The collision of the India and Asia has created the Himalaya, the highest mountains in the world, over the last 57 million years. Convergence between the two tectonic plates continues today, at 4cm (1 ½ inches) per year, deforming the earth?s crust in the Himalaya, and creating great earthquakes. The recent devastating earthquakes in Nepal (April 25, 2015 magnitude 7.8; and May 12, 2015 magnitude 7.3) are examples of this activity, though scientists believe that far larger earthquakes have happened in the past, up to magnitude 8.8, and will inevitably occur again sometime in the future. The biggest of these earthquakes could kill as many as 1 million people in northern India and Nepal. These large earthquakes rupture faults over very large areas, perhaps 100 to 500 km West-East along the Himalaya. The biggest fault, on which the recent Nepal earthquakes occurred, is called the Main Himalayan Thrust. Scientists do not know why the magnitude 7.8 ?main shock? earthquake initiated exactly where it did, 100 km (60 miles) northwest of Kathmandu; nor why the earthquake fault stopped moving about 160 km (100 miles) to the east-south-east. Scientists believe earthquakes start and stop at locations (called ?asperities?) where the fault-plane changes geometry, perhaps where it becomes steeper or less steep. If these asperities are a long way apart, the earthquake can be devastatingly large; but if the asperities are close together, then the earthquakes are likely to be smaller. Hence, in order to quantify seismic hazard in the Himalaya, scientists need to first understand the geometry of the Main Himalayan Thrust. During Project NAMASTE, US scientists will work alongside Nepali seismologists and students to understand this fault geometry, while at the same time building the Nepali scientific capacity.
In response to the April 25, 2015 M=7.8 earthquake on the Main Himalayan Thrust in Nepal, scientists from UTEP and Stanford are urgently deploying ~20 broadband and short-period seismometers in an areal array across eastern Nepal, spanning the region of the largest aftershocks. Historically, aftershocks of large Himalayan earthquakes occur on both 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. Detailed location of the aftershock seismicity will provide unprecedented sub-surface resolution of the geometry of these faults that at present are known almost entirely from surface mapping. Knowing which faults are active at the present day ? a subject of ongoing controversy ? will lead to better kinematic descriptions of the India-Asia collision. Knowing the down-dip ?ramp-and-flat? geometry of the Main Himalayan Thrust, and particularly whether and where along-strike lateral ramps exist, will lead to better understanding of the historical record of great Himalayan earthquakes, and potential future rupture zone dimensions. The 20-station University of Texas at El Paso/Stanford University array will complement an Oregon State/University of California Riverside array of similar size and areal dimension to together acquire a comprehensive image of the entire aftershock zone that extends somewhat in all directions beyond the initial rupture area. Both arrays will remain in place for about six months. This dataset will be submitted as rapidly as possible to the IRIS Data Management Center for analysis by all interested seismologists.
