特異な震源分布を示す甑海峡北部の地震活動

抄録

On January 30, 1992, a shallow earthquake of magnitude 4.9 followed by about 300 aftershocks occurred in a northern area of the Koshiki channel, north-western Kagoshima Prefecture. We located seismic events observed at two stations of NOEV (Nansei-toko Observatory for Earthquakes and Volcanoes) and four stations of SEVO (Shimabara Earthquake and Volcano Observatory), using Joint hypocenter determination (JHD). Hypocenters of the mainshock and its aftershocks were nearly vertically distributed at depths from 5km to 13km in a small area.
Initial motions at the seismic stations of NOEV, SEVO and FMO (Fukuoka Meteorological Observatory) suggest a focal mechanism of strike slip fault type with a T-axis of NNW-SSE direction: the mechanism is very similar to those reported for the earthquakes in and around the area. The nodal plane striking in NE-SW agrees with trends of the fault system in the channel and the other WNW-ESE plane is parallel to the earthquake alignment along Amakusanada-Izumi-Kakuto areas. Hypocenters of the event and aftershocks nearly vertically distributing are, however, not consistent with any of the planes.
In March of 1991, about 10 months before the M 4.9 event, an earthquake swarm (M_max 2.9) occurred at depths around 5km almost within the same epicentral area. Namely, two different types of earthquake sequence occurred at different depths in the same area: the swarm occupied a shallower zone than the focal zone of the M 4.9 event. Although some volcanic process may be inferred from hypocenters vertically aligning, it is probably difficult to explain the fact that the earthquake swarm at shallow depths occurred about 10 months before the M 4.9 event at a deeper depth without accompanying any notable foreshocks.