HR: 0800h
AN: S31A-0212    [Abstracts]
TI: Temporal variations of crustal structure in the source region of the 2007 Noto Hanto and Niigata-ken Chuetsu-oki Earthquakes, central Japan, with passive image interferometry
AU: * Ohmi, S
EM: ohmi@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institue, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan
AU: Hirahara, K
EM: hirahara@kugi.kyoto-u.ac.jp
AF: Graduate School of Science, Kyoto University, Kitashirakawa, Sakyou, Kyoto, 606-8502, Japan
AU: Wada, H
EM: hiroo@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institue, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan
AU: Ito, K
EM: ito@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institue, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan
AB: The passive image interferometry technique (Sens-Sch{\" o}nfelder and Wegler, 2006) is preliminarily applied to the continuous seismic waveform data obtained around the source region of the 2007 Noto Peninsula Earthquake (M6.9, Noto EQ) and 2007 Niigata-ken Chuetsu-oki Earthquake (M6.8, Chuetsu-oki EQ), central Japan, to detect the temporal variation of the subsurface structure around the source region. We computed the autocorrelation function (ACF) of band-pass (1.5 or 2.0 Hz ~ 10.0 Hz) filtered seismic noise portion recorded with each short-period seismometer at several seismic stations for each one day. There are two remarkable features in the observed ACFs. One is that phases with a respective predominant frequency (around 2.5 Hz ~ 3.5 Hz) in ACF at one station are coherent over different days, though the shape of ACF at each station is different to each other. The other is that the temporal evolutions of particular phases in ACF are observed around the occurrence of the earthquake in some cases. In general, the increase lag time of phases in ACF is caused by decrease of seismic wave velocity, and vice versa. Change of wave velocity will be caused by several reasons. Some possible candidates proposed in the previous studies are as follows; (1) change in stress in the Earth's interior caused by the earthquake, (2) change in the near-surface material properties such as generation of damage or cracks caused by the strong shaking of the earthquake, and (3) change of degree of water saturation in the near-surface. Sudden changes of ACF are detected associated with the occurrence of the main shocks. Followings are some results together with epicentral distances (EPD) and changes of volumetric strain caused by mainshocks. In the source region of the Noto EQ, station N.TGIH (EPD = 4km, Dilatation) exhibits the increase of lag time of the phases. ACFfs of the two stations around the source region of the Chuetsu-oki EQ also show the change of lag time. Station IZUMOZ (EPD = 9 km, Contraction) exhibits increase of lag time while station N.KZKF (EPD = 30 km, Dilatation) shows decrease of the lag time of the ACFs. Additionally, in previous study, Wegler and Sens-Sh{\" o}nfelder (2007) detected the increase of lag time at the station N.KZKF associated with the occurrence of 2004 Niigata-ken Chuetsu earthquake (M6.8, Chuetsu EQ), whose EPD is 31 km and located in the area of dilatation. If we assume the change of lag time of the ACFs are caused by change of stress, the tendencies observed in the source region of Noto EQ and Chuetsu EQ are consistent, while that of Chuetsu-oki EQ is not. Temporal variations of ACFs associated with these three M6 class inland earthquakes indicate that change of seismic velocity structure associated with mainshock can be detected with this method. At present, however, it is hard to give concrete interpretations to the observed features. It is essential to make quantitative analyses to identify what the phases in ACF observed at each stations mean, as well as identifying the cause of temporal evolution of ACFs. In some stations, temporal evolution of ACFs preceding the mainshock is also detected, which would be of great importance for understanding the stress state before occurrence of earthquakes.
DE: 0520 Data analysis: algorithms and implementation
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7299 General or miscellaneous
DE: 8164 Stresses: crust and lithosphere
SC: Seismology [S]
MN: 2007 Fall Meeting