Tetsuro Tsuru

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.

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.

Structural characteristics controlling the seismicity crustal structure of southern Japan Trench fore-arc region, revealed by ocean bottom seismographic data

Abstract The Japan Trench is a plate convergent zone where the Pacific Plate is subducting below the Japanese islands. Many earthquakes occur associated with plate convergence, and the hypocenter distribution is variable along the Japan Trench. In order to investigate the detailed structure in the southern Japan Trench and to understand the variation of seismicity around the Japan Trench, a wide-angle seismic survey was conducted in the southern Japan Trench fore-arc region in 1998. Ocean bottom seismometers (15) were deployed on two seismic lines: one parallel to the trench axis and one perpendicular. Velocity structures along two seismic lines were determined by velocity modeling of travel time ray-tracing method. Results from the experiment show that the island arc Moho is 18–20 km in depth and consists of four layers: Tertiary and Cretaceous sedimentary rocks, island arc upper and lower crust. The uppermost mantle of the island arc (mantle wedge) extends to 110 km landward of the trench axis. The P-wave velocity of the mantle wedge is laterally heterogeneous: 7.4 km/s at the tip of the mantle wedge and 7.9 km/s below the coastline. An interplate layer is constrained in the subducting oceanic crust. The thickness of the interplate layer is about 1 km for a velocity of 4 km/s. Interplate layer at the plate boundary may cause weak interplate coupling and low seismicity near the trench axis. Low P-wave velocity mantle wedge is also consistent with weak interplate coupling. Thick interplate layer and heterogeneous P-wave velocity of mantle wedge may be associated with the variation of seismic activity.

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.

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.

Detailed plate boundary structure off northeast Japan coast

In 1997, a seismic experiment using an airgun array and ocean bottom seismographs (OBSs) was performed in the forearc region of the northeast Japan (NEJ) arc. The objectives of this experiment were to clarify whole of the velocity structure around the forearc region of NEJ arc including a detailed plate boundary structure and the heterogeneous structure. In this paper, we estimated the heterogeneous velocity structure around the forearc region of northeast Japan by applying 2-D ray tracing and travel time inversion to the airgun-OBS data. The depth where the island arc Moho comes into contact with the subducting oceanic crust is about 20 km. We suggest the existence of an oceanic layerl overlying the oceanic crust subducting down to at least near 18 km by comparing observed waveforms with these calculated using the reflectivity method.

Subsurface structure of the Myojin Knoll pumiceous volcano obtained from multichannel seismic reflection data

The Myojin Knoll is a submarine volcano that has a classically beautiful conical-shaped silicic caldera whose surface is covered by pumice. To determine the tectonic structure inside the caldera wall and beneath the caldera floor of this pumicious submarine volcano, we carried out a structural interpretation study using newly collected deep-penetrating multichannel seismic (MCS) reflection data. We also conducted a detailed velocity analysis of the MCS data, which facilitated the interpretation study. The results demonstrate that approximately 90% of the caldera wall is composed of pumiceous volcanic breccia. This finding supports those of previous researchers who, based on seafloor observations, single-channel seismic reflection, and gravity and geomagnetic data, concluded the Myojin Knoll is a knoll having a pumiceous caldera wall underlain by a pre-caldera rhyolitic stratovolcano edifice. We also determined a down-warping reflector approximately 800 m beneath the caldera floor. A seismic unit immediately above the reflector has a higher P-wave velocity than the pumice units and shows a chaotic seismic reflection pattern. We interpreted the reflector to be the bottom of a possible shallow magma chamber where the magma would undergo repeated expansion and contraction as a result of recurrent eruption activities.

Geophysical imaging of subsurface structures in volcanic area by seismic attenuation profiling

Geophysical imaging by using attenuation property of multichannel seismic reflection data was tested to map spatial variation of physical properties of rocks in a volcanic area. The study area is located around Miyakejima volcanic island, where an intensive earthquake swarm was observed associated with 2000 Miyakejima eruption. Seismic reflection survey was conducted five months after the swarm initiation in order to clarify crustal structure around the hypocenters of the swarm activity. However, the resulting seismic reflection profiles were unable to provide significant information of deep structures around the hypocenters. The authors newly applied a seismic attribute method that focused seismic attenuation instead of reflectivity to the volcanic area, and designed this paper to assess the applicability of this method to subsurface structural studies in poorly reflective volcanic areas. Resulting seismic attenuation profiles successfully figured out attenuation structures around the Miyakejima volcanic island. Interestingly, a remarkable high-attenuation zone was detected between Miyakejima and Kozushima islands, being well correlated with the hypocenter distribution of the earthquake swarm in 2000. The high-attenuation zone is interpreted as a fractured area that was developed by magma activity responsible for the earthquake swarms that have been repeatedly occurring there. The present study can be one example showing the applicability of seismic attenuation profiling in a volcanic area.

Faulting and Bending of Oceanic Crust Around Japan Trench

This study was made as one of oceanic crust dynamics researches which Japan Marine Science and Technology Center (JAMSTEC) currently conducts.

Visualization of attenuation structure and faults in incoming oceanic crust of the Nankai Trough using seismic attenuation profiling

Seismic attenuation properties were tested as indicators of lateral variation in geological structures and detection of faults within poorly reflective oceanic crust, on a seismic survey line along the Nankai Trough. We can specify both sedimentary structures by configuration of reflections and faults by offsetting of reflections on seismic reflection profiles. This procedure is often applied to analyze geological structures and existence of faults within sedimentary layers; however, it is almost impossible to analyze them within igneous oceanic crust because seismic reflections are inherently invisible there. Therefore, we applied seismic attenuation profiling to visualize geological structures and faults within poorly reflective oceanic crust. As a result, oceanic crust altered by late-coming volcanisms as well as damaged by intraplate earthquakes was imaged as extremely high-attenuation property, which was clearly distinguished from normal oceanic crust. Many faults were observed in the sedimentary unit on the seismic reflection profile, whereas possible lower segments of the faults were imaged as high-attenuation stripes in the oceanic crust on the seismic attenuation profile. Thus, the effectiveness of seismic attenuation profiling to structural and fault imaging within poorly reflective oceanic crust was successfully demonstrated.