[SSS15-P10] Spatio-temporal Change of Seismic Wave Field Induced by Slow Earthquake Revealed from Seismic Interferometry
Keywords:seismic interferometry, slow slip event, ambient noise
Between the end of January and the occurrence of the largest foreshock on March 9 that preceded the 2011 Tohoku-Oki earthquake, SSE and low-frequency tremors have been detected off Miyagi (Ito et al., 2013, 2015; Katakami et al., 2016). We apply a technique in the seismic interferometry using ambient noise to data from 17 OBSs that were installed above the focal region before the 2011 Tohoku-Oki earthquake. We measure temporal variations in seismic wave field with measuring temporal variations of Auto-Correlation Function (ACF) calculated from ambient noise. All OBSs with three components have short-period seismometers with an eigenfrequency of 4.5 Hz and a sampling rate of 100Hz.
The method to calculate ACFs is as follows. First, we apply a band-pass filter of 0.25-2.0Hz in the frequency domain, and compared this with a one-bit technique in the time domain to the ambient noise signal. Second, we calculate 120 seconds ACFs in the time domain using continuous data for fifteen days of time series. Finally, we compare ACFs calculated from fifteen-day data to a reference ACF that calculated from all of the observed data.
As results, we successfully detected a temporal decrease and increase in seismic velocity at the landward and trenchward of the SSE fault. These temporal variations may be related to the temporal variation of distribution of sources constituting ambient noise, owing to the occurrence of low-frequency tremors occurs. This suggests the possibility to detect low-frequency tremors based on a detection of ACF temporal variations.