ALBERT

Description: We present a systematic study of seismic velocity changes associated with a megathrust and five strong crustal earthquakes in Japan. We perform both cross-correlation and single-station cross-correlation analysis for station pairs and stations, respectively. The correlation of ambient seismic noise allows us to reconstruct the Green's functions of the wave propagation. 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. We analyse data from four areas in Japan where large earthquakes occurred: Iwate-Miyagi (2008 M W 6.9 Iwate-Miyagi Nairiku earthquake), Niigata (2004 M W 6.6 Chūetsu, 2007 M W 6.6 Chūetsu-oki and 2011 M W 6.2 Nagano/Niigata earthquakes), Noto Peninsula (2007 M W 6.7 Noto Hantō earthquake) and Fukuoka (2005 M W 6.6 Fukuoka earthquake). In all areas, we analyse time-series which start before the respective earthquakes and last until after the 2011 M W 9.0 Tōhoku-oki earthquake. The analysis in five different frequency ranges between 0.125 and 4.0 Hz yields time-series of the velocity changes for the different station pairs or stations. At the time of the respective earthquakes, we observe coseismic velocity drops in all areas which are followed by a partial post-seismic recovery process. For the Tōhoku-oki earthquake, coseismic velocity drops can also be observed in all regions. There is a general trend of increasing coseismic velocity drops with frequency in all four areas. The largest coseismic drops are observed close to the fault zones. Over the observed time range, the post-seismic recovery is only partial and around half of the coseismic velocity drops do not recover. The characteristic recovery times for the recovering part are similar in all areas and frequency ranges, with an average value of 0.55 yr. We model the volumetric strain changes for the different earthquakes and find that the observed pattern of the coseismic velocity drops cannot be explained by these models. The coseismic velocity drops at the different stations are better related with the peak ground velocities and the associated dynamic strain than with the peak ground accelerations, but the correlation is still poor. This suggests that non-linear effects caused by the strong ground motion during the earthquake can explain at least part of the coseismic velocity drops.