long-term evolution of the mountain range to the dynamics of seismic ruptures. We use all available space-based geodetic data recording surface displacements during the 2015 Gorkha earthquake, including InSAR, GPS and optical imagery, to explore the space of possible geometric configurations of the MHT in the region of Kathmandu. From the Main Frontal Thrust (MFT), the surface expression of the MHT, our preferred fault geometry includes a steep (30°) ramp down to 5-km-depth, prolonged at depth by a shallow (5-8°) thrust extending 85 km north from the MFT where it steepens to form a 20-30° mid-crustal ramp, linking the seismic portion of the MHT to its deep, aseismic, root. This geometry is consistent with earlier geophysical measurements, such as electro-magneto-telluric data, the InDepth seismic reflection profile and the depth of relocated microseismicity. Moreover, the position of the mid-crustal ramp is consistent with the broad anticlinal shape of foliation north of the Kathmandu klippe. Such geometric configuration would suggest a structural control of the generation of high frequencies during the propagation of the rupture, as high-frequency radiators coincide with the up-dip edge of the mid-crustal ramp. Our proposed fault structure is meant to be used for further modeling of the earthquake cycle in the area, including the propagation of dynamic ruptures on a complex fault plane and the long-term behavior of the MHT, controlling the growth of the Himalayas.