[J08-2] Imaging and interpreting lithospheric structures using seismic and geodetic approaches II

[J08-2-04] Seismic structure around the slow slip source in the northeastern Japan forearc by an airgun-ocean bottom seismometer survey

^○Ryosuke Azuma¹, Ryota Hino¹, Kimihiro Mochizuki², Yoshio Murai³, Hiroshi Yakiwara⁴, Toshinori Sato⁵, Masanao Shinohara³ (1.RCPEVE, Tohoku University, 2.ERI, University of Tokyo, 3.ISV, Hokkaido University, 4.Kagoshima University, 5.Chiba University)

Various kinds of slips have been observed along the plate boundary (PB) in the Japan Trench subduction zone; slow slip events (SSEs) happened in 2008 and 2011 at 37.7–38.7N in the source area of the 2011 Tohoku-oki earthquake. Faults hosting SSEs are often characterized by low seismic velocity anomalies. Although it is expected that the distribution of the interplate low velocity material can be identified by strong seismic reflectivity of the interface, a detail structure in the SSE source area remains unknown. To clarify seismic structure near the PB source, which will improve understanding of the mechanism of SSE generation, we made an airgun-OBS (Ocean Bottom Seismometer) survey in 2014. Survey lines covered the SSE source area in their southern sections.
The P-wave velocity (Vp) model estimated by a first arrival traveltime tomography (Fujie et al., 2013) showed distinct lateral Vp variation in the overriding plate; the high Vp (> ~ 4 km/s) island arc upper crust (IUC) material was estimated to the south of 39 N, whereas the low Vp (< ~ 3 km/s) unconsolidated sediment (US) distributed in the northern part of the survey line. A reflection traveltime mapping method (Fujie et al., 2006) imaged the PB as a continuous reflector at depths from 8 km to 14 km, where Vp is ~ 5 km/s. The PB is more reflective beneath the IUC than that beneath the US. Since the Vp contrast between the IUC and the oceanic layer 2 may be small, the clear reflections from the PB suggest the existence of a low Vp channel layer along the PB. The spatial extent of high-Vp overriding crust and the highly reflective PB, identified to the south of 39N, almost coincides with the SSE source location. Therefore, our survey results suggest that the SSE source area is characterized by the presence of low-Vp channel layer along the PB beneath the high Vp overriding crust.