HR: 11:05h
AN: S52B-04    [Abstracts]
TI: Comparison of Early Aftershocks for the 2004 Mid-Niigata and 2007 Noto Hanto Earthquakes in Japan
AU: * Mori, J
EM: mori@eqh.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan
AU: Kano, Y
EM: kano@eqh.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan
AU: Enescu, B
AF: GeoForschungsZentrum, Telegrafenberg, Potsdam, D-14473, Germany
AB: We compared the aftershock sequences of the similar 2004 Mid-Niigata (Mw6.6) and 2007 Noto Hanto (Mw6.7) earthquakes in central Japan. Although the two mainshocks had similar size, depth, and focal mechanisms, the numbers of aftershocks were quite different, with the Niigata mainshock producing a much stronger sequence. We examined the continuously recorded data from nearby Hi-Net stations operated by the National Institute for Earth Science and Disaster Prevention (NIED), to identify the early aftershocks following both mainshocks. A 5 hz high-pass filter was chosen to facilitate identification of the high-frequency arrivals from individual aftershocks. We used 6 stations distributed at distances within about 30 km. Aftershocks were identified by looking at large printouts of the continuous records for the six stations and peak amplitudes were measured to calculate the magnitude. The magnitude determination using these high-pass filtered records was calibrated by using a set of 30 earthquakes that were also listed in the catalog of the Japan Meteorological Agency (JMA). We estimate that the completeness level of small aftershocks is about Mj3.5. The event counts show that the aftershock sequences of the two earthquakes were quite similar for about the first 7 minutes. Following that time, the Niigata aftershocks clearly continue at a much higher rate which is about 3 times the rate of the Noto earthquake. The time where the rates diverge corresponds to the occurrence of a Mj6.3 earthquake in the Niigata sequence. This pattern can be seen in both the plots for the Mj¡Ý3.5 and Mj¡Ý4.0 events. Since there are more earthquakes for the Mj¡Ý3.5 data set, the time resolution is better. These results show an enhanced triggering of aftershocks for the Niigata sequence several minutes after the mainshock. The Niigata region is an area of hydrocarbon production with regions of high pressure fluids, and Sibson (2007) proposes that the swarm-like behavior is due to upward discharge of fluids from a deeper overpressured region. Fluid may flow into openings of fault zones caused by strong earthquake shaking, resulting in a reduction of the normal stresses on the faults.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
DE: 8164 Stresses: crust and lithosphere
SC: Seismology [S]
MN: 2007 Fall Meeting