Abstract
AbstractA seismic tomography study has revealed a detailed three-dimensional seismic velocity structure along a highstrain-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.
Highlights
The GEONET (GPS Earth Observation NETwork) operated by the Geographical Survey Institute of Japan has revealed that a zone of high strain rates, called the NiigataKobe Tectonic Zone (NKTZ), extends from Niigata to Kobe (Fig. 1) (Sagiya et al, 2000)
It is evident that the velocity structures vary considerably along the Niigata-Kobe Tectonic Zone (NKTZ)
Note that we carried out the inversion without crustal discontinuities in order to assess the effect of fixed crustal discontinuities on velocity structures and obtained results that show almost the same features, indicative of the robustness of the obtained velocity structures
Summary
Iio et al (2002, 2004) proposed a model in which a weak zone with low viscosity exists in the lower crust. Using a threedimensional (3-D) viscoelastic finite element model, Hyodo and Hirahara (2003) showed that the high strain rates in the zone can be attributed to the 15-km-thick viscoelastic lower crust having almost the same viscosity as that of the upper mantle.
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