COLLISION AND FAN-SHAPED COMPRESSIONAL STRESS PATTERN IN THE IZU BLOCK AT THE NORTHERN EDGE OF THE PHILIPPINE SEA PLATE

The Izu block in central Japan, which is surrounded by the Nankai-Suruga trough and the Sagami trough, is considered to be colliding with the Eurasian (EUR) plate at the northern edge of the Philippine Sea (PHS) plate. The stress state in the Izu block is investigated from focal mechanism data mainly obtained by the seismic network of the National Research Institute for Earth Science and Disaster Prevention. The predominant focal mechanisms in the central to eastern part of the Izu block indicate strike-slip faulting with P axes oriented N-S to NW-SE and T axes oriented E-W to NE-SW, whereas focal mechanisms in the western part are strike-slip and reverse faulting with P axes oriented N-S to NE-SW, indicating that the compressional axes remain horizontal throughout the region. The compressional stress trajectories estimated from the P axes show a fan-shaped pattern, radiating from the northern edge of the Izu block where the collision is considered to be occurring. The fan-shaped pattern of compressional stress trajectories is interpreted to be the result of the collision of the Izu block with EUR plate. Plane stress analysis by the finite element method supports this conclusion. The stable distribution of the T axis directions in the central to eastern part is basically interpreted as resulting from the slab pull force by the subducting PHS plate along the Sagami trough. Furthermore this model can explain the seismic quiescence in the northern part of the Suruga Bay region.