Inferred Harmonic Tremor
Since May of 1998, both SOSUS and the Eastern Equatorial Pacific autonomous hydrophone array have detected extremely loud volcanic tremor-like signals from the Volcano Island chain south of Japan. The signals are characterized by a high amplitude fundamental at around 10 Hz and three harmonics at 20, 30, and 40 Hz. The tremor signals typically appear as discrete packets that last for 4-5 minutes, with brief periods of quiescence for roughly 30 seconds followed by the beginning of the next signal packet. During the record of each signal packet, the spectral peaks change frequency (typically increasing by 5-10Hz) as a function of time while maintaining their harmonic spacing. These spectral characteristics are very distinctive and have been previously identified in volcanic tremor recorded using both seismic and airborne acoustic records at Arenal Volcano in Costa Rica (Garces et al., 1998) and at Pavlof Volcano, Alaska (Garces and Hansen, 1998). Unfortunately, the source of these signals is outside of optimum array coverage, so the estimated locations are not accurate. The best locations place it between 22-27N latitude and 138-141E longitude.
The tremor has been occurring intermittently since May 1998, but as of late 1999 was still being recorded. During this time, periods of intense tremor activity have been recorded on 30 different days. Peak amplitudes and durations occurred on August 16-18, and 31 in 1998, and on April 22, August 20-27, and October 10-11, 1999. December 10-12, 1999 produced some of the loudest signals yet detected.