11:00 〜 11:15
[SCG45-20] Correlations between dominant styles of near-trench prism deformation and tsunamigenic earthquakes along the Japan Trench
キーワード：Seafloor mapping, Japan Trench, accretionary prism, deformation, tsunami earthquakes
The seismic hazards of northern Japan received global attention after the 2011 Tohoku-Oki earthquake and tsunami, where a Mw 9.1 megathrust earthquake produced up to 50–60 m of coseismic slip at the trench, and resulting tsunami runups along the Sanriku coast exceeding 20–30 m. The unpredictability of hazardous, large magnitude, near-trench deformation, as observed in the Tohoku-oki earthquake and tsunami, has resulted in increased efforts to characterize near-trench deformation in the accretionary prism that may be capable of hosting shallow slip. However, seafloor structures that may contribute to tsunami excitation are poorly resolved by existing seismic reflection surveys, largely due to the acoustically-chaotic nature of the accretionary prism. In this study, we use high-resolution (80 m) bathymetry of the Japan Trench accretionary prism, acquired by JAMSTEC, to generate a tectonogeomorphic map of upper plate faults that may contribute to seismic and tsunamigenic hazards in northern Japan. We remove long wavelength topography from the bathymetric grids using high-pass and band-pass filtered DEMs, and identify the slope, curvature, relief, and the geometry of ridgelines and trough lines associated with major faults and folds. We use these DEM derivatives to identify the dominant patterns and magnitude of near-trench deformation, and interpret variations in SHmax orientation at the seafloor.
From our tectono-geomorphic seafloor mapping of the Japan Trench accretionary prism, we identify three primary styles of upper plate deformation. We characterize the first deformation style where a wide (60–80 km) frontal prism contains alternating zones of compression (e.g., SHmax oriented E-W) and extension (e.g., SHmax oriented N-S) over length scales of 5–10 km. In these zones, frequent tectonic overprinting occurs along a convergence-parallel trend, suggesting a strongly heterogeneous distribution of stress in the upper plate near the trench. This deformation style is strongly spatially correlated to the slip contours of the Tohoku-oki megathrust earthquake, at latitudes between 37°–39° N. A second deformation style occurs along the northern Japan Trench accretionary prism (between 38°30’–40°15’ N), where the width of the prism narrows to 30–50 km, where distinct, localized zones of frontal compression and upper slope extension are separated by a semi-continuous, N-S trending, trench-parallel axis of slope failure. We suggest that this pattern of deformation may be associated with an active out-of-sequence splay fault. Furthermore, this style of deformation is strongly spatially correlated to the estimated slip contours for the 1896 Sanriku-oki tsunamigenic earthquake. Finally, we identify a third style of upper plate deformation along the northernmost (north of 40°15’N) and southernmost (south of 37°N) extents of the Japan Trench accretionary prism, where a narrow (4–14 km-wide) frontal zone of reverse faulting is subsequently overprinted by a 50–100 km-wide zone of abundant normal faulting and slope failure. Here, the SHmax orientations appear to form a curvilinear arc around the trench axis, suggesting that this style of deformation may be related to subducting structures (e.g., seamounts) on the incoming plate. Notably, the slip patches estimated for the Tohoku-oki and Sanriku-oki tsunamigenic earthquakes terminate at latitudes where the upper plate is characterized by this third deformation style. Overall, we show how tectono-geomorphic mapping can constrain the dominant style of permanent deformation in the frontal prism, and how these deformation patterns may relate to slip along the plate boundary fault. We emphasize that the upper plate deformation recorded in the accretionary prism may inform the hazard characterization of shallow tsunamigenic slip along the Japan Trench, and in other analogous subduction systems.
From our tectono-geomorphic seafloor mapping of the Japan Trench accretionary prism, we identify three primary styles of upper plate deformation. We characterize the first deformation style where a wide (60–80 km) frontal prism contains alternating zones of compression (e.g., SHmax oriented E-W) and extension (e.g., SHmax oriented N-S) over length scales of 5–10 km. In these zones, frequent tectonic overprinting occurs along a convergence-parallel trend, suggesting a strongly heterogeneous distribution of stress in the upper plate near the trench. This deformation style is strongly spatially correlated to the slip contours of the Tohoku-oki megathrust earthquake, at latitudes between 37°–39° N. A second deformation style occurs along the northern Japan Trench accretionary prism (between 38°30’–40°15’ N), where the width of the prism narrows to 30–50 km, where distinct, localized zones of frontal compression and upper slope extension are separated by a semi-continuous, N-S trending, trench-parallel axis of slope failure. We suggest that this pattern of deformation may be associated with an active out-of-sequence splay fault. Furthermore, this style of deformation is strongly spatially correlated to the estimated slip contours for the 1896 Sanriku-oki tsunamigenic earthquake. Finally, we identify a third style of upper plate deformation along the northernmost (north of 40°15’N) and southernmost (south of 37°N) extents of the Japan Trench accretionary prism, where a narrow (4–14 km-wide) frontal zone of reverse faulting is subsequently overprinted by a 50–100 km-wide zone of abundant normal faulting and slope failure. Here, the SHmax orientations appear to form a curvilinear arc around the trench axis, suggesting that this style of deformation may be related to subducting structures (e.g., seamounts) on the incoming plate. Notably, the slip patches estimated for the Tohoku-oki and Sanriku-oki tsunamigenic earthquakes terminate at latitudes where the upper plate is characterized by this third deformation style. Overall, we show how tectono-geomorphic mapping can constrain the dominant style of permanent deformation in the frontal prism, and how these deformation patterns may relate to slip along the plate boundary fault. We emphasize that the upper plate deformation recorded in the accretionary prism may inform the hazard characterization of shallow tsunamigenic slip along the Japan Trench, and in other analogous subduction systems.