Seismic velocity structure of the central Himalayas revealed by a joint inversion of ambient noise and earthquake surface wave tomography

Abstract:

The collision of the Indian plate with Asian plate has formed the highest mountain range on Earth, the Himalayas. The Himalayas is currently rising by more than 1 cm per year as the Indian plate resumes to move northward into the Asia and hosts frequent earthquake activities, some of which are devastating, the 1934 M_s8.0 Bihar earthquake and the 2015 M_S8.1 Gorkha earthquake. An understanding of dynamic processes responsible for the uplift of the Himalayas and earthquake generation would require a detailed subsurface image under this region. However, the structure of the crust and upper mantle in this region is still unclear from existing studies, especially the lack of detailed images of the three-dimensional distribution of several large faults (such as the main Himalayan thrust (MHT)) in the crust along the parallel development of the orogenic belt. Based on seismic networks (i.e. NAMASTE and Hi-CLIMB) deployed in Nepal, central segment of the Himalayas (Figure 1.), in this study we first obtained Rayleigh-wave phase velocity maps at periods of 6-14 s using Ambient Noise surface wave Tomography. Subsequently, we combined with previously obtained Rayleigh-wave phase velocity at periods of 20-87s using Two-Plane-Wave earthquake surface wave Tomography to construct a 3-D shear-wave velocity models at depths of 5-96km using a generalized linear inversion.

Our result shows that velocity variations in shallow crust well correlates with the distribution of sedimentary thickness. In the middle and lower crust, two convex high-speed anomalous bodies are imaged in the northwest and southeast of the study area, well correlating with distribution of earthquakes. The main earthquake and the largest magnitude aftershock of Ms8.1 Nepal earthquake seem to locate at the top of the two high-speed anomalies. The 3-D seismic velocity model could provide crucial constraints on the geometry of the major thrust faults, such as Main Himalayas Thrust (MHT), and crust-mantle transition zone under the Himalayas, which would shed lights on robust features of dynamic processes responsible for the generation of large earthquake and the uplift of the Himalayas.