Observation of coseismic and postseismic velocity changes for deep borehole seismic stations in the Kanto area

Abstract

Using Passive Image Interferometry (PII), we analyze long time series of seismic noise data from Hi-net seismometers in the Kanto Basin in and around Tokyo. The 17 deep borehole seismometers used in this study are located in depths between 1200 and 3500 m below the Earth's surface. The analyzed time series span 9.5 years (January 2003 to June 2012) and include the recordings of the MW 9.0 Tohoku-oki earthquake on 2011 March 11 and also an M 5.7 earthquake in Southern Ibaraki prefecture on 2004 October 6. Single-station cross-correlations and cross-correlations were calculated for the different stations and station pairs, respectively, in frequency ranges between 0.125 and 4.0 Hz. Significant coseismic velocity drops are observed, followed by postseismic recovery that can be modeled by an exponential function. Using the frequency-dependence of the velocity changes, we can constrain the depth range of the observed velocity changes. Furthermore, we correlate the observed velocity changes with the pga and pgv values at the respective sites. As this study uses very deep sensors compared with previous studies, we anticipate that the observed noise field differs from the one observed at surface stations. Consequently, the depth range of the observed coseismic velocity changes can be constrained more precisely combining the results of this study with previous results.