Migration of low-frequency tremors revealed from multiple-array analyses in western Shikoku, Japan

Multiple-array observation above a belt-like tremor zone was conducted to investigate the detailed location and migration of tremor activity in western Shikoku, Japan. In March 2007, an episodic tremor and slip event occurred, and highly coherent waveforms were recorded at three arrays. Multiple signal classification analysis for the data from each array enabled measuring precise arrival directions. The majority of tremor signals suggested relatively low slowness. The arrival directions of tremor signals were used to locate tremor sources by the grid search method. Tracking the tremor activity showed that the tremor migrated within several hours in the northeast-southwest direction over a distance of 12-15 km, and its migration velocity was 1-2.5 km h(-1). This migration velocity is more rapid than the mean velocity of 0.5 km h(-1) over the whole tremor episode lasting several days. Such a short-timescale migration may represent fluctuation of slip acceleration during the slow slip event. Whenever a tremor migrates southwestward, very low frequency earthquakes occur in the vicinity of the tremor migration terminus. This indicates that the tremor migration is related to the occurrence of very low frequency earthquakes and slow slip events.