Slip pulse characteristics, Kathmandu basin resonance and high-frequency waves radiation during unzipping of locked MHT by the 2015, Mw 7.8 Gorkha earthquake, Nepal.
Thursday, 17 December 2015: 08:15
305 (Moscone South)
We use high-rate GPS, seismological and Synthetic Aperture Radar imagery (SAR) measurements to produce a detailed image of the seismic rupture during the 2015 Mw 7.8 Gorkha earthquake, Nepal. The earthquake ruptured a 150x50km elliptical patch striking parallel to the Himalayan front located north of Kathmandu. This asperity represents only a small fraction of the previous locked portion of the Main Himalayan Thrust (MHT) along which the Himalaya is thrust over India. The earthquake initiated at western end of the ruptured patch, 75km northwest of Kathmandu. It produced a slip pulse of ~20 km width, ~6 s duration with peak sliding velocity of ~1 m/s which propagated eastwards at ~2.8 km/s. High frequency seismic waves (~ 1 Hz) were radiated continuously as the earthquake unzipped the northern edge of the locked portion of the of the MHT, a zone of presumably high and heterogeneous pre-seismic stress. Most of the moment was actually released south, hence, updip, of the sources of high frequency seismic waves. The slip pulse there shows a remarkable smooth onset indicating a large effective slip-weakening distance of several meters. This smooth onset can explain the moderate ground shaking at high frequencies (>1Hz) and the limited damage to regular few-storey high dwellings within Kathmandu basin. By contrast, the entire basin resonated at ~4-5 s for 30s resulting in the collapse of some tall buildings. The study suggests a deterministic control, of probably structural origin, of the source characteristics and induced ground shaking.