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

[SCG40-P37] 日本海溝沈み込み帯浅部スロー地震に関する地質学的研究・実験的研究

*上田 瑞貴¹、氏家 恒太郎¹、高橋 美紀² (1.筑波大学、2.国立研究開発法人産業技術総合研究所　活断層・火山研究部門)

キーワード：断層の強度回復特性、浅部スロー地震、遠洋性粘土、チャート砕屑岩複合体

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.