M 7.3 - Nepal

Tectonic Summary

The May 12, 2015, M 7.3 Nepal earthquake (southeast of Zham, China) occurred as the result of thrust faulting on or near the decollément associated with the Main Himalayan Thrust, which defines the interface between the underthrusting India plate and the overriding Eurasia plate to the north. At the location of this earthquake, approximately 80 km to the east-northeast of the Nepalese capital of Kathmandu, the India plate is converging with Eurasia at a rate of 45 mm/yr towards the north-northeast—a fraction of which (about 18 mm/yr) is driving the uplift of the Himalayan Mountain Range. The May 12, 2015, event is the largest aftershock to date of the M 7.8 April 25, 2015, Nepal earthquake—known as the Gorkha earthquake—which was located 150 km to the west, and which ruptured much of the decollément between the hypocenters of these two earthquakes.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Thrust-faulting events of the size of the May 12, 2015, earthquake are typically about 55x30 km (length x width). The April 25, 2015, M 7.8 mainshock had approximate dimensions of about 120x80 km, directed from its hypocenter eastward, and towards Kathmandu. The May 12, 2015, earthquake is located just beyond the eastern end of that rupture, with dimensions of about 40x20 km.

  The boundary region of the India and Eurasia plates has a history of large and great earthquakes. Prior to April 25, four events of M 6 or larger had occurred within 250 km of this area over the past century. One, a M 6.9 earthquake in August 1988, 140 km to the south-southeast of the May 12th event, caused close to 1,500 fatalities. The largest, a M 8.0 event known as the 1934 Nepal-Bihar earthquake, ruptured a large section of the fault to the south of this May 2015 event, and east of the April 2015 mainshock, in a similar location to the 1988 earthquake. It severely damaged Kathmandu, and is thought to have caused about 10,600 fatalities. Prior to the 20th century, a large earthquake in 1833 is thought to have ruptured a similar area as the April 25, 2015, event. To date, there have been almost 100 M3+ aftershocks of the Gorkha earthquake. In the first 2 hours after the May 12th event, 6 further aftershocks have occurred to the southwest and southeast of that earthquake.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)