東北地方地殻深部に発生する低周波微小地震の活動とモーメントテンソル解

抄録

Most of shallow inland earthquakes in northeastern Japan are distributed at depths shallower than 15 km, but some occur at anomalously large depths (20-45 km). They have low predominant frequencies for both P and S waves. We confirmed these deep low-frequency microearthquakes (deep LF events) and relocated swarms of deep LF events in the period from 1976 to May 1999 by station corrections obtained for events located accurately using dense seismic network. They are isolated from normal shallow earthquakes and occur beneath active volcanoes and/or in or around P wave low-velocity zones. The seismic activity of deep LF events varies in time. Most seismically active areas for deep LF events are two areas near Iwate volcano and Kurikoma/Narugo volcanoes, where large earthquakes with magnitudes 6.1 and 5.9 occurred recently. We determined moment tensors of three deep LF events near Narugo volcano by waveform inversion. Some of these include non-double couple components.