Aftershocks of the M7.8 Gorkha (Nepal) Earthquake: Early Results from Project NAMASTE

Abstract:

In response to the April 25, 2015 M7.8 earthquake on the Main Himalayan Thrust in Nepal, NSF Geosciences funded a rapid seismological response, project NAMASTE (Nepal Array Measuring Aftershock Seismicity Trailing Earthquake). We deployed an array of 41 broadband and short-period seismometers and 14 strong motion sensors across eastern and central Nepal. The ~100x200 km-sized array spans the primary aftershock region and covers the rupture area. Sites include twenty-five broadband, six medium-band, ten co-located short-period and strong motion, and four strong-motion only installations. The array will remain in place for 6-12 months. We will present initial results from the first few months of the deployment.

The April 25 earthquake and the May 12, 2015 M7.3 aftershock are examples of seismic activity resulting from the ongoing collision between the Indian and Eurasian plates. The two tectonic plates converge at ~4 cm per year, deforming the Earth’s crust and uplifting the Himalayan Mountains. Historically, aftershocks of large Himalayan earthquakes occur on the principal subduction-zone thrust (the Main Himalayan Thrust), and also on splay thrust faults such as the Main Central Thrust, Main Boundary Thrust, and Main Frontal Thrust as well as along faults within the overriding plate. Detailed location of aftershocks will provide unprecedented sub-surface resolution of the geometry of these faults that at present are known mainly from surface mapping. Structures down-dip and along-strike of the main thrust that may limit the extent of rupture, or control the nucleation pattern of future earthquakes, require high-resolution studies that cannot be conducted with teleseismic recordings of the main shock and its aftershocks. Detailed aftershock imaging or controlled-source profiling is required to solve this problem.