Abstract
AbstractTravel-time tomography reveals the presence of highly heterogeneous seismic velocity structures around the source areas of the 2004 Chuetsu and 2007 Chuetsu-oki earthquakes. A prominent low-velocity zone present below the Moho under the source areas of the two events is probably formed by fluids being conveyed through a small-scale upwelling in the mantle wedge. A low-velocity zone distributed in the lower crust below each source area is attributable to fluids supplied from the uppermost mantle to the source area. Since this region with these two source areas is exceptional in that the low-velocity zone is distributed in the uppermost mantle but no Quaternary volcanoes are formed at the surface, a relatively large amount of fluids may be stored below the source areas. These localized fluids may have reduced the strength in the lower crust as well as in the fault zones in the upper crust and, consequently, have promoted a brittle failure in the seismogenic layer. Relocated hypocenters of aftershocks of the two events become several kilometers shallower than the original locations determined by the Japan Meteorological Agency, suggesting the importance of using the three-dimensional velocity model in locating the hypocenter.
Highlights
The Chuetsu-oki earthquake (M 6.8) occurred in the Chuetsu region of Niigata Prefecture, Japan, on July 16, 2007 and caused serious damage to the surrounding area
Lowvelocity zones at a depth of 40 km are generally correlated spatially with the distribution of Quaternary volcanoes, but the source areas of the two events are exceptional regions in that a prominent S-wave low-velocity zone exists in the uppermost mantle but no Quaternary volcanoes are distributed at the surface
For the first time, the existence of a low-velocity zone even under the source area of the 2007 Chuetsu-oki earthquake
Summary
The Chuetsu-oki earthquake (M 6.8) occurred in the Chuetsu region of Niigata Prefecture, Japan, on July 16, 2007 (black star in Fig. 1) and caused serious damage to the surrounding area. Wang and Zhao (2006) found a zone with a pronounced low velocity and high Poisson’s ratio in the lower crust of the source area of the 2004 event. Nakajima and Hasegawa (2007a) revealed the existence of low-velocity zones in the lower crust and uppermost mantle along the NKTZ, but any discussion of velocity structures around the source areas of the two events was difficult because the region is located at the edge of their study area.
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