Abstract
Earthquake swarms repeatedly occurred in Chijiwa Bay and around Unzendake volcano in west Kyushu, Japan. The volcano erupted in November 1990 and lava domes began to emerge in May 1991 after a quiescence of 198 years. It was investigated whether the change of the volcanic activity, such as the occurrences of eruption and lava domes, were easily detected by monitoring the time variations of coda Q-1 values (Qc-1s). In this study, Qc-1s were estimated from the earthquakes around the Unzen area in the period from 1984 to 1991. According to single isotropic scattering approximation, the Qc-1s were calculated from seismic records with sampling rate 30 Hz at Nagasaki and Unzen stations, which were located 40km and 4km away from the summit of the volcano, respectively. We obtained the following two conclusions. The Qc-1s for Unzen station were independent of the frequencies in the range from 2 to 3Hz. No remarkable changes were detected for the Qc-1s, their variances and their frequency dependence at Nagasaki and Unzen stations associated with the changes of the volcanic activity in the period from 1984 to 1991.
