Crustal structure in the Tohoku region before and after the Tohoku-Oki earthquake with P to S receiver functions

Abstract:

The M9 Tohoku-Oki earthquake is the best-recorded megathrust earthquake to date. The effects of the strong shaking experienced throughout Japan include a decrease in shear velocity along the volcanic arc and changes in the crustal seismicity patterns. Here, we utilize a total of 181 Hi-Net and F-Net stations from the National Research Institute for Earth Science and Disaster Prevention (NIED) to investigate the crustal discontinuity structure in the Tohoku region before and after the earthquake. We calculate P to S receiver functions (PRFs) for ~3400 teleseismic events from 2004 to early 2015. The ~500,000 radial component PRFs are stacked with a common conversion point method to create a new three-dimensional model of the Tohoku region discontinuity structure and to map the Mohorovičić Discontinuity (Moho). The Moho is ~5 km deeper than previous estimates using travel time and local earthquake information, indicating the two estimates correspond, potentially, to the top and bottom of a crust-mantle transition zone. In addition to the time invariant Moho structure, we investigate the temporal variation of the PRFs. We observe an increase in the high frequency (>1 Hz) signal within the first 10 seconds of the receiver functions for teleseismic events after the Tohoku-Oki earthquake relative to those events before the mainshock. This change in the signal lasts for at least 10 days after the earthquake, but is not observed at all stations. Mapping the stations that exhibit increased high frequency signal highlights two areas of active volcanism and the Oga-Ojika Tectonic Line (OOTL), which separates northern Tohoku from southern Tohoku. The increased high frequency signal in the volcanic regions may reflect fluid migration in the crust induced by the strong shaking, while the signal along the OOTL may reflect an increase in fault-zone damage in the seismogenic zone. This suggests that analysis of time-varying receiver functions provides important information on the formation and evolution of the crust.