Earth Planets Space, Vol. 59 (No. 2), pp. e5-e8, 2007E-LETTER
Junichi Nakajima and Akira Hasegawa
Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
(Received October 12, 2006; Revised December 8, 2006; Accepted December 13, 2006; Online published March 21, 2007)
A seismic tomography study has revealed a detailed three-dimensional seismic velocity structure along a high-strain-rate zone, the Niigata-Kobe Tectonic Zone (NKTZ), located in Japan. The results show that the depth extent of the low-velocity zone varies along the NKTZ. We divided the NKTZ into three regions on the basis of the velocity structure. A low-velocity anomaly observed in the lower crust beneath the southwestern part of the NKTZ is probably attributable to the fluids derived from the Philippine Sea slab, while a prominent low-velocity anomaly extending from the upper crust to the uppermost mantle in the volcanic region, the middle part of the NKTZ, may be caused by the existence of melts and a higher-temperature condition that results from magmatic activity. The northeastern part exhibits low-velocity anomalies in the upper crust and the uppermost mantle, which are probably due to the thick sediment and fluids related to the back-arc volcanism, respectively. The strength of the crust and uppermost mantle along the NKTZ may have been weakened by the concentration of the fluids, which in turn facilitates the large contraction there. The heterogeneous structures revealed in this study suggest that the origin of the high-strain-rate zone varies along the NKTZ.
Key words: Seismic tomography, low-velocity zone, Niigata-Kobe Tectonic Zone, deformation, fluids.