[1] Passive Image Interferometry (PII) is a seismological method employing ambient seismic noise to monitor the temporal evolution of mean shear wave velocity within the Earth. First, the elastic Green's tensor between two seismometers is computed from the cross correlation of seismic noise recorded during a certain period. Second, the constructed seismograms of different time periods are treated as earthquake multiplets, and small time shifts in their coda are used to invert a relative change in mean shear wave velocity. When applied to the source region of the 2004 M w = 6.6 mid-Niigata earthquake, Japan (centroid depth 5 km), we used noise recorded at six seismometers located at a distance of less than 25 km from the epicenter. Daily inversions during the 2 months before and after the earthquake show a sudden drop of mean shear wave velocity of some tenths of a percent at the time of the mid-Niigata earthquake. Using noise in two frequency bands, 0.1-0.5 Hz and 2-8 Hz, we find similar amplitudes for the velocity drop, which indicates that changes are not restricted to the shallow subsurface. Possible interpretations of this velocity decrease are a decrease of crustal stress after the earthquake, a nonlinear site response in the shallow subsurface layer due to strong ground motion, or structural weakening due to the creation of new fractures in the source area of the earthquake.

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.

[1] We analyze coseismic and postseismic velocity variations caused by the June 13, 2008 Iwate-Miyagi Nairiku earthquake (M W = 6.9) using Passive Image Interferometry (PII). Seismic noise is correlated in order to reconstruct the Green's function between two sensors. Shear wave velocity changes are determined by relating the coda parts of the daily Green's functions to a long-term reference Green's function. Our study extends from January 2008 to August 2010 and includes the correlations for 190 station pairs in three different frequency ranges from 0.125 to 1.0 Hz. We show that combining the 9 different component cross-correlation functions stabilizes the velocity change estimation and increases analysis resolution. The observed velocity change curves can be fitted by model time series consisting of a coseismic velocity drop followed by logarithmic postseismic recovery and seasonal velocity variations. The coseismic velocity drops are stronger at higher frequencies and are concentrated in the southern part of the fault zone. A tomography algorithm was developed to reproject the observed velocity variations of the different sensor pairs onto the single sensors. The depth distribution of coseismic changes was modeled for the three stations with the largest velocity drops. At two stations, the coseismic velocity changes are located in the upper several hundred meters. The third station shows indications for deeper changes, in the order of kilometers. Postseismic recovery takes significantly longer than the analyzed two year period. Seasonal velocity variations with periods of one year are observed at all analyzed frequencies for most station pairs.

We introduce the single-station cross-correlation (SC) technique of processing ambient seismic noise and compare its results with the established cross-correlation (CC) and autocorrelation (AC) techniques. While CC is the correlation of the signals of two seismic stations with each other and AC is the correlation of a signal with itself, SC is the correlation of two different components of a single three-component seismic sensor. The comparison of the three different correlation techniques shows that CCs give the best results at frequencies below 0.5 Hz and that SCs give the best results at higher frequencies. In all three processing techniques, ambient seismic noise is correlated in order to reconstruct the Green's function describing the wave propagation between the first and the second sensor. By relating the coda parts of the daily Green's functions with the long-term reference Green's functions, shear wave velocity changes are determined. Here, we apply this technique to the data of 20 seismic stations in the surroundings of the fault zone of the Iwate-Miyagi Nairiku earthquake (M W = 6.9), which occurred on 2008 June 13, UTC (2008 June 14, Japan Standard Time) in the northern part of the Japanese island Honshu. The data range from 2008 January to 2011 June and therefore include the Tohoku earthquake (M W = 9.0), which occurred on 2011 March 11, off the coast of northern Honshu. The data are analysed in five different frequency ranges between 0.125 and 4.0 Hz. The data show coseismic velocity changes for both earthquakes followed by a postseismic velocity recovery. In general, the coseismic velocity changes increase with frequency. For the Iwate-Miyagi Nairiku earthquake, the strongest velocity changes occur close to the fault zone. Quickly recovering coseismic velocity changes can be separated from changes not recovering during the study period. For the Tohoku earthquake, the complete area is affected by coseismic velocity changes. A modelling of the depth of the coseismic velocity changes indicates that the Iwate-Miyagi Nairiku earthquake can be explained either by large shallow velocity changes or by small, but deep changes. For one station, the observations can only be explained by assuming deeper changes. For the Tohoku earthquake, the modelling shows that different parts of the study area are affected in different ways, some showing shallow changes, others deeper changes. Furthermore, seasonal velocity variations occur, which are compatible for the different stations above 0.5 Hz, with velocity maxima in autumn.

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.