Earth Planets Space, Vol. 57 (No. 6), pp. 557-561, 2005LETTER
Satoshi Matsumoto1, Yoshihisa Iio2, Takeshi Matsushima1, Kenji Uehira1, and Takuo Shibutani 2
1Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shin'yama, Shimabara, Nagasaki 855-0843, Japan
2Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
(Received February 14, 2005; Revised April 23, 2005; Accepted May 16, 2005)
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.
Key words: Inhomogeneity, reflector, mid Niigata, crustal structure.