Crustal structure and discontinuities beneath the Nepal Himalaya using seismic ambient noise and teleseismic P wave coda autocorrelation

Nepal is an actively deforming region because of its tectonic setting as demon-strated by many destructive seismic events it hosted in the past with the most re-cent Gorkha 2015 magnitude 7.8 earthquake. For a better understanding of the crustal structure, of the physics of earthquakes as well as their detailed high-resolution location and monitoring, and to mitigate the real-time seismic hazard, an adequate 3-D regional velocity model is needed. So far, a country-wide 3-D velocity structure is unavailable. Here, we present a new high-resolution 3-D shear-wave velocity structure down to 60 km depth beneath the Nepal Himalaya using ambient noise cross-correlations. Our results show significant lateral varia-tion in crustal structure and thus, correlate well with the known geological and tectonic features of the study area. The shear wave velocity structure of shallow depth (top 5 km) shows the low shear wave velocity beneath the epicenter of 1934, 1833, 2015 earthquakes, and beneath the region which were hardly hit by 2015 Gorkha Earthquakes (for example, Sindhuplachok, Kathmandu valley, etc.). Mapping of crustal discontinuities is crucial to decipher the dynamics of the continental deformation as well as better constrain the earthquake potential. Ac-cordingly, we also use autocorrelation of teleseismic P coda to recover the shal-low and deeper interfaces in the crust beneath Nepal. The results show the high-resolution images of four major seismic reflectors in the crust beneath Central Nepal. The shallow discontinuity recovered from our study is well supported by the geology. The result obtained from our study shows the Moho depth variation from 40 km beneath South Nepal to 60 km beneath High Himalaya. Our results provide for the first-time good constraint on both the seismogenesis and on the earthquake hazard in the Nepal Himalaya.

Nepal is an actively deforming region because of its tectonic setting as demon-strated by many destructive seismic events it hosted in the past with the most re-cent Gorkha 2015 magnitude 7.8 earthquake. For a better understanding of the crustal structure, of the physics of earthquakes as well as their detailed high-resolution location and monitoring, and to mitigate the real-time seismic hazard, an adequate 3-D regional velocity model is needed. So far, a country-wide 3-D velocity structure is unavailable. Here, we present a new high-resolution 3-D shear-wave velocity structure down to 60 km depth beneath the Nepal Himalaya using ambient noise cross-correlations. Our results show significant lateral varia-tion in crustal structure and thus, correlate well with the known geological and tectonic features of the study area. The shear wave velocity structure of shallow depth (top 5 km) shows the low shear wave velocity beneath the epicenter of 1934, 1833, 2015 earthquakes, and beneath the region which were hardly hit by 2015 Gorkha Earthquakes (for example, Sindhuplachok, Kathmandu valley, etc.). Mapping of crustal discontinuities is crucial to decipher the dynamics of the continental deformation as well as better constrain the earthquake potential. Ac-cordingly, we also use autocorrelation of teleseismic P coda to recover the shal-low and deeper interfaces in the crust beneath Nepal. The results show the high-resolution images of four major seismic reflectors in the crust beneath Central Nepal. The shallow discontinuity recovered from our study is well supported by the geology. The result obtained from our study shows the Moho depth variation from 40 km beneath South Nepal to 60 km beneath High Himalaya. Our results provide for the first-time good constraint on both the seismogenesis and on the earthquake hazard in the Nepal Himalaya.