Combined use of repeated active shots and ambient noise to detect temporal changes in seismic velocity: application to Sakurajima volcano, Japan

Coda-wave interferometry is a technique to detect small seismic velocity changes using phase changes in similar waveforms from repeating natural or artificial sources. Seismic interferometry is another technique for detecting seismic velocity changes from cross-correlation functions of ambient seismic noise. We simultaneously use these two techniques to clarify seismic velocity changes at Sakurajima volcano, one of the most active volcanoes in Japan, examining the two methods. We apply coda-wave interferometry to the records of repeated active seismic experiments conducted once a year from 2011 to 2014, and seismic interferometry to the ambient seismic noise data. We directly compare seismic velocity changes from these two techniques. In coda-wave interferometry analyses, we detect significant seismic velocity increases between 2011 and 2013, and seismic velocity decreases between 2013 and 2014 at the northern and eastern flanks of the volcano. The absolute values are at a maximum 0.47 +/- 0.06% for 2-4 Hz, 0.24 +/- 0.03% for 4-8 Hz, and 0.15 +/- 0.03% for 8-16 Hz, respectively. In seismic interferometry analyses, vertical-vertical cross-correlations in 1-2, 2-4, and 4-8 Hz bands indicate seismic velocity increases and decreases during 3 years of 2012-2014 with the maximum amplitudes of velocity change of +/- 0.3% for 1-2 Hz, +/- 0.4% for 2-4 Hz, and +/- 0.2% for 4-8 Hz, respectively. Relative velocity changes indicate the almost annual change. These periodical changes are well matched with volcano deformation detected by GNSS receivers deployed around the volcano. We compare the results from coda-wave interferometry with those from seismic interferometry on the shot days and find that most of them are consistent. This study illustrates that the combined use of coda-wave interferometry and seismic interferometry is useful to obtain accurate and continuous measurements of seismic velocity changes.