10:45 〜 12:15
[SSS08-P05] Seismic structural characteristics illuminating a large shallow slip zone in the 17th-century Kuril earthquake
キーワード：地殻構造、千島海溝沈み込み帯、プレート境界浅部大すべり
In the southern Kuril subduction zone, A great earthquake is predicted with a high probability in the next 30 years. A tsunami deposit observation suggests that the previous earthquake in the 17th century caused a large slip in the shallow plate boundary fault and induced a tsunami like the 2011 Tohoku earthquake. Specifying a precise area causing a large shallow slip is essential for tsunami mitigation. This study investigates structural characteristics illuminating the large shallow slip zone by quantitative Vp estimation using seismic survey data.
We model a seismic structure from airgun-dense OBS and MCS data, which profile crosses the trench axis, in the Nemuro-oki region. A previous seismic survey neighbor to our profile is less resolutive at the shallowest portion of the plate boundary technically restricted by high water pressure at the trench axis for normal OBS. The new survey data has overcome this problem by installing higher-pressure-protected OBSs and thus can model the frontal part of the forearc with high resolution.
For estimating a 2-D Vp model, we handle a tomographic inversion by a Monte Carlo (MC) approach, using traveltime data of first arrivals and Moho reflections recorded on OBSs and MCS. We conducted inversion to 500 of starting model considering the dependency of results on the starting model and took an average of 500 results as the final model. As a result, we obtained the Vp distribution of the frontal 50-km part of the forearc with a good model uncertainty of < 0.2 km/s, whereas characteristics of Vp beneath the forearc upper slope basin and the subducting Pacific Plate including the seamount agree with the previous studies.
The new model definitively shows a moderate Vp zone of 4–5.5 km/s at 30–60 km from the trench, which distributes between the high Vp (> 5.5 km/s) forearc crust and the low Vp (< 3 km/s) frontal wedge. The moderate Vp zone is characterized by a discontinuous reflective zone (RZ). Whereas the RZ is a familiar feature in tectonic erosional margins like the northern Japan Trench, our survey is the first study to quantitatively estimate the Vp of the RZ. Since the reflectors in the RZ distribute near-parallel to the plate boundary, the nature of the moderate Vp zone may be an imprint of the old consolidated accretionary sediment, otherwise the fractured island arc crust. In either case, revealed seismological characteristics suggest that composed rocks of the frontal 60 km part of the forearc would be weaker strength than the island arc crust.
We consider the moderate Vp zone with RZ as a key characteristic to discuss the cause of a large slip at the shallow plate boundary. Ioki and Tanioka argue that a slip of 25 m at the shallow part of the plate boundary is necessary to explain tsunami deposits relate to the 17th earthquake, and the amount of shallow slip is larger than the deep part (5 m). Such contrast in slip amount along the dip is commonly indicated for the 2011 Tohoku earthquake; slip amount increases toward the trench and keeps large over the most near-trench 40 km of the plate boundary (Sun et al., 2017; Kubota. Et al., 2022). Hence, seismic reflection images at the shallow part of the 2011 rupture area display the RZ with Vp ~ 5 km/s existing 30–80 km from the trench axis (Kodaira et al., 2017), and the relative position of the RZ against the trench axis is almost common in the Nemuro-oki. Finally, considering the spatial consistency between the RZ and the large shallow slip of the 2011 earthquake, we suggest that the RZ would relate to the cause of giant tsunamis. The obtained similarity of the near-trench forearc structure explains the potential to cause a large shallow slip by giant earthquakes in the Nemuro-oki.
We model a seismic structure from airgun-dense OBS and MCS data, which profile crosses the trench axis, in the Nemuro-oki region. A previous seismic survey neighbor to our profile is less resolutive at the shallowest portion of the plate boundary technically restricted by high water pressure at the trench axis for normal OBS. The new survey data has overcome this problem by installing higher-pressure-protected OBSs and thus can model the frontal part of the forearc with high resolution.
For estimating a 2-D Vp model, we handle a tomographic inversion by a Monte Carlo (MC) approach, using traveltime data of first arrivals and Moho reflections recorded on OBSs and MCS. We conducted inversion to 500 of starting model considering the dependency of results on the starting model and took an average of 500 results as the final model. As a result, we obtained the Vp distribution of the frontal 50-km part of the forearc with a good model uncertainty of < 0.2 km/s, whereas characteristics of Vp beneath the forearc upper slope basin and the subducting Pacific Plate including the seamount agree with the previous studies.
The new model definitively shows a moderate Vp zone of 4–5.5 km/s at 30–60 km from the trench, which distributes between the high Vp (> 5.5 km/s) forearc crust and the low Vp (< 3 km/s) frontal wedge. The moderate Vp zone is characterized by a discontinuous reflective zone (RZ). Whereas the RZ is a familiar feature in tectonic erosional margins like the northern Japan Trench, our survey is the first study to quantitatively estimate the Vp of the RZ. Since the reflectors in the RZ distribute near-parallel to the plate boundary, the nature of the moderate Vp zone may be an imprint of the old consolidated accretionary sediment, otherwise the fractured island arc crust. In either case, revealed seismological characteristics suggest that composed rocks of the frontal 60 km part of the forearc would be weaker strength than the island arc crust.
We consider the moderate Vp zone with RZ as a key characteristic to discuss the cause of a large slip at the shallow plate boundary. Ioki and Tanioka argue that a slip of 25 m at the shallow part of the plate boundary is necessary to explain tsunami deposits relate to the 17th earthquake, and the amount of shallow slip is larger than the deep part (5 m). Such contrast in slip amount along the dip is commonly indicated for the 2011 Tohoku earthquake; slip amount increases toward the trench and keeps large over the most near-trench 40 km of the plate boundary (Sun et al., 2017; Kubota. Et al., 2022). Hence, seismic reflection images at the shallow part of the 2011 rupture area display the RZ with Vp ~ 5 km/s existing 30–80 km from the trench axis (Kodaira et al., 2017), and the relative position of the RZ against the trench axis is almost common in the Nemuro-oki. Finally, considering the spatial consistency between the RZ and the large shallow slip of the 2011 earthquake, we suggest that the RZ would relate to the cause of giant tsunamis. The obtained similarity of the near-trench forearc structure explains the potential to cause a large shallow slip by giant earthquakes in the Nemuro-oki.