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Abstract: At Aso volcano, Kyushu, 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 and 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 in 1999. We locate the source of the continuous tremor by seismic array data processing. We use the semblance coefficients in order to estimate the arrival azimuth and apparent slowness by grid search. The epicenters of the continuous tremor are located around the currently active crater, and the source depths are likely to be shallower than 600 m. 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 s, 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 s. 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 (we call the two sources as “NW source” and “SE source”, respectively), and the amplitude of the SE source changes with time. Consequently, the SE source dominates the seismogram when the observed amplitude is larger, whereas the NW 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 400 m to agree with the observation. We also analyze the seismic array data observed in 2001, and infer that the NW source in 1999 may be identical to the tremor source of the 2001 data. [Copyright &y& Elsevier]

Abstract: In order to investigate the continuous volcanic tremor of Aso Volcano in Japan, we performed a series of temporary short-period seismic array observations near the Nakadake first crater (the active crater) during five years from 1999 to 2003. We deployed in all of the temporary observations a seismic array at the same location about 700 m west of the active crater, in order to investigate long-term changes in the tremor activity. In 1999 and 2003, another array was simultaneously deployed at a different location 700 m north of the crater to help locate the tremor sources. We developed a frequency domain semblance method and applied it to the waveform data of the frequency range where the continuous tremor is dominant (3–6 Hz). We measured arrival azimuths and slownesses of the continuous tremor signals as functions of frequency, which are then used to locate the epicenters of the tremor signals corresponding to the principal peaks of the power spectra. For the observations in 1999 and 2002, the continuous tremor amplitudes are relatively small, and the slowness of the tremor signal observed at the west array takes a local minimum (0.5 to 0.6 s/km) near the frequency (∼4.7 Hz for 1999 and ∼4.8 Hz for 2002) which corresponds to the highest spectral peak. This implies that body waves dominate the tremor signals at the west array around the frequency. The tremor epicenters corresponding to 4.7 Hz for the observation in 1999 are located at the west rim of the currently active crater. While the surface crater activity of Aso remains low and the tremor activity is not clearly linked with the surface activity until early 2003, a close link between the tremor and crater activity appears in the middle of 2003, when a small phreatic eruption occurred a month before the array observation (July 10, 2003). Tremor signals of the observation in 2003 show a large spectral peak (4.2 Hz) where the slowness measured for the west array is very large (1.1 s/km), clearly suggesting that surface waves are dominant. The epicenter is again located at the western rim of the active crater. We interpret these observations as follows: in 1999 and 2002 when the surface activities of Aso were low, the continuous tremor excitation was deep and inactive. In the middle of 2003 when Aso Volcano became active with a series of phreatic eruptions a shallower tremor source was activated, possibly masking the deeper sources. This shallowing of the dominant tremor source could be due to the increase in the volcanic gas flow rate triggered by the phreatic eruptions. [Copyright &y& Elsevier]