Abstract
Abstract An S-wave reflector is considered to relate to the existence of liquid in the seismogenic zone of the crust, which plays an important role in understanding the mechanism of earthquakes. We studied a distribution of S-wave reflectors in and around the hypocentral zone of the 2004 mid Niigata Prefecture Earthquake (M6.8). The earthquake was followed by several aftershocks that were greater than M6. Moreover, the aftershocks were not only located on the fault plane of the main shock but also on conjugate fault planes and on a parallel plane to that of the main shock. In order to discuss the relationship between this complex activity and the crustal heterogeneities, we analyzed the seismograms observed at the seismic stations in this region. Normal moveout processing (NMO) was applied to the data of the aftershock. Several S-wave reflectors could be identified from the NMO sections for every station. In particular, relatively strong S-wave reflectors exist in the lower crust at a depth of approximately 20-25 km in the middle part of the aftershock region. Additionally, reflectors were found beneath the fault planes of the main shock and the largest aftershock. This suggests a possibility of the correlation of the crustal heterogeneities to the occurrence of an earthquake.
Highlights
The 2004 mid Niigata Prefecture Earthquake (M6.8) occurred on 23 Oct., 2004 in the central part of the Niigata Prefecture, Honshu, Japan
For an Normal moveout processing (NMO), it is assumed that the seismic signal in horizontal component is composed of many S waves reflected by a horizontal reflector (Sx S phases)
To check the assumption for the reflected phases to be Sx S waves, a NMO section of the vertical component for the event in region B at HIROKA is plotted in left side of Fig. 3
Summary
The 2004 mid Niigata Prefecture Earthquake (M6.8) (referred to as the Niigata EQ. in this paper) occurred on 23 Oct., 2004 in the central part of the Niigata Prefecture, Honshu, Japan. In order to determine the precise aftershock location, we deployed online and offline temporal seismic stations above the hypocentral region of the earthquake. For an NMO, it is assumed that the seismic signal in horizontal component is composed of many S waves reflected by a horizontal reflector (Sx S phases).
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