The Main Himalayan Thrust beneath Nepal and Southern Tibet illuminated by seismic ambient noise and teleseismic P wave coda autocorrelation

Nepal is an actively deforming region due to its tectonic setting that hosts many destructive earthquakes including the most recent 2015 Gorkha earthquake of magnitude 7.8. To better understand the physics of earthquakes and their precise location as well as monitoring of seismicity and real-time seismic hazard in the region, a highly resolved 3-D structure of the crust is essential. This study presents a new 3-D shear S -wave velocity structure of the crust using group and phase velocity dispersions obtained from ambient noise tomography. This study further constrains the discontinuities beneath Himalaya Nepal using teleseismic compressional P-wave coda autocorrelation. Our results show significant variation in the crustal structure within the region and correlate well with known geological and tectonic features present there. The results from the P-wave coda autocorrelation identify major seismic discontinuities in the crust including the Main Himalayan Thrust (MHT). The MHT with two ramps correlates well with a low S-wave velocity layer obtained from the ambient noise tomography. The first ramp agrees with the duplex structure in the MHT beneath Lesser Himalaya while the second ramp connects flat low velocity beneath High Himalaya to a broad low-velocity zone beneath South Tibet. Moreover, the High Himalaya low-velocity layer is located where the GPS data show creeping north of the coseismic rupture of the 2015 Gorkha earthquake. The lateral variation of S-wave velocity on the MHT surface provides the details of lateral transitions that might have potentially controlled the rupture pattern of the 2015 Gorkha earthquake.

The geometry and extent of the High Himalaya low-velocity layer mimics the decollement coupling zone inferred from GPS data with widths of 50 to 70 km north of the nucleation of the 2015 Mw 7.8 Gorkha earthquake and 90 to 100 km north of the source of the Mw 8.4 1934 earthquake. The occurrence of millenary Mw>9.0 earthquakes in Central and Eastern Nepal would require either a wider coupling low velocity zone compared to the ones identified in this work or the involvement of southernmost Tibet low velocity decoupling zone so to store enough elastic energy.