Earth Planets Space, Vol. 60 (No. 2), pp. 151-154, 2008LETTER
Jim Mori1, Yasuyuki Kano1, and Bogdan Enescu2
1Disaster Prevention Research Institute, Kyoto University, Japan
2GeoForschungsZentrum, Potsdam, Germany
(Received July 17, 2007; Revised August 10, 2007; Accepted August 16, 2007; Online published February 19, 2008)
We examined continuously recorded seismograms of the 2004 Mid-Niigata and 2007 Noto Hanto earthquakes to compare the early aftershocks for these two similar earthquakes in central Japan. Although the two mainshocks had similar size, depth, and focal mechanisms, the aftershock levels were quite different, with the Niigata case being much stronger. The results of our analyses show that the early aftershock occurrences for the two events were quite similar for the first several minutes. About 7 minutes after the Niigata mainshock, significantly enhanced aftershock triggering began and continued for the duration of that aftershock sequence. We speculate that the change that occurred in the Niigata region may be due to fluid effects in the fault zone.
Key words: Aftershocks, Mid-Niigata earthquake, Noto Hanto earthquake, triggering.