Stress states at the subduction input site, Nankai Subduction Zone, using anelastic strain recovery (ASR) data in the basement basalt and overlying sediments

The three-dimensional orientations of stress and stress magnitudes in the basement basalt and overlying sediments at the subduction input site, IODP Site C0012, have been determined using anelastic strain recovery (ASR) analyses. The ASR results in the sedimentary sequence indicate that σ1 is nearly vertical. The magnitudes of σ2 and σ3 are very similar, indicating that the stress state in the sedimentary sequence is “at rest”. On the other hand, ASR analyses in the basement basalt show that σ1 is nearly horizontal and oriented NE–SW, almost parallel (or slightly oblique) to the trench axis. σ3 plunges moderately to the NW. The stress state of the basement basalts suggests a strike-slip or thrust (reverse fault) regime, which is very different from a “state at rest” condition, which is the theoretical stress condition for the ocean floor far from a subduction zone. The basement basalt at the subduction input site (C0012) has experienced trench-parallel compression and trench-normal extension, consistent with the focal mechanisms of earthquakes in the vicinity. The estimated stress magnitudes show only small variations between the principal stresses, implying that the directions of principal stress could be easily rotated in association with any tectonically induced local stress variation. The stress orientation in the basement basalt seems to be the result of hinge extension during bending of the Philippine Sea Plate, either in association with subduction or with the formation of an anticline during intraoceanic thrusting.

► Stress state in the sedimentary sequence is “at rest”.
► Basement basalt has experienced trench-parallel compression/trench-normal extension.
► The estimated stress magnitudes show small variations between the principal stresses.
► Stress orientation in basement seems to be result of bending of subducting plate.