Spatial variation in coda Q and stressing rate around the Atotsugawa fault zone in a high strain rate zone, central Japan

We investigated a detailed spatial distribution of coda Q around the Atotsugawa fault zone in a high strain rate zone, central Japan, using waveform data from dense seismic observations. Low coda Q at lower frequencies is localized along the fault zone, showing a good spatial correlation with a low velocity zone in the lower crust. On the other hand, we find no characteristic spatial pattern of coda Q at higher frequencies. The spatial correlation between the low coda Q at the lower frequencies, and the low velocity zone, suggests that ductile deformations below the brittle-ductile transition zone in the crust contribute to the variation in coda Q at lower frequencies. We estimated a spatial variation in the stressing rate of 15-18 kPa/year in the crust from that of coda Q in the analyzed region. This value is greater than that estimated from GPS data. We conclude, therefore, that a high deformation rate below the brittle-ductile transition zone causes the high stressing rate, which results in the high strain rate along the fault zone observed by GPS. © The Society of Geomagnetism and Earth Planetary and Space Sciences (SGEPSS) The Seismological Society of Japan The Volcanological Society of Japan The Geodetic Society of Japan The Japanese Society for Planetary Sciences TERRAPUB

Microseismic observations during a water injection experiment in 2000 at the Nojima Fault, Japan

After the 1995 Hyogo-ken Nanbu earthquake, a scientific drilling program called the Nojima Fault Zone Probe was carried out on the Nojima fault which ruptured during the 1995 earthquake. In 1997, a water injection experiment was carried as part of the program. A second water injection experiment was conducted from January to March 2000 (1/11: failed, 1/22-26: 3MPa, 1/31-2/5: 4MPa, 3/3-3/11: 4.5 MPa). We monitored changes of earthquake activity before and after the water injections. We deployed a temporary seismic network around the water injection site consisting of four telemetered seismic stations, six borehole seismometers, and five off-line seismographs. The 800-m-deep borehole seismometer detected 711 events with an S-P time of less than 1.0 s from December 1, 1999 to May 18, 2000. Clustered seismicity occurred several times during the period. Among the detected microearthquakes, 329 events were located with locational errors of less than 1 km. Most of these are distributed in three regions at depths of less than 5 km