Structural evolution in accretionary prism toe revealed by magnetic fabric analysis from IODP NanTroSEIZE Expedition 316
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Accretionary wedge
Prism
Two 3D seismic reflection surveys across the Nankai Trough have imaged the deep structure of the subduction zone megathrust from the trench into the up dip end of the seismogenic zone where slip occurred during the most recent megathrust earthquakes. In 1999 we acquired a 3D seismic volume across the Muroto transect in the vicinity of the Muroto peninsula, offshore Shikoku Island, Japan. These data imaged 70 km of the subduction thrust from the toe of the accretionary wedge, across the up dip rupture area of the 1946 Nankaido M 8.1 earthquake, to 8 km depth below seafloor. At the down dip edge of the 3D volume we see a 1 km high seamount that has subducted to 7 km below seafloor, producing several very distinctive structures that imply uplift of the overriding accretionary wedge, removal of ∼ 1 km of material from the overriding plate, and a 1 km thick underthrust sediment sequence that appears to have been subducted to 5 km below seafloor in the wake of the subducted seamount.
Seafloor Spreading
Seamount
Accretionary wedge
Trough (economics)
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Accretionary wedge
Thrust fault
Prism
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Abstract Sumatra subduction zone is one of the most seismically active zones on Earth. After having produced three Mw > 8.4 earthquakes and several Mw > 7.5 earthquakes, including the Mw = 7.8 2010 tsunami earthquake, the northern Mentawai segment is still locked and is capable of generating a great earthquake, possibly a disastrous tsunami. We analyzed ultralong offset seismic reflection data from this locked zone to characterize the nature of the accretionary prism and the plate interface using a combination of traveltime tomography, full waveform inversion, and prestack depth migration. In order to enhance the refractions, we downward extrapolate the streamer data to the seafloor, allowing the refraction arrivals to be observed from near‐zero offset up to far offset, and use a traveltime tomography to determine the background velocity in the upper sediments. Starting from these velocities, we perform a multiscale elastic full waveform inversion to determine the detailed P wave velocity structure of the subsurface. Based on this velocity, we perform a prestack depth migration to obtain seismic image in the depth domain and compute the porosity of the sediments to determine fluid content along faults. Our results show a low‐velocity subduction channel with high porosity at the plate interface that connects the likely active frontal thrusts at the toe of accretionary wedge, suggesting that the frontal section of the prism is seismogenic. We have also observed a low‐velocity layer in the middle of wedge separating old sediments below from new sediments above, defining the roots of bivergent thrust faults up to the seafloor, which can be interpreted as a psudo‐décollement.
Accretionary wedge
Seafloor Spreading
Wedge (geometry)
Seismic Tomography
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Abstract The Semidi segment of the Alaska convergent margin appears capable of generating a giant tsunami like the one produced along the nearby Unimak segment in 1946. Reprocessed legacy seismic reflection data and a compilation of multibeam bathymetric surveys reveal structures that could generate such a tsunami. A 200 km long ridge or escarpment with crests >1 km high is the surface expression of an active out‐of‐sequence fault zone, recently referred to as a splay fault. Such faults are potentially tsunamigenic. This type of fault zone separates the relatively rigid rock of the margin framework from the anelastic accreted sediment prism. Seafloor relief of the ridge exceeds that of similar age accretionary prism ridges indicating preferential slip along the splay fault zone. The greater slip may derive from Quaternary subduction of the Patton Murray hot spot ridge that extends 200 km toward the east across the north Pacific. Estimates of tsunami repeat times from paleotsunami studies indicate that the Semidi segment could be near the end of its current inter‐seismic cycle. GPS records from Chirikof Island at the shelf edge indicate 90% locking of plate interface faults. An earthquake in the shallow Semidi subduction zone could generate a tsunami that will inundate the US west coast more than the 1946 and 1964 earthquakes because the Semidi continental slope azimuth directs a tsunami southeastward.
Escarpment
Accretionary wedge
Seafloor Spreading
North American Plate
Fracture zone
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Accretionary wedge
Seafloor Spreading
Thrust fault
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To reveal the origin of a backstop and its influence on the evolution of the accretionary prism, we analyzed reflection seismic data acquired in the Nankai Trough off the Kii Peninsula. The deformation features of the forearc basin sequence show that the landward accretionary prism close to the coast was not deformed after the development of the forearc basin about 2–4 Ma. The surface of the landward prism can be identified as strong amplitude reflector, indicating that the landward prism has higher seismic velocity. Therefore, the landward accretionary prism inferred to be of higher strength constitutes a static backstop. Based on seismic and geologic observations, we interpret that the backstop was generated due to the large age differences of accreted material resulting from an inferred hiatus in subduction between ∼13 and 6 Ma. The time-dependent processes such as the igneous activity in middle Miocene further contribute to the development of the backstop. A ridge structure beneath the forearc basin located trenchward of this backstop and running roughly parallel to it appears to reflect activity on an ancient splay fault. The strike of the ancient splay fault runs parallel to the backstop identified in this study and oblique to the current trench. This geometry suggests that location and mechanical behavior of this splay fault system is influenced by the backstop, and its distribution could be related to the coseismic rupture area.
Accretionary wedge
Forearc
Prism
Trough (economics)
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Accretionary wedge
Décollement
Wedge (geometry)
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Accretionary wedge
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Forearc
Accretionary wedge
Martinique
Slab
Volcanic arc
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