Features of Long-Period Ground Motion on the Kathmandu Valley for the 2015 Gorkha Nepal Earthquake Sequence

Abstract eng:
On 25 April 2015, the Gorkha earthquake (M W 7.8) occurred in the Himalayan Range of Nepal. Major damage occurred in central and eastern Nepal. The focal area estimated by U.S. Geological Survey was about 200 km long and 150 km wide, with a large slip area near the Kathmandu Valley. The devastating earthquake was followed by a series of aftershocks: there were four large aftershocks (Mw 6.3~7.3). The aftershock of M W 6.6 occurred on 25 April 2015 ~80 km northwest of Kathmandu at epicenter near to that of the main shock. The other three large aftershocks were originated ~80 km east of Kathmandu; the aftershock of M W 6.7 occurred on 26 April 2015 and the aftershocks of M W 7.3 and M W 6.3 occurred on 12 May 2015. The aftershock of M W 7.3 brought about more damages to infrastructures already vulnerable due to the main shock. The Kathmandu Valley, formed by soft lake sediments of Plio-Pleistocene origin, consists of thick soft sediment below the center of the valley. To understand the site effect of the Kathmandu Valley, we installed continuous recording accelerometers in different parts of the valley. We observed the strong motion records from the 2015 Gorkha Nepal earthquake and the aftershock sequence. In this paper, we examine characteristics of long-period ground motion in the Kathmandu Valley based on the strong motion records from four large aftershocks. First we examine the velocity Fourier spectra of the radial, transverse, and vertical components at all stations for the four large aftershocks. The velocity Fourier spectra around 0.1 Hz for the three components show nearly the same spectral peaks at all stations including a station on a rock site for three aftershocks except the M W 6.7 event. In order to investigate the nature of the long-period ground motions with a period of about 10 sec for four aftershocks, we compare time-frequency analysis diagrams of three components at all stations using the multiple filter analysis following Dziewonski et al. (1969). The diagrams for three aftershocks show the same peaks around 0.1 Hz just after S-wave arrival on the radial and vertical components. Next, we investigate these long-period ground motions on the radial and vertical components in detail. Narrow bandpass filters are applied to the radial and vertical velocity waveforms and the particle motions on the radial-vertical plane are drawn. The particle motions of the long-period motion appearing after S-wave arrival has retrograde elliptical motion, which indicates the long-period motion is due to Rayleigh wave. Finally, we applied the semblance analysis to the narrow band-pass filtered vertical-component velocity waveforms for the M W 7.3 and M W 6.3 aftershocks. The phase velocity of the Rayleigh wave was estimated to be about 3 km/s at 10 sec period. We compare these phase velocities with the theoretical ones based on the one-dimensional velocity structure of crust and upper mantle in the Himalayas of eastern Nepal by Monsalve et al. (2006). The estimated values are a bit lower than those from Monsalve et al. (2006) in the frequency range of 0.05-0.10 Hz.

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