Macroseismic Intensities from the 2015 Gorkha, Nepal, Earthquake

Abstract:

The Mw 7.8 Gorkha, Nepal, earthquake, the largest central Himalayan earthquake in eighty-one years, yielded few instrumental recording of strong motion. To supplement these we collected 3800 detailed media and first-person accounts of macroseismic effects that included sufficiently detailed information to assign intensities. Our resultant macroseismic intensity map reveals the distribution of shaking in Nepal and the adjacent Gangetic basin. A key observation was that only in rare instances did near-field shaking intensities exceed intensity 8 on the European Macroseismic Scale (EMS), a level that corresponds with heavy damage or total collapse of many unengineered masonry structures. Within the Kathmandu Valley, intensities were generally 6-7 EMS, with generally lower intensities in the center of the valley than along the edges and foothills. This surprising (and fortunate) result can be explained by the nature of the mainshock ground motions, which were dominated by energy at periods significantly longer than the resonant periods of vernacular structures throughout Kathmandu. Outside the Kathmandu Valley the earthquake took a heavy toll on a number of remote villages, where many especially vulnerable masonry houses collapsed catastrophically in shaking equivalent to 7-8 EMS. Intensities were also generally higher along ridges and small hills, suggesting that topographic amplification played a significant role in controlling damage. The spatially rich intensity data set provides an opportunity to consider several key issues, including amplification of shaking in the Ganges basin, and the distribution of shaking across the rupture zone. Of note, relatively higher intensities within the near-field region are found to correlate with zones of enhanced high-frequency source radiation imaged by teleseismic back-projection (Avouac et al., 2015). We further reconsider intensities from a sequence of earthquakes on 26 August 1833, and conclude the largest of these ruptured at least part of the same plate boundary segment as did the Gorkha earthquake.