Lateral variation of the Main Himalayan Thrust in the source area of the 2015 Mw7.8 Gorkha, Nepal earthquake
Abstract
The Himalaya orogenic belt is the largest continental collision zone on Earth with high population density and high risk of large earthquakes. In the central region, the April 25, 2015 Mw7.8 Gorkha, Nepal earthquake partially ruptured a 120-km by 80-km patch of the north-dipping Main Himalayan Thrust (MHT), propagating eastwards from the hypocenter located 75 km WNW of Kathmandu, Nepal at 16 km depth. The main shock was followed by a large number of aftershocks including a May 12, 2015 Mw7.3 aftershock that initiated near the eastern termination of the main shock rupture. In this study we combine seismic waveforms recorded both north of the aftershock zone in Tibet, and above and south of the aftershock zone in Nepal. We use 22 seismic stations deployed in China along the China-Nepal border by the Institute of Tibetan Plateau Research, Chinese Academy of Sciences and the 46-station NAMASTE (Nepal Array Measuring Aftershock Seismicity Trailing Earthquake) network deployed across the Gorkha earthquake rupture area in Nepal. We also used waveform and arrival time data from the China National Seismic Network and the National Seismic Network of Nepal. Based on these data, we relocated earthquakes using a multiscale double-difference method, calculated fault plane solutions using a waveform modeling, and derived three-dimensional P- and S-wave velocity models using a regional-scale double-difference seismic tomography to study the source and structural properties of the region of the Gorkha earthquake.
The Mw7.8 main shock and the Mw7.3 aftershock initiated on the MHT. However, most of our well-located aftershocks are shallower than the main shock. The deeper edge of the distribution of aftershocks may align with the geometry of the MHT. In the source area, the geometry of the MHT (as defined by the deepest aftershock hypocenters and the focal mechanisms) and of the Moho (as defined by a velocity increase in our tomographic image) exhibit clear lateral variations. As expected, both structures deepen to the NNE parallel to the plate convergence direction as one moves from the foreland to the hinterland, but both MHT and Moho also deepen toward the ESE along strike of the collision zone. Our tomographic model shows lower velocities in the upper crust (< 20 km) east and west of a higher-velocity region containing the aftershocks are observed. At greater depths around 60 km, prominent high P-wave velocity anomalies are present to the south and to the west of the rupture area, but not to the east, further demonstrating along-strike changes in the subducting Indian plate.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T11A..05B
- Keywords:
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- 8102 Continental contractional orogenic belts and inversion tectonics;
- TECTONOPHYSICSDE: 8104 Continental margins: convergent;
- TECTONOPHYSICSDE: 8149 Planetary tectonics;
- TECTONOPHYSICSDE: 8150 Plate boundary: general;
- TECTONOPHYSICS