Collaborative Research: The Nepal Earthquake and Limits on Moment, Fault Geometry and Time Dependent Stress Changes

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The M7.8 earthquake occurred on 25 April 2015 in Nepal is unique among recent Himalayan earthquakes, as it failed to release entirely the pent up stress accumulated since a previous similar earthquake there in 1833. Instead, stress was transferred from 40 km north of the capital, Kathmandu, to 8 km to the south, shifting the 2 million population of the Kathmandu Valley about 1.3 m to the southwest in the process. Of major concern is that this stress is now poised, either to diffuse benignly as slow slip, or to be released violently in a future Mw≤7.5 earthquake. US Embassy officials in Kathmandu are concerned that a major aftershock of this size occurring on the outskirts of the capital would have catastrophic consequences to the already weakened buildings of the city, and would greatly impair the rescue operation to the thousands of villages wiped out by the April 25, 2015 earthquake. In this collaborative rapid project, the PIs will install instruments in the region south of Kathmandu to identify what is exactly happening to this stress. The hope is that processes similar to those occurring on the San Andreas fault in California prevail, namely that stress are being released slowly, in which case it would be possible to infer that a future major earthquake is unlikely. The PIs also recognize that the 2015 earthquake may provide clues to future earthquakes near Kathmandu, in that it may be possible to gain insights into what may happen next by re-evaluating historical earthquakes occurred near Kathmandu in the period from 1833 to 1866. We are collaborating with Nepali students and faculty in Tribhuvan University to undertake these studies. The PIs plan to install two arrays of GPS units on stable monuments between Kathmandu and the frontal thrusts of the Himalaya- a 35 km wide distance with instrument spacing as close as 5 km. The narrow spacing will permit them to detect slip of the Himalayan décollement should it be occurring, and will permit them to distinguish various forms of spatial strain diffusion, or locally locked asperities should these exist. Data from each instrument will be telemetered to Boulder and the data will be processed daily and archived with easy public access. A number of campaign GPS sites will be also installed that will be measured at six-month intervals (or more frequently) by students from Tribhuvan University. The PIs envisage that up to eleven additional GPS instruments will be deployed. The PIs plan also to gather time-sensitive intensity data from the epicentral region that may be used to calibrate the attenuation of surface acceleration in the 2015 earthquake. They will use these data to calibrate historical intensity data from the 1833, 1835, 1866,1934 and 1936 Nepal earthquakes, that will used to constrain the rupture areas of these former events, and how they relate to the 2015 earthquake.