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.