SEISMOLOGICAL CHARACTERISTICS OF THE 25 APRIL 2015 MW 7.9 GORKHA, NEPAL EARTHQUAKE

The 25 April 2015 Gorkha, Nepal underthrusting earthquake ruptured along the down-dip edge of the locked region of the Main Himalayan Thrust. Long-period moment tensor inversions indicate a seismic moment M₀ = 7.8 – 9.1 x 10²⁰ Nm (Mw 7.9), fault dip of 5 to 7°, centroid depth 12 to 17.5 km, and centroid delay time of ~32.5 s. Back-projections of ~1 s period teleseismic P waves using large aperture arrays or globally distributed stations indicate unilateral eastward rupture expansion ~140 km from the epicenter located 80 km WNW of Kathmandu, with a rupture velocity of 3.0 ± 0.3 km/s. Finite-fault slip model inversions using teleseismic data indicate small slip near the epicenter gradually increasing to peak slip of about 6 to 7 m 50 km ESE of the epicenter with a total rupture duration of about 55 to 60 s. Joint inversions with SAR observations and/or regional high-rate GPS recordings indicate a narrow along-dip slip zone width of ~40 km along much of the rupture. The back-projections place most bursts of short-period energy along the deeper portion of the slip zone. The broadband radiated seismic energy Er = 9.8 x 10¹⁵ J, for signals up to 2 Hz, and the moment-scaled Er/M₀ = 1.26 x 10^-5. The hr-GPS indicate that the slip pulse extended 20 to 30 km along strike with a rise-time of 6 s in the large slip regions, with peak sliding velocity of about 1.1 m/s. Surface ground velocities of up to 0.7 m/s were recorded with modest peak ground accelerations of about 1 m/s². The narrow slip zone on the MHT was located north of Kathmandu, and 4 s period resonance was generated in the basin below the city, responsible for selective damage of taller structures in the basin. The smooth moment rate time function contributed to the modest ground accelerations, which were comparable to those for a Mw 7.2 aftershock on May 12 located eastward of the mainshock slip zone. The up-dip region appears to remain locked and stress has been concentrated on its margin.