Narumi Takahashi

The 2011 Tohoku-Oki earthquake: displacement reaching the trench axis.

Vertical and horizontal displacement that occurred up to the Japan trench likely contributed to formation of the tsunami. We detected and measured coseismic displacement caused by the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (MW) 9.0] by using multibeam bathymetric surveys. The difference between bathymetric data acquired before and after the earthquake revealed that the displacement extended out to the axis of the Japan Trench, suggesting that the fault rupture reached the trench axis. The sea floor on the outermost landward area moved about 50 meters horizontally east-southeast and ~10 meters upward. The large horizontal displacement lifted the sea floor by up to 16 meters on the landward slope in addition to the vertical displacement.

Continental Crust, Crustal Underplating, and Low-Q Upper Mantle Beneath an Oceanic Island Arc

A detailed structural model of the crust, subducting slab, and underlying upper mantle across the northern Izu-Ogasawara (Bonin) island arc system is derived from a marine seismic reflection and ocean bottom seismographic refraction survey and subsequent forward modeling combined with tomographic inversion. The model indicates that the crust is thickest beneath the presently active rift zone and a granitic crust may have formed in the mid-crust. A highly attenuative mantle (that is, one with low quality Q) seems to be confined mainly beneath the presently active rift zone. In contrast, high P-wave velocity persists in the lower crust between the forearc and eastern margin of the back arc basin, suggesting a large-scale magma input responsible for the arc formation.

Crustal structure and evolution of the Mariana intra-oceanic island arc

A new high-resolution velocity model of the Mariana arc-backarc system obtained from active-source seismic profiling demonstrates velocity variations within the arc middle and lower crusts of intermediate to felsic and mafic compositions. The characteristics of the oceanic-island-arc crust are a middle crust with velocity of ∼6 km/s, laterally heterogeneous lower crust with velocities of ∼7 km/s, and unusually low mantle velocities. Petrologic modeling suggests that the volume of the lower crust, composed of restites and olivine cumulates after the extraction of the middle crust, should be significantly larger than is observed, suggesting that a part of the lower crust, especially the cumulates, is seismically a part of the mantle.

Seismological evidence for variable growth of crust along the Izu intraoceanic arc

[1] The processes that create continental crust in an intraoceanic arc setting are a matter of debate. To address this issue, we conducted an active source wide-angle seismic study to examine along-arc structural variations of the Izu intraoceanic arc. The data used were acquired over a 550-km-long profile along the volcanic front from Sagami Bay to Tori-shima. The obtained structural model showed the existence of felsic to intermediate composition middle crust with a P wave velocity (Vp) of 6.0-6.5 km s -1 in its upper part and 6.5-6.8 km s -1 in its lower part. The thickness of the middle crust varied markedly from 3 to 13 km. The underlying lower crust also consisted of two layers (Vp of 6.8-7.2 km s -1 in the upper part and Vp of 7.2-7.6 km s -1 in the lower part). The upper of these layers was interpreted to consist of plutonic gabbro, and the lower layer was interpreted to be mafic to ultramafic cumulates. Average crustal velocities calculated from our model showed remarkable lateral variation, which correlated well with arc volcanism. Low average crustal seismic velocities (∼6.7 km s -1 ), due to thick middle crust, were obtained beneath basaltic volcanoes (e.g., O-shima, Miyake-jima, Hachijo-jima, Aoga-shima), while higher average velocities (∼7.1 km s -1 ) were obtained beneath rhyolitic volcanoes (e.g., Nii-jima, Kurose, South Hachijo caldera, Myoji knoll, and South Sumisu caldera). We concluded from these observations that continental crust grows predominantly beneath the basaltic volcanoes of the Izu arc and that rhyolitic volcanism may be indicative of a more juvenile stage of crustal evolution, or remelting of preexisting continental crust, or both.

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.

New seismological constraints on growth of continental crust in the Izu-Bonin intra-oceanic arc

The process by which continental crust has formed is not well understood, though such crust mostly forms at convergent plate margins today. It is thus imperative to study modern intra-oceanic arcs, such as those common in the western Pacific Ocean. New seismic studies along the representative Izu-Bonin intra-oceanic arc provide unique along-strike images of arc crust and uppermost mantle to complement earlier, cross-arc lithospheric profiles. These reveal two scales (1000–10 km scale) of variations, one at the scale of the Izu versus Bonin (thick versus thin) arc crust, the other at the intervolcano (∼50 km) scale. These images show that: (1) the bulk composition of the Izu-Bonin arc crust is more mafic than typical continental crust, (2) the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes, (3) the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments, and (4) a process to return lower crustal components to the mantle, such as delamination, is required for an arc crust to evolve into continental crust. Continued thickening of the Izu-Bonin crust, accompanied by delamination of lowermost crust, can yield velocity structure of typical continental 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.

Three-dimensional crustal structure of the Mariana island arc from seismic tomography

[1] A three-dimensional (3-D) seismic refraction survey was acquired over the Mariana volcanic arc at 14.5–18.5N and 145–147E. First-arrival traveltimes from this survey and from a separate 2-D survey acquired approximately perpendicular to the arc have been simultaneously inverted for a 3-D P wave velocity model using seismic tomography subject to smoothness constraints. The active arc, which initiated only 3–4 Ma ago, has an average crustal thickness of 18 km. Approximately 40 km to the east the inactive remnant of the rifted Eocene arc has an average crustal thickness of 21 km, due primarily to a thicker lower-crustal layer with velocities of 6.5–7.0 km s � 1 . Crustal production clearly varies both temporally and spatially, with some crustal layers, including the igneous forearc crust, varying in thickness by a factor of up to 2 along strike. Average P wave velocities within the upper crust of the modern arc are 240–360 m s � 1 lower than in the Eocene arc but are 280 m s � 1 higher within the lower crust. Middle crust with velocities of 6.0–6.5 km s � 1 is best developed beneath the Eocene arc. These results suggest an evolution of arc structure with increasing age: We infer closure of fractures and porosity in the upper crust through hydrothermal circulation and a reduction in the mafic character of the middle to lower crust as a result of intracrustal differentiation. Although tonalitic rocks may predominate in the transition from upper to middle crust, the bulk of the crust is essentially basaltic.

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.