2 0 0 0 OA 付加型沈み込み帯浅部の地質構造:房総半島南部付加体-被覆層システム
- 著者
- 山本 由弦 千代延 俊 神谷 奈々 濱田 洋平 斎藤 実篤
- 出版者
- 一般社団法人 日本地質学会
- 雑誌
- 地質学雑誌 (ISSN:00167630)
- 巻号頁・発行日
- vol.123, no.1, pp.41-55, 2017-01-15 (Released:2017-04-18)
- 参考文献数
- 44
- 被引用文献数
- 2 1
三浦・房総半島では,世界的にも珍しい,ごく若く埋没深度の浅い付加体-被覆層システムが,後生の変成作用を経験しないまま陸上に露出している.本見学旅行では,房総半島に絞って見学する.沈み込み帯のごく浅部(1km程度)から中深部(2-4km)の付加体に発達するin-sequence thrust,付加体を不整合に覆う海溝斜面堆積物,これらを切るout-of-sequence thrust,それに前弧海盆堆積物を見学し,付加型沈み込み帯の基本システムを網羅する.
Pore fluid pressure is important for understanding generation of both megathrust and slow earthquakes at subduction zones. However, its occurrence and quantitative constraints are quite limited. Here, we report the estimate of pore pressure by the analysis of transient upwelling flow from the borehole, that was observed while drilling the underthrust sediments in the Nankai Trough off Cape Muroto during IODP Expedition 370. In order to interpret the observed velocity and duration of the flow, we have solved a radial diffusion equation to estimate pore pressure before penetrating an aquifer. The calculation yields that the pore pressure exceeded ~3 MPa above hydrostatic and the size scale of the aquifer is several hundred meters, in case of an aquifer permeability of 10-13 m2. Our result suggests that the underthrust sequence is currently composed of patchily-distributed high-pressure aquifers.In the neighborhood of the drilling site, very low frequency (VLF) earthquakes have been reported (e.g., Obara and Kato, 2016). Seismic survey has suggested a possible linkage between high pore pressure zone and the distribution and generation of slow earthquakes (e.g., Kodaira et al., 2004). Furthermore, high temperature and pressure friction experiments by Sawai et al. (2016) suggested that a transition from stable to unstable slip behavior appears with increasing pore fluid pressure that is a prerequisite for the generation of slow earthquakes. Our result implies that the slow earthquakes at off Cape Muroto can be attributed with slip behaviors along not only décollement but also the patchily distributed high-pore-pressure aquifers in the underthrust sediments.