Apparent migration of tremor source synchronized with the change in the tremor amplitude observed at Aso volcano, Japan, ,


Takagi, N., S. Kaneshima, H. Kawakatsu , M. Yamamoto, Y. Sudo, T. Okura, S. Yoshikawa, T. Mori
J. Vol. Geothermal Res., , in press, 2006.

Abstract

At Aso volcano, Kyusyu, Japan, several different types of volcanic tremor have been observed for many years. One of them is the continuous tremor, the ground vibration which has dominant frequency between 3 to 10 Hz and has approximately constant amplitudes without any clear beginning and ending. We observed the continuous tremor at Aso using short period seismometer arrays for 3 days both in 1999 and 2001. We locate the source of the continuous tremor by seismic array data processing. We assume the arrivals of cylindrical waves and use the semblance coefficients in order to estimate the arrival azimuth, distance, and apparent slowness by grid search. For both of the two observations the epicenters of the continuous tremor are located around the currently active crater, and the source depths are likely to be shallower than 600m. For the 2001 observation, the location of the epicentral area is estimated to be at nearly 300 m west of the active crater within a narrow band of 40m by 360m. For the 1999 observation, we find that the estimated epicenter clearly migrates synchronized with the change in the tremor amplitude. The migration often occurs periodically with a period of about 80 seconds, but aperiodic occurrence of the migration is also often seen. In both cases, the epicenter is located southeastward (northwestward) when the amplitude is larger (smaller). We propose that there are two or more independent tremor sources with fixed locations, and that their amplitudes modulate either aperiodically or periodically with periods nearly 80 seconds. The tremor signals from those sources are mixed at the arrays, and the estimated epicenter parameters vary according to which signal dominates the seismograms. The simplest model is that there are two point sources, one at west and the other at south of the crater, and the amplitude of the southern source changes with time. Consequently, the southern source dominates the seismogram when the observed amplitude is larger, whereas the western source dominates when the amplitude is smaller. We generate synthetic seismograms, and apply the location technique to them to verify the validity of the two point source model and to search the locations of two sources which can explain the observed synchronization between the amplitude and the apparent epicenter location. We find that the distance between the two sources needs to be more than 450m to agree with the observation.