Surface topography explains weak ground shaking in Kathmandu valley during the 2015 Gorkha earthquake, Nepal

The reasons behind weak and limited ground shaking in Kathmandu valley during the April 25, 2015 Mw 7.8 Gorkha, Nepal, earthquake remain elusive. We have performed spectral element numerical simulations which show that during this earthquake surface topography restricted the propagation of seismic energy into the valley, leading to low peak ground displacement (PGD) within the valley, compared to a situation without surface topography. The mountains north of Kathmandu diverted the incoming seismic wave mostly to the eastern and western margins of the valley. This resulted in de-amplification of PGD in most parts of valley interior, which indicates that surface topography reduced seismic shaking and lessened earthquake impact within the valley. However, tests show that rupture at other locations along the same fault line would affect Kathmandu valley in a much stronger way than the 2015 Gorkha earthquake and, therefore, the earthquake risk in this area remains very high.