The 2015 Mw 7.8 Gorkha Earthquake in Nepal and its aftershocks: analysis of strong ground motion

Abstract

Various aspects of the 2015 Gorkha Earthquake and its aftershocks are presented, and different hypotheses about the nature of long period pulses observed in ground motion record in Kathmandu Valley are critically discussed and reviewed. The frequency-size distribution of aftershocks, as well as the temporal distribution within one year of the mainshock is discussed. Some interesting features of the strong ground motion of the mainshock recorded at different places in Kathmandu Valley are discussed. Although the root-mean-square horizontal ground accelerations recorded at rock and soft sites were similar, the peak accelerations varied significantly. Elastic response spectra of the mainshock near the centre of the Valley (KATNP station operated by USGS) contained two very distinct peaks: a narrower and larger peak at ~0.4 s and a smaller, but very wide, peak at ~5 s. The peak of the spectra was significantly larger than what is commonly adopted in design codes. The latest published uniform hazard spectra for a 475-year return period were found to significantly over-estimate seismic demands on stiff structures (fundamental period less than ~0.5 s,) and underestimate the demands on more flexible structures. The resonant site frequencies computed from microseismic recordings, as reported in published literature, are found to be much larger than what was observed in the spectral ratio of ground motion recorded in soft sediments and exposed bedrock in Kathmandu. Response spectra of aftershock motions recorded at two stations lacked the peculiar peak at 5 s observed in the spectra of mainshock motion. The relatively high Fourier spectral amplitudes (at frequencies near 0.2–0.3 Hz) of ground acceleration recorded at the rock site indicate that the long-period pulse prominent at the soft site was not entirely due to site effects. The polarization pattern of the 5 s pulse at the rock site and at soft site KATNP were found to be very different. Comparison of polarization patterns of mainshock ground motion at rock and soft sites indicate that the 5 s pulse observed at some locations in Kathmandu Valley is a combination of shear waves (the amplitudes of which were amplified by the basin layers) and Love waves.