Time-Lapse Changes of Seismic Velocity in the Shallow Ground Caused by Strong Ground Motion Shock of the 2000 Western-Tottori Earthquake, Japan, as Revealed from Coda Deconvolution Analysis

摘要

A large earthquake shock often drops the seismic velocity of the shallow ground. However, it is not clear whether the dropped velocity recovers shortly after the earthquake shock or not. The purpose of this article is to report the time-lapse changes of seismic velocity in the shallow ground after the 2000 Western-Tottori earthquake, Japan. We deconvolve the coda record of small earthquakes registered on the ground surface by that registered at the 100 m depth in a borehole at a station that experienced a strong shock from the mainshock. Because coda waves are mostly composed of randomly scattered S waves, deconvolution of the two coda records enables us to obtain a robust image of the ground structure. Assuming that the shear modulus was reduced at the depth of 0-11 m, we estimate the shear modulus change in each time period after the mainshock by fitting synthetic coda deconvolution to the observed one from 1 to 16 Hz. As a result, the shear modulus dropped to 52% of the value obtained before the mainshock a few minutes after the strong earthquake shock. This caused a decrease in the S-wave velocity of 30% and an increase in S-wave travel time of 17 msec. The shear modulus continued to recover for over 1 yr following the logarithm of the lapse time. It recovered to 69%, 83%, 87%, and 97% of the value obtained before the mainshock in the periods of 0 to 1 week, 1 week to 1 month, 1 month to 1 yr, and 1 to 4 yr after the mainshock, respectively.