10S-PERIOD VOLCANIC TREMORS OBSERVED OVER A WIDE AREA IN SOUTHWESTERN JAPAN

Unusual seismic oscillations with a period of 10 seconds were observed over a wide area of south-western Japan. Several lines of evidence indicate that the origin of these 10s waves must be around the Aso volcano, located in the central part of Kyushu Island, Japan. Although a 10s period is unusually long for a volcanic tremor and no corresponding surface activity at the crater was reported by the local observatory, the volcanic origin of the tremor is indisputable. A further astonishing aspects of the observation is that there exists a strong correlation between the onset of 10s waves observed remotely and cessation of high-frequency (> 1Hz) volcanic tremors at the Aso volcano. The Aso volcano appears to be trembling with a 10s period without this having been noticed. Since seismic monitoring of volcanic activity is limited to high frequencies (above 1 Hz) at the most of the volcanoes in the world, long-period tremors of the type observed here might be a widespread phenomenon that has been overlooked.