Abstract
Seismic interferometry is one of the most effective techniques for detecting temporal variations in seismic velocity caused by large earthquakes. Before the 2011 Tohoku-Oki earthquake (Mw9.0) near the Japan Trench, a slow slip event (SSE, Mw7.0) and low-frequency tremors were observed near the trench. Here, we applied a seismic interferometry technique using ambient noise to data from 17 ocean bottom seismometers (OBSs) installed above the focal region before the main shock. We used our technique to detect temporal variations in seismic velocity caused by the main shock, SSE, and low-frequency tremors. In the region above the large coseismic slip area, we detected a 1–2% seismic velocity decrease after the main shock. In addition, we observed very small temporal increases in seismic velocity near the SSE fault during the initial SSE stage. Moreover, for most of the OBSs, we observed temporal variations in the autocorrelation functions (ACFs) during the low-frequency tremors. These may have been caused by temporal variations in the ambient noise source distributions, resulting from low-frequency tremors. These results suggest the possibility of detecting low-frequency tremors using ACF monitoring.
Highlights
Seismic interferometry is one of the most powerful techniques for obtaining Green’s functions
We investigate spatio-temporal variations of three functions: Autocorrelation Function (ACF) of ambient noise, cross-correlation coefficients of the cross-correlation function (CCF) between a 15-day ACF and a reference ACF, and phase shift based on the CCFs
The cross-correlation coefficient at zero lag time between the 15-day and reference ACFs during later lapse times decreased after the main shock (Fig. 2b)
Summary
Seismic interferometry is one of the most powerful techniques for obtaining Green’s functions. Other studies have reported decreases in seismic velocity accompanied by large earthquakes in several regions, such as Japan and Sumatra (e.g., Nimiya et al 2017; Sawazaki et al 2016; Takagi et al 2012; Xu and Song 2009). A wide variety of techniques have been used in many regions, and most studies have reported variations in the seismic velocity structure accompanying large. A number of studies have reported variations in seismic velocity of between 0.1 and 5% after a main shock and accompanying SSE. Sawazaki et al (2016) detected velocity decreases of 3.1% and 1.4% 1 week after the 2014 northern Nagano Prefecture earthquake (Mw6.2), and recovery to velocity decreases of 1.9% and 1.1% 4 weeks after the main shock, using KiK-net data and autocorrelation functions (ACFs). During SSEs, changes are observed in Uemura et al Progress in Earth and Planetary Science (2018) 5:87 seismic velocities, which decrease during the early parts of an SSE period and subsequently recover in the later parts
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