The 2015 Gorkha Earthquake: Earthquake Reflection Imaging of the Source Fault and Connecting Seismic Structure With Fault Slip Behavior

A seismic array observation across the central focal area of the 2015 Gorkha earthquake was conducted to investigate aftershock distribution and crustal structure. Most aftershocks near Kathmandu were located above the Main Himalayan Thrust (MHT). Earthquake reflection imaging revealed the geometry of the MHT, which lies at similar to 10-km depth at 75 km away from the Main Frontal Thrust. We found three areas of seismic velocity variability around the MHT above 15 km depth: (1) a high-Vp zone where the MHT changes its dip angle from 5 degrees to 13 degrees, coinciding with a local maximum in coseismic slip, (2) a low-Vp zone showing less coseismic slip, and (3) a high-Vp zone where coseismic slip decreased further but afterslip occurred. These differences in slip behavior may indicate changes in frictional properties on the fault, which are reflected in the heterogeneity of the velocity structure. Plain Language Summary On 25 April 2015, the Gorkha earthquake (M-w 7.8) struck central Nepal and resulted in nearly 9,000 fatalities. Destructive earthquakes have occurred repeatedly in the Himalayan region. The Himalayan seismogenic zone is located in the India-Eurasia plate collision zone where the Indian plate underthrusts the Eurasian plate along the Main Himalayan Thrust. Revealing the crustal structure of the India-Eurasia plate collision zone is important to constraining the process of earthquake occurrence and the orogenic process. Seismic array observation is an effective tool to investigate the crustal structure. Here, we present the results of seismic array observation across the central focal area of the 2015 Gorkha earthquake, which shows the geometry of the Main Himalayan Thrust and seismic velocity variability along its length. We find a high-Vp zone which coincides with a large coseismic slip zone (>6m). A low-Vp zone shows less coseismic slip. Another high-Vp zone located in the southern side of the low-Vp zone corresponds to an area of afterslip. These results connect heterogeneous crustal structure with fault slip behavior and suggest that variability in the seismic properties of the crustal structure around the plate boundary may affect fault slip behavior.