1990 Volume 35 Issue 2 Pages 163-173
During the 1988-1989 eruptive activity of Mt.Tokachi, repeated field observations and strengthened efforts for seismometrical observations were made by the National University Team. Nine eruptions during the later eruptive period were observed by three temporary seismic projccts; (1) a new seismic station (HNG) installed at the possible nearest site to the 62-II crater, (2) a small array and (3) nine temporary seismic stations using direct recording system at the volcano and surroundings. These observations revealed that both low-frequency earthquakes (LF) and volcanic tremors (TR) were characterized by Rayleigh type surface waves. The incident azimuths and the apparent velocities at the array are common for both LF and TR. They are inferred to be originated from the limited small region at the shallow depth beneath the 62-II crater. On the other hand, the wave characteristics and the sources of explosion earthquakes (EX) are different from those of LF and TR. EX are inferred to be located beneath the crater, but their depths are somewhat variable.
