HR: 1330h
AN: S52F-0185    [PDF]
TI: Swarm Activity of Low Frequency Earthquakes beneath Ashio Region, Central Japan
AU: * Ueda, H
EM: bono@eri.u-tokyo.acjp
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, 113-0032 Japan
AU: Takeo, M
EM: takeo@eri.u-tokyo.ac.jp
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, 113-0032 Japan
AU: Hagiwara, H
EM: hagiwara@eri.u-tokyo.ac.jp
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, 113-0032 Japan
AB: It is well known that the depths of almost all inland earthquakes are shallower than 15 km, and this cut-off depth can be interpreted as a brittle-ductile transition zone. However, a few earthquakes with depths ranging 20 km to 40 km have been observed. These anomalously deep events have lower dominant frequencies for both P and S waves than the corner frequencies expected from those magnitudes which based on ordinary earthquake source spectrum model. These events are termed Deep Low-Frequency earthquakes (DLF). Recently, a high sensitivity seismograph network (Hi-net) is installed by National Research Institute for Earth Science and Disaster Prevention (NIED), and we can obtain high-density high-gain seismic data in Japan. On 2002_@March 11, we recognized swarm activity of DLF in Ashio region, central Japan. More than 140 DLF events were observed within only 4 hours. The first time we locate the hypocenters, it seemed that the hypocenters migrated westward about 2 km during this activity. This is very fast migration compared with previous study which reported that DLF hypocenters migrated toward west about 2 km during half a year. Then we obtain high resolution relative hypocenters of similar earthquakes (22 events) by cross spectral analysis method and double-difference earthquake location algorithm. Relocated DLF hypocenters didn_ft show remarkable migration. The distribution of DLF hypocenters like a vertical disk (with radius 700 m and thickness 200 m and the strike EW) which suggest some structure of lower crust. We analyzed some DLF events to get focal mechanisms by using a waveform inversion method. The result is that some of DLF focal mechanisms have almost the same P-axis directed west-southwest.
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics and mechanics
DE: 7280 Volcano seismology (8419)
SC: Seismology [S]
MN: 2003 Fall Meeting