Abstract
Autocorrelation analysis using ambient noise is a useful method to detect temporal changes in wave velocity and scattering property. In this study, we investigated the temporal changes in seismic wave velocity and scattering property in the focal region of the 2018 Hokkaido Eastern Iburi Earthquake. The autocorrelation function (ACF) was calculated by processing with bandpass filters to enhance 1–2 Hz frequency range, with aftershock removal, and applying the one-bit correlation technique. The stretching method was used to detect the seismic wave velocity change. After the mainshock, seismic velocity reductions were observed at many stations. At N.AMAH and ATSUMA, which are located close to the mainshock, we detected 2–3% decreases in seismic wave velocity. We compared parameters indicating strong ground motion and showed the possibility of correlations with peak dynamic strain and seismic velocity reduction. We also investigated the relationship between waveform correlation and lag time, using ACFs from before and after the mainshock, and detected distortion of the ACF waveform. The source of the waveform decorrelation was estimated to be located near the maximum coseismic slip, at around 30 km depth. Thus, distortion of the ACF waveform may reflect the formation of cracks, due to faulting at approximately 30 km depth.
Highlights
Earthquakes, and their genesis processes, change the internal state of the Earth, via stress state changes, pore fluid movement, fractures around the fault, and shallow ground damage
The dv/v decreases in the autocorrelation function (ACF) estimated by the present study may be due to the shallow ground damage caused by strong ground motions
The velocity drop with a similar magnitude to previous works, and correlation with peak dynamic strain (PDS), implied that damage in the shallow layers due to strong ground motion was the main cause of the velocity drop
Summary
Earthquakes, and their genesis processes, change the internal state of the Earth, via stress state changes, pore fluid movement, fractures around the fault, and shallow ground damage. The Earth’s interior state affects seismic wave velocity and scatterer distribution, or scattering properties. We can understand the temporal evolution of the Earth’s interior state, when associated with earthquakes, better, by monitoring changes in the seismic wave propagation process over time. Seismic interferometry is a useful method with which to monitor temporal change in the seismic wave propagation process (e.g., Sens-Schönfelder and Wegler 2006). Repeating earthquakes and artificial explosions have been used to detect seismic velocity changes associated with large earthquakes or volcanic activity (e.g., Nishimura et al 2000; Poupinet et al 1984). Since repeating earthquakes do not occur frequently, and artificial explosions are expensive, the temporal and spatial resolution of velocity changes has been low in these studies. Methods using auto- and crosscorrelation functions of the continuous ambient noise record (ACFs and CCFs) can estimate temporal changes
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