16:30 〜 16:45
[SCG40-11] Upper-Crustal Structure of Central Nepal inferred from Local Earthquake Tomography
Central Nepal is one of the most studied sections of the Himalayas. Multiple geological and geophysical campaigns in central Nepal have proposed a variety of models for seismo-tectonics and geometries of the Main Himalayan Thrust (MHT). However, most of them focused on the two-dimensional model across the MHT. After the 2015 Gorkha earthquake (Mw 7.8), several studies have suggested considerable lateral variation on the crustal structure in both cross-strike and along-strike directions. In this study, we utilize the recordings of numerous local events by the dense network focusing on (1) imaging 3D physical heterogeneities of the upper crust in central Nepal and (2) its role in controlling the rupture of the 2015 earthquake.
We used manually picked P- and S- arrivals from around 1700 events of the Gorkha earthquake sequence for a simultaneous inversion of the seismic velocities and hypocenters using the local earthquake tomography technique. These events were recorded in central Nepal by a temporary network of 42 broadband and short-period stations from June 2015 to April 2016. We applied a gradational approach starting with inversion of a 1D velocity model and progressively updated to a detailed 3D model in several steps. This allows us to have a reasonable model even in areas where we have lower resolution. Moreover, we also obtained a Qp model of the region using vertical-component waveforms of relocated earthquakes and the Vp model. Our results not only provide a detailed velocity structure and aftershock distribution within the source region of the Gorkha earthquake but also are useful for other applications such as numerical simulations.
Preliminary results of our velocity model show three main domains. (1) Zone of low Vp and low Vs in the southern part of the area at shallow depth (<10 km). This zone is spatially well-correlated with sedimentary rocks of the sub-Himalaya. (2) A zone of remarkably low Vp/Vs ratio (<1.65) in the central-northern part of the area from surface to the depth of 10 km. This anomalously low ratio may represent bodies of felsic (quartz-rich) rocks as mapped at the surface. (3) A region of high Vp, at the depth of the Gorkha Earthquake rupture, coincides with the area of large (>4 m) co-seismic slip. This high-velocity zone may represent an asperity on the fault which possibly controlled the rupture of the mainshock.
We used manually picked P- and S- arrivals from around 1700 events of the Gorkha earthquake sequence for a simultaneous inversion of the seismic velocities and hypocenters using the local earthquake tomography technique. These events were recorded in central Nepal by a temporary network of 42 broadband and short-period stations from June 2015 to April 2016. We applied a gradational approach starting with inversion of a 1D velocity model and progressively updated to a detailed 3D model in several steps. This allows us to have a reasonable model even in areas where we have lower resolution. Moreover, we also obtained a Qp model of the region using vertical-component waveforms of relocated earthquakes and the Vp model. Our results not only provide a detailed velocity structure and aftershock distribution within the source region of the Gorkha earthquake but also are useful for other applications such as numerical simulations.
Preliminary results of our velocity model show three main domains. (1) Zone of low Vp and low Vs in the southern part of the area at shallow depth (<10 km). This zone is spatially well-correlated with sedimentary rocks of the sub-Himalaya. (2) A zone of remarkably low Vp/Vs ratio (<1.65) in the central-northern part of the area from surface to the depth of 10 km. This anomalously low ratio may represent bodies of felsic (quartz-rich) rocks as mapped at the surface. (3) A region of high Vp, at the depth of the Gorkha Earthquake rupture, coincides with the area of large (>4 m) co-seismic slip. This high-velocity zone may represent an asperity on the fault which possibly controlled the rupture of the mainshock.