A Trial for Detecting the Temporal Variation in Seismic Velocity Accompanied by a Slow Slip Event, Using Seismic Interferometry of Ambient Noise

Seismic interferometry is one of the most effective techniques to detect temporal variations in seismic velocity before or after a large earthquake. Some previous studies have been reported on seismic velocity reductions due to the occurrences of large earthquakes (e.g., Wegler et al., 2009; Yamada et al., 2010) and preceding them (e.g., Lockner et al., 1977; Yoshimitsu et al., 2009). However, only a few studies accompanying slow slip events have been conducted. Here we report preliminary results on velocity reduction accompanying a slow slip event based on ambient noise seismic interferometry from ocean bottom seismometers (OBSs) in a subduction zone. Between the end of January and the occurrence of the largest foreshock on March 9 that preceded the 2011 Tohoku-Oki earthquake, slow slip events and low-frequency tremors were detected off Miyagi (Ito et al., 2013, 2015; Katakami et al., 2016). We apply seismic interferometry using ambient noise to data from OBSs that were installed above the focal region before the 2011 Tohoku-Oki earthquake. All OBSs with three components are short-period seismometers with an eigenfrequency of 4.5 Hz that were deployed off Miyagi between November 2010 and April 2011. First, we estimated an installed azimuth of two horizontal components for all OBSs, by using particle orbits of some direct P waves from natural earthquakes, to analyze one vertical and two horizontal components. Next we calculated auto- and cross-correlation functions among radial and transverse components using ambient noise after applying a band-pass filter of 0.25-2.0 Hz and a one-bit technique. Finally, we calculated correlation coefficients using a 5 s time window with lag time from -30 s to 30 s at intervals of 0.1 s, as auto- and cross-correlation functions. The preliminary results of auto- and cross-correlation functions show small changes before and after the slow slip event terms. However, these changes may be due to temporal variations in local seismicity. We will remove natural earthquakes from this analysis and proceed to them in further studies.