Variability of deconvolved bedrock motion of the 2011 off Pacific Coast of Tohoku Earthquake around the K-NET Tsukidate station considering uncertainty in shallow S-wave velocity model from inversion of Rayleigh wave phase velocity

摘要

A large acceleration with a peak amplitude of 2700 cm/s(2) was recorded at the K-NET Tsukidate station (MYG004) during the 2011 off Pacific Coast of Tohoku Earthquake. Nonetheless, there was little structural damage around the station. In this study, we estimate the engineering bedrock's motion variability due to the main shock, deconvolved from the observed surface motions, considering uncertainties in shallow S-wave velocity (Vs) profiles. The Vs profile uncertainty at the strong motion station is estimated with observed Rayleigh wave phase velocities, from microtremor explorations, using the Markov-chain Monte Carlo inversion. We obtain upper and lower possible values of the estimated bedrock spectral accelerations and the ground motion proxies at MYG004 using the uncertainty of the Vs profile. The estimated bedrock peak ground acceleration ranges are 610-930 cm/s(2) and 2000-3100 cm/s(2) for the east-west (EW) and north-south (NS) directions, respectively. Meanwhile, the ranges of the peak ground velocities are about 35-40 cm/s and 90-120 cm/s for the EW and NS components, respectively. The results indicate that the minimum and maximum possible values of the deconvolved spectral acceleration and the ground motion proxies are not only affected by Vs profile uncertainty but also by surface ground motion characteristics. Moreover, the spectral velocities at a station in the main part of Tsukidate Town are also estimated with the observed phase velocity. They have predominant peaks at periods shorter than 0.5 s, suggesting only minor structural damage in the city center.