Abstract
AbstractA shallow destructive inland earthquake of M6.8 occurred on Oct. 23, 2004 (JST) in the middle part of Niigata prefecture, Japan. The aftershock activity was high as compared to those of the other shallow inland earthquakes in Japan. We relocated the sources of the main shock and aftershock activity by adopting the double-difference earthquake location algorithm. The precisely relocated hypocenter distribution shows that there are a doubleplanar structure with a distance 5 km in parallel dipping in WNW with about 50 degrees and a single-planar structure dipping in ESE with about 15 degrees. The upper and lower planes of the double plane began to be formed with the main shock and the largest aftershock of M6.5, respectively, while the ESE dipping plane began to be formed with the 3rd largest aftershock of M6.1. The three planes represent the fault planes of the large events mentioned above. The aftershock distribution is found to be a superposition of the fault planes of the main shock and the large aftershocks. The high aftershock activity is attributed to the formation of the fault planes.
Highlights
A shallow destructive inland earthquake of M6.8 occurred at 17:56 on Oct. 23, 2004 (JST) in the middle part of Niigata prefecture, Japan
The main purpose of this study is to understand the reason of the high aftershock activity of the Mid Niigata prefecture Earthquake in 2004 in relation with its aftershock distribution
We can summarize characteristics of this activity as follows: (1) the distribution of the hypocenters has a double-planar structure in parallel dipping in WNW and a single-planar structure dipping in ESE, (2) the upper and lower planes of the double plane began to be formed with the main shock (α) and the largest aftershock (β), and the ESE dipping plane began to be formed with the 3rd largest aftershock (γ ), and (3) the areas of the planes are almost proportional to the magnitudes of these events
Summary
A shallow destructive inland earthquake of M6.8 occurred at 17:56 on Oct. 23, 2004 (JST) in the middle part of Niigata prefecture, Japan (hereafter referred to as the Mid Niigata prefecture Earthquake in 2004). The focal mechanism of this earthquake was of a reverse fault type with a compression axis in the WNW-ESE direction. This earthquake was the largest in the shallow inland earthquakes of reverse fault type in Japan since Oct. 1997, when JMA began to compile all the seismic waveform data in Japan and make a catalog of seismological data in Japan (hereafter referred to as the integrated seismological catalog) in cooperation with the National University Corporations and the National Research Institute for Earth Science and Disaster Prevention (NIED). The aftershock area is located in the high strain rate zone in the eastern margin of the Japan Sea, which was found based on the geological data (Okamura, 2002) and on the data of the dense GPS array (Sagiya et al, 2000)
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