Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.s53a1808o&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #S53A-1808
Physics
0520 Data Analysis: Algorithms And Implementation, 7230 Seismicity And Tectonics (1207, 1217, 1240, 1242), 8164 Stresses: Crust And Lithosphere
Scientific paper
The passive image interferometry technique is preliminarily applied to the continuous seismic waveform data obtained around the source region of the 2007 Noto Peninsula Earthquake (M6.9, occurred on March 25, 2007, Noto 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 (1-10Hz) filtered seismic noise portion recorded with short-period seismometer at several seismic stations for each one day. Ohmi et al (2008, Earth, Planets, Space, in press) already reported that sudden changes of ACFs are detected associated with the occurrence of the main shock. Around the source region of the Noto EQ, station N.TGIH (epicentral distance 4km), DP.NNJ (36 km), and DP.HRJ (45 km) exhibit the change of ACFs. In these stations, changes of lag time of the particular phases in ACF are observed. They are attributed to the change of seismic wave velocity in the volume considered. In some stations, temporal evolution of ACFs preceding the mainshock is also detected. In this paper, we investigated the temporal variation of the decay factor of the ACFs. We calculated the band-pass (1-3 Hz) ACFs of each station for each one day obtained in the previous study. It is also indicated that decay of ACF is equivalent to that of coda waves. Thus we assume the envelope of ACF E(t) should obey the typical relation of the coda Q theory; E(t) ∝ (1/tn)exp(-2πft/Q), where t is lapse time, f is the central frequency, and the Q is the decay rate. We tried n=2, 1.5, and 1 to adopt the model to the observed ACFs, however, we could not obtain appropriate Q values because of too rapid decay of the ACFs. Therefore, we assume n=0 to obtain 'pseudo' decay factor ~{Q}, which is not the coda Q value in the strict meaning. We adopted above model with n=0 to the observed ACFs of the above three stations and obtained decay factor ~{Q} of each station for each one day. The decay factors during a period of one year (September 2006 - August 2007) including the mainshock exhibit temporal variations. At station N.TGIH, ~{Q} gradually decreases since September 2006 and kept lowermost values from mid November 2006 to mid March 2007, and then gradually increased after the mainshock. At station DP.NNJ, gradual decrease of ~{Q} with slight undulation is observed. The lowermost values were observed at the end of February 2007 and then gradual increase with undulation is observed. At station DP.HRJ, these tendencies are hardly seen. Many previous studies reported the temporal change of coda Q values associated with seismic activity including precursory change (e.g. Gusev and Lemzikov, 1985). In our result, decrease of ~{Q} before the occurrence of the mainshock is observed in some stations, which is consistent with previous studies using coda Q. Although ~{Q} values in our analysis is not identical to the coda Q, the decay rate of ACFs would be also a powerful tool for monitoring the stress state of the crust. In our analysis, decay of ACFs is not expressed as surface, diffusive, nor body wave. It is also a future issue to investigate the source of the ambient seismic noises used in the study.
Hirahara Kazuro
Ohmi Shiro
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