Jin-Oh Park

Tectonic features of the Japan Trench convergent margin off Sanriku, northeastern Japan, revealed by multichannel seismic reflection data

Near the Japan Trench convergent plate margin the seaward edge of the continental plate is deformed by subduction of the oceanic plate. We report the results of a multichannel seismic survey in the northern Japan Trench in which this deformed zone is demarcated from the rigid continental framework by a pronounced landward dipping reflector. The oceanic plate also undergoes deformation as the two plates interact in the subduction processes, resulting in a progressive deformation or destruction of a horst structure along the top of the subducting oceanic crust. This may eventually lead to the formation of a smooth plate boundary at the greater depth. More than 45 km landward from the trench axis, a smooth reflector suggesting a stable slip plane is visible along and above the oceanic crust. Our data indicate that the deformed zone pinches out landward ∼60 km from the axis at 13 km depth and the slip plane becomes less obvious there. Seismicity of interplate earthquakes rapidly increases landward from this location, leading us to speculate that this is where coupling at the plate boundary becomes strong enough for earthquakes to occur. We conclude that the updip limit of the seismogenic zone of interplate earthquakes in the study area is characterized by the tectonic feature of a pinchout of the deformed sediments which overlie the subducting oceanic crust.

Western Nankai Trough seismogenic zone: Results from a wide‐angle ocean bottom seismic survey

The Nankai Trough, southwestern Japan, is recognized as a vigorous seismogenic zone with well-studied historic earthquakes. This paper presents results of a wide-angle ocean bottom seismographs (OBS) study at the western Nankai Trough seismogenic zone. The OBS data used were acquired on a profile (250 km long) across the presumed coseismic slip zone of the 1946 Nankaido earthquake (Ms = 8.2). The main purpose of the seismic study is to obtain an entire crustal cross section of the seismogenic zone for the 1946 earthquake. The crustal model is characterized by a gentle sloping of subducting oceanic crust and thick overlying sedimentary wedge. P wave seismic velocities of the subducting oceanic crust show normal oceanic crustal velocities (Vp = 5.0–5.6 km/s and 6.6–6.8 km/s in oceanic layers 2 and 3, respectively). The maximum thickness of the sedimentary wedge is 9 km at 70 km from the trough axis with Vp = 3.4–4.6 km/s in the deeper part. The subducting oceanic crust traced down to 25 km depth shows that the subduction angle becomes steeper landward: 3.2°and 7.2° at 0–50 km and 50–100 km from the trough axis, respectively. The oceanic crust is smooth to the hypocenter zone, down to 40 km depth beneath Shikoku Island. Our crustal model shows that the downdip limit of the coseismic slip area does not extend to the deep end of the oceanic crust-island arc crust contact zone. Even though there is large uncertainty about the seaward limit of the coseismic slip zone, the crustal model clearly indicates that the updip limit of the coseismic slip zone extends beneath the young accretionary prism.

A subducting seamount beneath the Nankai Accretionary Prism off Shikoku, southwestern Japan

A multi-channel seismic (MCS) reflection survey was conducted to study the structure of the Nankai convergent margin off Shikoku Island in July 1997. Based on reflection characteristics, we could identify three major seismic reflection units, i.e., Units A, B, and C. The MCS data as well as swath-bathymetric data reveal a buried circular seamount subducting beneath the Nankai accretionary prism. The subducting seamount is responsible for deformation of the accretionary wedge, resulting in a compressed uplifted sediment knoll and many steep escarpments with NE-SW strike. A possible thrust fault was identified on the seaward flank of the seamount, indicating compressive deformation. A tectonic model for subduction of the seamount is presented based on the interpretation of the MCS data.

Seismic record of tectonic evolution and backarc rifting in the southern Ryukyu island arc system

Abstract The southern Ryukyu island arc system is located at a convergent plate margin where the Philippine Sea Plate is subducting under the Eurasia Plate. We have conducted multi-channel seismic reflection surveys to study tectonic evolution and backarc rifting of the southern Ryukyu island arc system using the R/V Tansei-maru of the Ocean Research Institute, University of Tokyo, in June 1993 and 1994. We describe systematically a complete cross-section from the East China Sea continental shelf to the Ryukyu trench from the viewpoint of seismic stratigraphy. Seven major seismic units and three stages in the tectonic evolution of the system are identified: (1) during stage 1 from Late Miocene to earliest Pleistocene, pre-rift deposits of the Shimajiri Group accumulated over a wide region from the East China Sea continental shelf to the forearc region; (2) stage 2 is defined by a series of tectonic processes involving crustal doming, erosion, subsidence, and sedimentation, in association with initial rifting of the southern Okinawa Trough during most of Early Pleistocene time; and (3) the backarc rifting is still in progress and syn-rift sedimentation has been under way since the Late Pleistocene (stage 3). A new significant observation lies in the fact that the Pliocene Shimajiri Group is not distributed in most of the present-day southern Okinawa Trough. This implies that the backarc rifting of the southern Okinawa Trough was probably initiated after the deposition of the Shimajiri Group, that is, during the Early Pleistocene. Crustal-scale simple shear allowing an asymmetrical half-graben structure, rather than pure shear, governed the initial rifting of the southern Okinawa Trough. The possibility, however, cannot be excluded that pure shear associated with symmetrical rifting may be predominant in the present-day rifting of the southern Okinawa Trough as a result of northwestward migration of the rifting axis. The average extension rate of the southern Okinawa Trough is estimated to be approximately 1–2 cm/year on the basis of fault geometry observed on the acoustic basement.

A low-velocity zone with weak reflectivity along the Nankai subduction zone

Three-dimensional seismic reflection data reveal the presence of a low seismic velocity zone (LVZ) with weak reflectivity character along the Nankai accretionary prism. This LVZ is intercalated between an upper, offscraped layer and a lower, underthrusting layer in the outer accretionary wedge. Wide-angle ocean bottom seismograph data also support the presence of the LVZ, which is estimated to be a maximum of ∼2 km thick, ∼15 km wide, and ∼120 km long. The LVZ could be an underthrust package underplated in response to the lateral growth of the Nankai accretionary prism. Underplating of the underthrusting layer beneath the overlying offscraped layer would maintain a critical taper of the accretionary prism so that the offscraped layer can continue to grow seaward. The LVZ could have elevated fluid pressure, leading to rigidity reduction of the entire outer accretionary wedge. The rigidity-lowered outer wedge, containing the LVZ, may be more easily uplifted and thus eventually foster tsunami generation during a Nankai megathrust earthquake. If the fluid-rich LVZ supplies a significant amount of the fluid to the megasplay fault zone at depth, it may affect stick-slip behavior of the fault.

Out‐of‐sequence thrust faults developed in the coseismic slip zone of the 1946 Nankai Earthquake (Mw=8.2) off Shikoku, southwest Japan

A multi-channel seismic (MCS) reflection survey was conducted to study the subsurface structure of the coseismic slip zone of the 1946 Nankai earthquake (Mw=8.2) off Shikoku Island in 1997. We could identify a splay fault system consisting of several sigmoid out-of-sequence thrust (OST) faults dipping landward with slope 10∼25° on the poststack depth migrated MCS profiles. Most of the OSTs are apparently developed from the subducting oceanic basement to the seafloor in the forearc region, cutting both underthrust sediments and the overriding accretionary prism. In addition, most of the OSTs are within the coseismic zone of the 1946 Nankai earthquake and the interseismic locked zone. The OSTs are considered to be related to large interplate earthquakes including the 1946 Nankai earthquake. These OSTs may be responsible for tsunami generation following deformation of forearc accretionary wedge.

Deformable backstop as seaward end of coseismic slip in the Nankai Trough seismogenic zone

Abstract Wide-angle seismic surveys performed in the last decade have clarified the 3-D crustal structure along the Nankai Trough. The geometry and velocity structure of the southwestern Japan subduction zone are now well constrained. Comparing these observations with the rupture distribution of historic great thrust earthquakes, it appears that the coseismic rupture occurred along plate boundaries deeper than the wedge/backstop boundary (the boundary between the Neogene–Quaternary accretionary wedge and the crust forming the backstop). From the view of spatial relationship, both rupture distributions of the last two large events and the crust forming the backstop are considerably retreated from the trough axis in the west and east off the Kii Peninsula. In both areas, seamount or ridge subduction is apparent in seismic results, geomorphological data and geomagnetic data. The landward indentation of the deformable backstop, which corresponds to the crustal block of old accreted sediments, may be formed by seamount subduction according to published results of sandbox modeling. In particular, the subducted seamount may be a structural factor affecting the recession of the crustal block forming the backstop.

Massive methane release triggered by seafloor erosion offshore southwestern Japan

Vast amounts of methane hydrate exist beneath continental margins, but whether this methane releases from sediment on a large scale and affects the oceans and atmosphere remains unclear. Analysis of newly acquired three-dimensional seismic images and drilling data from a large gas hydrate province reveal a recently eroded v-shaped depression. The depression sharply cuts through a relic bottom simulating reflection (BSR) and hydrate-laden sediments. The shape of the relic BSR indicates that the seafloor depression was once a large anticline that has recently been eroded and released an estimated 1.51 × 10 11 m 3 of methane. We hypothesize that erosion of the seafloor via bottom-water currents unroofed buoyant hydrate-laden sediments and subhydrate overpressured free gas zones beneath the anticline. Once triggered, gas-driven erosion created a positive feedback mechanism, releasing gas and eroding hydrate-bearing sediment. We suggest that erosive currents in deep-water methane hydrate provinces act as hair triggers, destabilizing kilometer-scale swaths of the seafloor where large concentrations of underlying overpressured methane exist. Our analysis suggests that kilometer-scale degassing events are widespread, and that deep-water hydrate reservoirs can rapidly release methane in massive quantities.

A subducted oceanic ridge influencing the Nankai megathrust earthquake rupture

Megathrust earthquake rupture is highly influenced by seafloor bathymetric features such as oceanic fracture zones, seamounts, and ridges in convergent plate margins where these features subduct. Recent seismic reflection and refraction data reveal a trough-parallel subducted oceanic ridge that is attached to the descending Philippine Sea Plate beneath the accretionary wedge of the overriding Eurasian Plate in the eastern Nankai Trough subduction zone consisting of the Tonankai and Tokai segments. The seismic survey data confirm the existence of subducted paleo-Zenisu ridge that was suggested by just magnetic data, and as a result, expands its distribution to the west. The newly constrained subducted ridge that spans the two segments is estimated to be a maximum of ∼2.5 km high, ∼20–30 km wide, and ∼200 km long. Spatial mapping of the ridge shows that it is located roughly at the seaward edge of the coseismic rupture zone of the 1944 Tonankai earthquake (M=8.1). This ridge appears to be in close contact with the seaward end of the rigid backstop in the Tonankai segment. Both the spatial correlation and the ridge–backstop collision geometry suggest that the subducted ridge might be strongly mechanically coupled and may thus play a significant role as a seaward barrier inhibiting the 1944 earthquake rupture from propagating farther seaward.

Deformation of a seamount subducting beneath an accretionary prism: Constraints from numerical simulation

We examined the process of seamount subduction via a numerical simulation using the finite element method, applying a frictional force on the plate interface that is proportional to the normal stress. We calculate the incremental stress due to infinitesimal deformation of the seamount associated with subduction, and consider the implications for stress buildup and fracturing of the seamount itself. Our results show that the maximum shear stress concentrates at both flanks of the seamount, which suggests that fracturing will start there. We can surmise that, eventually, the seaward flank may be more apt to break than the landward flank at shallow depth if the confining pressure there is sufficiently low. We consider this to be a possible scenario for the generation of a thrust fault imaged at the seaward flank of the Muroto seamount, which is subducting under the Nankai trough accretionary prism.