2006 Volume 59 Issue 1 Pages 1-18
We relocated interplate and intraslab earthquakes beneath the Tohoku district of NE Japan using the Double-Difference location method [Waldhauser and Ellsworth (2000)]. Relative earthquake arrival times were determined by waveform cross-spectrum analysis and catalog-picking data. Our relocations show that between 50 and 150 km in depth, most of the upper-plane seismicity occurs in the slab crust, and most of repeating earthquakes seem to occur along the upper boundary of the Pacific plate, where the intraslab seismicity is active. Obtained result shows that both the lower-plane and upper-plane seismicity are distributed unevenly in space. Moreover, there is a spatial correlation between clusters of upper-and lower-plane seismicity. If intermediate-depth earthquakes are caused by dehydration and/or CO2-bearing devolatilization of hydrated minerals [Kirby et al. (1996), Kirby et al. (2004), Peacock (2001), Seno and Yamanaka (1996), Yamasaki and Seno (2003)], the present result suggests that hydrated and/or carbonate minerals are distributed unevenly but in common in both the crust and deeper mantle of the slab. In the southern part of Tohoku district, where some seamounts subduct, seismicity between upper and lower seismic planes is active. If the hydrated and carbonate minerals are abundantly distributed in the slab crust and the slab mantle due to the volcanic activities of the seamount formation, dehydration and devolatization of these minerals may cause intraslab earthquakes.
