[S-CG40] Science of slow-to-fast earthquakes

[SCG40-P37] Geological and experimental investigations of slow slip in the Japan Trench subduction zone

*Mizuki Ueda¹, Kohtaro Ujiie¹, Miki Takahashi² (1.University of Tsukuba, 2.Research Institute of Earthquakes and Volcano Geology, AIST)

Keywords:healing properties, shallow slow earthquakes, pelagic clay, chert-clastic complex

In the Japan Trench subduction zone off Miyagi, slow slip updip of the seismogenic zone is thought to occur in the region of silica diagenesis along the subduction plate boundary where temperature is ~100 °C. However, the physical conditions and properties responsible for slow slip remain poorly understood. We investigated the deformation and fluid pressure conditions during silica diagenesis in the Jurassic chert-clastic complex in central Japan, which formed in a cold slab environment, similar to the Japan Trench subduction zone. The results show that bedded chert records bedding parallel slip along pelagic clay layers with cumulative displacements of 2-35 mm, which occurred in association with flexural-slip folding. The quartz- and chalcedony-filled extension veins develop nearly perpendicular to the bedding, often showing vein arrays at high angles to the bedding. Stylolite is commonly recognized parallel to bedding parallel slip zones. The veins and bedding parallel slip zones are mutually crosscut. These features indicate that bedding parallel slip along the pelagic clay layer occurred during opal-CT to quartz transformation under near lithostatic fluid pressure with maximum principal stress perpendicular to the subducting bedded chert, which may correlate with slow slip in the Japan Trench subduction zone. To evaluate whether pelagic clay hosting bedding parallel slip can accommodate slow slip, we tested frictional healing of pelagic clay with holding time ranging from 10-3000 s at temperatures of 25, 50, 100, 200 °C and fluid pressure ratios of 0.5 and 0.9. Since pelagic clay in the Jurassic chert-clastic complex underwent smectite to illite transformation, the smectite-rich (~80 % smectite) materials in the incoming sediments off Sanriku were sampled and set in a triaxial apparatus. The results indicate that pelagic clay exhibits near-zero frictional healing rates under all conditions tested. This behavior may account for the frequent generation of slow slip events with very small stress drops or aseismic creep at shallow portions of the subduction plate boundary. However, the near-zero frictional healing rates at 200 °C appear to be inconsistent with the infrequent generation of regular earthquakes at deeper portions of the subduction zone, where time-dependent frictional strengthening and larger stress drop are expected. Further tests are needed to determine whether near-zero frictional healing rates are true properties of pelagic clay.