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.

The spectral ratios of coda waves of local earthquakes have been often used as measures of relative amplification factors of different sites. Applying this method to coda waves registered by seismometers installed on the surface and at the bottom of a borehole, we succeeded in stably measuring the temporal change in site response associated with the occurrence of a large earthquake strong motion. A remarkable drop of coda spectral ratio and a shift of the peak frequency were observed during strong shake at two sites by the 2000 Western Tottori Earthquake and at a site by the 2003 Tokachi‐Oki Earthquake in Japan. The reduction of the peak frequency reached 30–70% at all the sites. After that, the peak frequency logarithmically recovered to the value before the strong motions for a few years at two sites, whereas the other one quickly recovered in a few tens of minutes.

A shallow M7.3 event with a M6.5 foreshock occurred along the Futagawa‐Hinagu fault zone in Kyushu, SW Japan. We investigated the spatiotemporal variation of the stress orientations in and around the source area of this 2016 Kumamoto earthquake sequence by inverting 1218 focal mechanisms. The results show that the σ3 axis in the vicinity of the fault plane significantly rotated counterclockwise after the M6.5 foreshock and rotated clockwise after the M7.3 main shock in the Hinagu fault segment. This observation indicates that a significant portion of the shear stress was released both by the M6.5 foreshock and M7.3 main shock. It is estimated that the stress release by the M6.5 foreshock occurred in the shallower part of the Hinagu fault segment, which brought the stress concentration in its deeper part. This might have caused the M7.3 main shock rupture mainly along the deeper part of the Hinagu fault segment after 28 h.

We detect time-lapse changes in P-and S-wave velocities (hereafter, V P and V S , respectively) and shear wave splitting parameters associated with the 2011 Tohoku earthquake, Japan, at depths between 0 and 504 m. We estimate not only medium parameters but also the 95 per cent confidence interval of the estimated velocity change by applying a new least squares inversion scheme to the deconvolution analysis of KiK-net vertical array records. Up to 6 per cent V S reduction is observed at more than half of the analysed KiK-net stations in northeastern Japan with over 95 per cent confidence in the first month after the main shock. There is a considerable correlation between the S-wave traveltime delay and the maximum horizontal dynamic strain (MDS) by the main shock motion when the strain exceeds 5 × 10 −4 on the ground surface. This correlation is not clearly observed for MDS at the borehole bottom. On the contrary, V P and shear wave splitting parameters do not show systematic changes after the Tohoku earthquake. These results indicate that the time-lapse change is concentrated near the ground surface, especially in loosely packed soil layers. We conclude that the behaviour of V P , V S and shear wave splitting parameters are explained by the generation of omnidirectional cracks near the ground surface and by the diffusion of water in the porous subsurface. Recovery of V S should be related to healing of the crack which is proportional to the logarithm of the lapse time after the main shock and/or to decompaction after shaking.

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To understand the energy release process that operates at the end of the main shock rupture and start of the aftershock activity, we propose an inversion method that uses continuous high-frequency seismogram envelopes of the main shock and early aftershocks (i.e., events that occur at short times after the main shock). In our approach, the aftershock sequence is regarded as a continuous energy release process, rather than a discrete time series of events. To correct for the contribution of coda wave energy excited by multiple scattering, we use the theoretical envelope synthesized on the basis of the radiative transfer theory as a Green's function. The site amplification factors are corrected considering the conservation of energy flux and using the coda normalization method. The inverted temporal energy release rate for the 2008 M W 6.9 Iwate-Miyagi Nairiku earthquake, Japan, decays following t À1.1, at the lapse time t of 40-900 s after the main shock origin time. This exponent of the decay rate is similar to the p value of the modified Omori law. The amount of estimated energy release is consistent with that calculated from the magnitude listed in the aftershock catalog. Although the uncertainty is large, the location of large energy release at the lapse times of 40-900 s approximately overlaps to that of the aftershocks, which surrounds the large energy release area during the main shock faulting. The maxima of the energy release rate normalized by the average decay rate distributes following a power law, similar to the Gutenberg-Richter law.