Title: Study of Iwasaki Earthquake Swarm in Aomori Prefecture (I)
Abstract:An active earthquake swarm have been occurring in and near Iwasaki, Aomori Prefecture, the northeast of Honshu, Japan from September 16, 1978 with precursory earthquake swarm before 3 and 9 days. The ...An active earthquake swarm have been occurring in and near Iwasaki, Aomori Prefecture, the northeast of Honshu, Japan from September 16, 1978 with precursory earthquake swarm before 3 and 9 days. The magnitude of the largest event was 4.3. A temporal seismic network using wireless telemetry was developed to precisely determine the distribution of hypocenter locations. Hypocenter locations of 2000 events were determined for the period of the observation over 80 days. Most of hypocenters were located at a distance less than 2km from the coast line and in a small region beneath the Sea of Japan of water depth shallower than 50m. An extent of the earthquake swarm region was about 2km in the north-south and 4km in the east-west direction. We found hypocenter distribution of shallow earthquakes having planed structure, which had not been found in the Tohoku district so far. Hypocenter locations projected on the vertical section in the northwest-southeast direction clearly showed that the events occured in a region of planed structure dipping about 40 degrees in the direction of the northwest. The events were at depths ranging from 4km to 7km. A high seismic activity was found at deeper focal depth from 6km to 7km in the first stage of the swarm and decreased rapidly from the latter half of October. However, the seismic activity of shallower depth did not decrease till the end of November.Read More