HR: 0800h
AN: T21A-0352    [Abstracts]
TI: Multiple seismic array analysis of low frequency tremors in western Shikoku, Japan
AU: * UENO, T
EM: ueno@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, IBARAKI, 305-0006, Japan
AU: MAEDA, T
EM: maeda@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, IBARAKI, 305-0006, Japan
AU: OBARA, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, IBARAKI, 305-0006, Japan
AU: ASANO, Y
EM: asano@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, IBARAKI, 305-0006, Japan
AU: TAKEDA, T
EM: ttakeda@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, IBARAKI, 305-0006, Japan
AB: In southwest Japan, low frequency tremors (LFTs) have been detected at deeper part of the seismogenic zone on the subducting Philippine Sea plate interface by high sensitivity seismograph network Hi-net operated by NIED. The tremor source area migrates during each episode along the strike of the subducting plate with a migration velocity of roughly 10 km/day, which was referred from source locations obtained by the envelope correlation method. In order to investigate the wave field propagation from the tremors in detail, we carried out a seismic observation campaign in western Shikoku, where the active tremor associated with the short-term slow slip event occurs with a recurrence interval of around six months. In this campaign, we deployed three seismic arrays above the belt-like LFT area with a spacing of about 20 km among arrays, during the period from February to May in 2007 because we anticipated the coming LFT episode to occur on March or April, 2007. Each array was composed of 32-channel receivers which mainly consist of vertical-component with natural frequency of 2 Hz. The receivers were placed with an average spacing of 30 m. Waveform data were recorded continuously with a sampling interval of 0.01 s. Since the active tremor episode with the short-term slow slip event occurred from 13 to 15 on March 2007 as expected, we performed frequency-wave number power spectrum analysis for each array recordings by the MUltiple SIgnal Classification (MUSIC) method in the period. At two seismic arrays, the apparent slowness were continuously low during the tremor episode. At the other array, the slowness was relatively high, and the arrival direction of the waves slightly changed with increasing time. In order to ascertain the change to be migration of the tremors, we located the sources of tremors by grid search method by using backazimuth and apparent slowness estimated at each array. We successfully located the sources of tremors, and detected the migration of the sources clearly with a velocity about 10 km/day. Furthermore we recognized that there were many arrival directions within short time window. This indicates that the tremors occur simultaneously at different places, which might suggest the inhomogeneity of the slip distribution on the plate interface.
DE: 7200 SEISMOLOGY
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting