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)
Jean-Philippe Avouac1,2, Lingsen Meng3, Diego Melgar4, Shenji Wei5, John R Elliott6, Romain Jolivet2, Teng Wang7, Yehuda Bock8, Victoria Stevens2, Jean Paul Ampuero9, John Galetzka10, Joachim F Genrich2, Jianghui Geng8, Susan E Owen11, Prithvi Lal Shrestha12, Angelyn W Moore11, Lok Bijay Adhikari12 and Kenneth W Hudnut13, (1)University of Cambridge, Cambridge, United Kingdom, (2)California Institute of Technology, Pasadena, CA, United States, (3)University of California Los Angeles, Los Angeles, CA, United States, (4)University of California Berkeley, Berkeley, CA, United States, (5)Nanyang Technological University, Singapore, Singapore, (6)University of Oxford, COMET, Department of Earth Sciences, Oxford, United Kingdom, (7)Southern Methodist University, Dallas, TX, United States, (8)University of California San Diego, La Jolla, CA, United States, (9)California Institute of Technology, Seismological Laboratory, Pasadena, CA, United States, (10)UNAVCO, Inc. Boulder, Boulder, CO, United States, (11)NASA Jet Propulsion Laboratory, Pasadena, CA, United States, (12)Nepal Department of Mines and Geology, Kathmandu, Nepal, (13)USGS Pasadena Field Office, Pasadena, CA, United States
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.