Comparison of early aftershock sequences for the 2004 Mid-Niigata and 2007 Noto Hanto earthquakes in Central Japan

Abstract

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. Copyright © 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.