Heterogeneity And Spatial Correlation Of Clusters Between The Upper And Lower Plane Seismicity Of The Double Seismic Zone Beneath NE Japan Revealed By Double-Difference Relocations

Seismicity within the subducted Pacific plate is very active beneath NE Japan, and this activity is expressed as a double seismic zone (Hasegawa et al., 1978). We relocated interplate and intraslab earthquakes in the entire Tohoku district of NE Japan using the Double-Difference method (Waldhauser and Ellsworth, 2000) where relative earthquake arrival times were determined by waveform cross-spectrum analysis. The result showed 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, 1995; Kirby et al., 1996; Seno and Yamanaka, 1996; Peacock, 2001; 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. This correlated heterogeneity of upper- and lower-plane seismicity may be associated with the subduction of structures produced by intraplate volcanism (Seno and Yamanaka, 1996; Kirby et al., 1996). Our relocations indicate that at depths between 50 and 150km, most of the background upper-plane seismicity occurs in the slab crust. Okada and Hasegawa (2003) and Sakoda et al. (2004) relocated intraslab earthquakes occurring around off the northern part of NE Japan, including the source region of the 2003 off Miyagi earthquake (M7.1), which occurred on May 26, 2003 in the upper plane. Their results show that many earthquakes occurred near the hypocenter of the main shock before the 2003 event. These events are distributed near the Moho and/or within the mantle of the slab. Anomalous 'deep' upper-plane earthquakes taking place within the slab mantle as well as the slab crust such as the source area of the 2003 Miyagi earthquake could also be caused by faulting in the thicker crust of a subducted seamount.