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The passive image interferometry technique is applied to the continuous seismic waveform data obtained around the source region of the 2007 Noto Hanto Earthquake, central Japan. We computed the autocorrelation function (ACF) of band-pass filtered seismic noise portion recorded with each short-period seismometer at several seismic stations for 1 day each. In some stations, comparison of each 1-day ACF shows temporal evolutions of the ACF, which are interpreted as the change of seismic velocity structure in the volume considered. Sudden changes of ACF are detected to be associated with the occurrence of the main shock in one station of the four stations analyzed. Gradual changes of ACFs in the preceding 2 weeks of the main shock are also recognized in two stations, which would be of great importance for understanding the stress state before the occurrence of earthquakes.

A large shallow earthquake occurred in the Noto Peninsula on March 25, 2007. The Japan Meteorological Agency (JMA) estimated the hypocenter and origin time to be 136.685E, 37.220N and 09:41:57.9 JST, respectively, and the depth to be 11 km. The magnitude of the main shock is Mj (JMA scale) 6.9 and Mw 6.6, and the focal mechanism of the main shock is a reverse fault with right-lateral components. The strike, dip and rake are 58◦, 66◦ and 132◦, respectively, according to the National Research Institute for Earth Science and Disaster Prevention (NIED). The two largest aftershocks of Mj 5.4 occurred at the northeast and southwest edges of the aftershock area, at 18:11 on May 25 and at 07:16 on May 26, respectively. A maximum seismic intensity, 6 upper on the JMA scale, was observed at several stations situated in the source area. The earthquake caused severe damage on the Noto Peninsula and in the surrounding prefectures of Ishikawa, Toyama, Niigata and Fukui. The final report of the Fire and Disaster Management Agency reported one human fatality, 356 people injured and 29,349 houses completely and partially collapsed. Although several moderate-sized earthquakes had occurred in the northeast and southern parts of Noto Peninsula prior to this event, there is no record of large earthquakes with M > 7 occurring in this area during the last 1500 years. In addition, events of M 6.0 ∼ 6.5 have never occurred in the source region of the March 25, 2007 earthquake, and even the seismic activity of small and microearthquakes has been very low for more than 25 years, based on the high-gain seismic-observations by the Kamitakara Observatory, Kyoto University (Ito et al., 2007). Large active faults have not been identified on land in the source area. Nevertheless, some offshore faults in west off Noto Peninsula have nearly the same strike as that of the aftershock area, which may be related to the March 25, 2007 earthquake. Well-arranged high-density nation-wide networks have been constructed, and these provided abundant data for the sequence of the earthquake. In particular, strong-motion and high-gain earthquake records as well as the GPS data provided fundamental information on the earthquake sequence and related phenomena from immediately after the main shock. In addition, temporary dense seismic stations were deployed for aftershock observations