Analysis of strong ground motions and site effects at Kantipath, Kathmandu, from 2015 Mw 7.8 Gorkha, Nepal, earthquake and its aftershocks

Yadab P Dhakal,Hiroyuki Fujiwara,Takashi Kunugi,Hisahiko Kubo,Wataru Suzuki,Shin Aoi

doi:10.1186/s40623-016-0432-2

Abstract

Strong ground motions from the 2015 Mw 7.8 Gorkha, Nepal, earthquake and its eight aftershocks recorded by a strong-motion seismograph at Kantipath (KATNP), Kathmandu, were analyzed to assess the ground-motion characteristics and site effects at this location. Remarkably large elastic pseudo-velocity responses exceeding 300 cm/s at 5 % critical damping were calculated for the horizontal components of the mainshock recordings at peak periods of 4–5 s. Conversely, the short-period ground motions of the mainshock were relatively weak despite the proximity of the site to the source fault. The horizontal components of all large-magnitude (Mw ≥ 6.3) aftershock recordings showed peak pseudo-velocity responses at periods of 3–4 s. Ground-motion prediction equations (GMPEs) describing the Nepal Himalaya region have not yet been developed. A comparison of the observational data with GMPEs for Japan showed that with the exception of the peak ground acceleration (PGA) of the mainshock, the observed PGAs and peak ground velocities at the KATNP site are generally well described by the GMPEs for crustal and plate interface events. A comparison of the horizontal-to-vertical (H/V) spectral ratios for the S-waves of the mainshock and aftershock recordings suggested that the KATNP site experienced a considerable nonlinear site response, which resulted in the reduced amplitudes of short-period ground motions. The GMPEs were found to underestimate the response values at the peak periods (approximately 4–5 s) of the large-magnitude events. The deep subsurface velocity model of the Kathmandu basin has not been well investigated. Therefore, a one-dimensional velocity model was constructed for the deep sediments beneath the recording station based on an analysis of the H/V spectral ratios for S-wave coda from aftershock recordings, and it was revealed that the basin sediments strongly amplified the long-period components of the ground motions of the mainshock and large-magnitude aftershocks.

Highlights

The 2015 Mw 7.8 Gorkha, Nepal, earthquake occurred at 11:56 local time (UTC + 05:45) on April 25, and several moderate- to large-magnitude aftershocks followed the event
Previous studies (e.g., Aguirre and Irikura 1997) have reported that verticalcomponent ground motions are negligibly affected by site response nonlinearity in comparison with horizontalcomponent motions. These findings are supported by the richer short-period ground motions and larger peak ground acceleration (PGA) of the vertical component in comparison with those of the Figure 5b shows that the mean H/V ratios of the coda waves achieve a larger peak at longer period, and the difference between the H/V ratios for the mainshock and aftershocks at short periods is not so strong as it was for the S-waves, suggesting that the coda waves were mainly composed of the long-period surface waves
Strong ground motions from the Mw 7.8 Gorkha earthquake and its eight aftershocks recorded by a strongmotion seismograph at the KATNP site were analyzed to understand the characteristics of strong ground motions and site effects

Summary

Introduction

The 2015 Mw 7.8 Gorkha, Nepal, earthquake occurred at 11:56 local time (UTC + 05:45) on April 25, and several moderate- to large-magnitude aftershocks followed the event. This paper compares the ground-motion parameters, namely the PGAs, peak ground velocities (PGVs), and response spectra, observed at the KATNP site with those obtained from the GMPE developed by Morikawa and Fujiwara (2013) for Japan for both plate interface and crustal events.

Results

Conclusion