Azusa Nishizawa

Crustal structure and seismicity beneath the forearc off northeastern Japan

Marine seismological and other data in the Japan Trench area (north of 38.5°N) were used to infer the relation of subducting plate structure, seismicity, and focal mechanisms especially at the aseismic to seismogenic zone along the interface between the two interacting plates. Previous crustal models from ocean bottom seismographic refraction (OBS) surveys were improved by taking into account the relative seismic amplitude characteristics and constraining the shallow structure using multichannel seismic data. A wave speed discontinuity in these models is interpreted to be the contact zone of the crust of the overriding plate and the subducting Pacific plate crust. Its dip angle increases to about 7°, 110 km landward of the trench axis. A large increase beneath the deep sea terrace is required to reach the well-defined angle of 25° beneath the Tohoku east coast. A large wave speed gradient within layer 2, commonly observed under normal oceans, seems to vanish beneath the inner trench slope at about 10-km depth. Within the overriding plate, apparently brittle material with a P wave speed of ∼6 km/s can be found as near as 35 km from the trench axis. The upper limit of the seismogenic zone of interplate low-angle thrust events is about 15-km depth from OBS seismicity and large-event analyses. Both the strength of the crust of the overriding plate and the characteristics of subducting sediments must be investigated to define the seismic coupling of interacting plates.

A detailed subduction structure in the Kuril trench deduced from ocean bottom seismographic refraction studies

Abstract In 1983, we conducted an ocean bottom seismograph (OBS) experiment in the southernmost part of the Kuril trench, beneath which the Pacific Plate is subducting under Hokkaido Island, Japan. The aim of this experiment was to determine crust and upper mantle structure from the oceanic basin to the continental slope by dense seismic refraction profiling, using explosives and airguns. The total length of the profiles was 560 km, along which ten OBSs were deployed. The observed data were of good quality, which enabled us to obtain a detailed velocity structure of the active margin down to 20–30 km. We constructed a velocity structure model by ray-tracing and amplitude modeling. In the oceanic basin, the crust has a typical oceanic structure characterized by three layers with P-wave velocities of 1.8, 3.8–6.5 and 6.5–7.0 km/s. The velocity gradient in layer 3 increases downward from 0.075 to 0.10 s −1 . The Moho discontinuity was well constrained by clearly observed P n and P m P phases. The total thickness of the oceanic crust was determined to be 8 km and almost constant in the oceanic basin. The P-wave velocity is 7.9 km/s beneath the Moho discontinuity, which increases downward with a rather small velocity gradient, 0.015–0.03 s −1 . Beneath the continental slope, we found relatively low velocity (2.5 to 5.5–5.8 km/s) material. The oceanic Moho discontinuity associated with the subducting plate was traced down to a depth of 25 km. Our seismic data strongly suggest that oceanic layer 2 is smoothly subducting and does not break up to form a wedge structure. This result is in remarkable contrast with the velocity structure in the active margin of the Ryukyu trench area (about 1000 km south of the present experimental area), where we found a 12 km-thick, prominent low-velocity wedge is situated 50–150 km landward from the trench axis. On the seaward side of the wedge, the surface of the igneous basement undulates severely. The wedge was probably formed by materials of oceanic origin. Such a difference in velocity structure suggests that the subduction mechanism in the trench area differs from region to region in the northwestern Pacific.

Variation in crustal structure along the Kyushu-Palau Ridge at 15–21°N on the Philippine Sea plate based on seismic refraction profiles

We acquired coincident wide-angle and multi-channel seismic reflection data along four profiles perpendicular to the Kyushu-Palau Ridge (KPR) between 15°N and 20°N on the Philippine Sea plate. The crustal thickness beneath the KPR, which is a remnant arc created in the Late Eocene, varies along the strike from 8 to approximately 20 km and is always thicker than the adjacent oceanic crust of the West Philippine Basin to the west and the Parece Vela Basin to the east. The thickest crust among the four profiles, which is primarily due to a thickening of the lower crust, is found where the KPR adjoins Oki-no-Tori-Shima Island. There is no clear evidence of the thick (>5 km) middle crustal layer with a P-wave velocity of 6.0–6.5 km/s that has been inferred beneath the conjugate rifted counterpart of the Izu-Ogasawara(Bonin)-Mariana Island-arc. Our results suggest that the crust of the KPR at 15–21°N represents a less mature island arc crust relative to that further north along the ridge where a mid-crustal layer of 6 km/s has been reported.

Spatial distribution of earthquakes associated with the Pacific plate subduction off northeastern Japan revealed by ocean bottom and land observation

Abstract Precise earthquake locations in the vicinity of a trench axis elucidate the characteristics of the shallower part of a subducting plate. We split the Japan Trench into two regions, off-Sanriku in the north and off-Fukushima in the south, on the basis of the seismic properties of the regions and the fine microearthquake structure investigated by ocean bottom seismographic surveys, as well as recent observation of major seismic activity. From ocean bottom seismograph data, the hypocentral distributions along a profile perpendicular to the trench axis, indicate that almost all earthquakes occur along the boundary separating the subducting Pacific plate and the landward plate. A difference of the hypocentral distribution between off-Fukushima and off-Sanriku is found at the shallow plate boundary level. The seismic thrust zone begins at more than 100 km landwards from the trench axis off-Fukushima, whereas it begins at 50 km off-Sanriku and is nearly flat between 50 and 100 km from the trench axis. This reflects regional variations of the interplate coupling. In spite of these variations, a common characteristic in recent observations indicates that in both regions almost all the hypocenters are deeper than 10 km. This suggests the plate boundary to be decoupled at least in the part shallower than 10 km.

Crustal structure of the ocean-island arc transition at the mid Izu-Ogasawara (Bonin) arc margin

Wide-angle refraction experiments were conducted to reveal the crustal structure at the transition between the intra-oceanic island arc crust of the mid Izu-Ogasawara (Bonin) arc and the backarc oceanic crust of the Shikoku Basin. The island arc crust consists of an upper crust about 5 km thick with a P-wave velocity <6.0 km/s, a middle crust 5 km thick with a P-wave velocity of 6.0–6.3 km/s, and a lower crust 10 km thick with a P-wave velocity of 6.8–7.2 km/s. The total crustal thickness is about 20 km. The thickness thins to approximately 6 km over a distance of 30 km at the western margin of the Izu-Ogasawara arc (IOA). These features are very similar to those of the northern IOA, which indicates that the crustal structure is relatively constant within 200 km at the northern and mid IOA. The Kinan Escarpment, a 500-km-long fault with a maximum offset of 800 m, characterizes the transition zone between the IOA and Shikoku Basin. The seismic crustal model indicates that the escarpment is a fault which tears the whole oceanic crust along the western margin of the IOA. However, no significant differences exist in the crustal structure on either side of the escarpment, and the Kinan Escarpment seems to be a zone of the structural weakness from its birth.

Deep crustal structure off Akita, eastern margin of the Japan Sea, deduced from ocean bottom seismographic measurements

Abstract A seismic exploration was carried out to detect the heterogeneity of the seismic wavespeed structure associated with the plate convergence at the eastern margin of the Japan Sea. Two airgun–OBS (ocean bottom seismograph) profiles were shot off Akita, Japan, where a seismic gap seems to exist but the location of the plate boundary has not been confirmed. One of the profiles was 60 km long, trending NNE–SSW, named Line OBS-9, at the northeastern end of the Yamato Basin and the other was 170 km long, trending WNW–ESE, Line NT95-1, parallel to the direction of the supposed plate convergence. The crustal structure beneath Line OBS-9 consists of six layers. The uppermost layer is sediment. Three layers are identified beneath the top sedimentary layer and their P wavespeeds are estimated to be 3.3–3.4 km/s, 5.1–5.4 km/s and 5.8–6.3 km/s, which corresponds to the upper crust. Underneath these layers, a layer with P wavespeed ranges of 6.3–7.2 km/s comprises the middle and lower crust. The depth of Moho is inferred to be 19 km. These wavespeed values are comparable with those of the present Japanese island arc, while the thickness of the crust is one-half of that of the Japanese arc. The crustal model supports the scenario that the Yamato Basin is formed by extension of the island arc. The crustal model for Line NT95-1 shows a transition from the extended island arc structure beneath the Yamato Basin to a thicker crust similar to the Japanese arc. P wavespeed heterogeneity related to the plate boundary is not detected. However, a significant change in the structural model along the profile is found around the region where the largest change in the seafloor topography exists. In that region, the wavespeeds in the middle crust have lower values than those of the neighboring area and the Moho begins to deepen towards the Japanese island arc. From comparison with the relationship between P wavespeed structure and aftershock distribution of the 1993 Hokkaido-Nansei-Oki earthquake (southwest of Hokkaido, M JMA 7.8), the region may correspond to a possible fault position of the pending earthquake.

Seismic structure of the subducting seamounts on the trench axis: Erimo Seamount and Daiichi-Kashima Seamount, northern and southern ends of the Japan Trench

We present detailed P-wave velocity models of subducting seamounts from two wide-angle seismic experiments across the Erimo Seamount and Daiichi-Kashima Seamount, northern and southern ends of the Japan Trench. Common characteristics of the velocity models of the seamounts are that the maximum crustal thicknesses of the seamounts are 12–17 km thicker than a typical oceanic crust and that Pn velocities beneath the seamounts are approximately 7.7 km/s, i.e., slower then those of the neighboring area. These features are very similar to the crustal models for the seamounts produced by the Cretaceous off-ridge volcanism on the Pacific Basin.

Wide-angle refraction experiments in the Daito Ridges region at the northwestern end of the Philippine Sea plate

Three large bathymetric highs (from north to south: the Amami Plateau, the Daito Ridge, and the Oki-Daito Ridge) originating from paleo-island arcs characterize the northwestern end of the Philippine Sea plate. We obtained 10 seismic refraction and multi-channel seismic reflection profiles across and along these bathymetric highs and obtained P wave velocity (Vp) models of the crust and the uppermost mantle. Although there are large variations in the crustal structure throughout this region, these bathymetric highs usually have a middle crust with Vp of 6.3 to 6.8 km/s, a lower crust with Vp of 6.8 to 7.2 km/s, a Pn velocity of 7.6 to 7.8 km/s, and a total crustal thickness of 15 to 25 km. These features are similar to those of the Izu-Ogasawara (Bonin)-Mariana island arc and the Kyushu-Palau Ridge, which are immature paleo-island arcs. However, the crust at the southwestern part of the Oki-Daito Ridge contains a relatively thin middle crust and a smaller total crustal thickness compared with other ridges in this region. In addition, we identified a deep reflector beneath the ridge, with these properties indicating a different origin, such as intraplate volcanism.

Structural evolution of preexisting oceanic crust through intraplate igneous activities in the Marcus‐Wake seamount chain

Multichannel seismic reflection studies and seismic refraction surveys with ocean bottom seismographs in the Marcus-Wake seamount chain in the northwestern Pacific Ocean reveal P wave velocity structures of hot spot-origin seamounts and adjacent oceanic crust. Inside the seamounts are central high-velocity (>6.5 km/s) structures extending nearly to the top that may indicate intrusive cores. Thick sediment layers (up to 4 km) with P wave velocities of 4–5 km/s have accumulated on seafloor that predates seamount formation. Downward crustal thickening of up to 2 km was documented beneath a large seamount cluster, but thickening was not confirmed below a small seamount cluster. Volume ratios of an intrusive core to a seamount body are 15–20%, indicating that most of the supplied magma was consumed in forming the thick sedimentary and volcaniclastic layer constituting the seamount flanks. Underplating and downward crustal thickening may tend to occur when second or later intrusive cores are formed in a seamount. P wave velocities in the lowest crust and in the uppermost mantle below the seamount chain are 0.1–0.2 km/s higher and 0.3–0.5 km/s lower, respectively, than velocities below oceanic crust. We explain this difference as a result of sill-like intrusion of magma into the lower crust and uppermost mantle. Reflected waves observed at offsets >200 km are from mantle reflectors at depths of 30–45 km and 55–70 km. The shallower reflectors may indicate structures formed by intraplate igneous activities, and the deeper reflectors may correspond to the lithosphere-asthenosphere boundary.

A new method for determining OBS positions for crustal structure studies, using airgun shots and precise bathymetric data

Abstract Ocean-bottom seismometer (OBS) positions are one of the key parameters in an OBS-airgun seismic survey for crustal structure study. To improve the quality of these parameters, we have developed a new method of determining OBS positions, using airgun shot data and bathymetric data in addition to available distance measurements by acoustic transponders. The traveltimes of direct water waves emitted by airgun shots and recorded by OBSs are used as important information for determining OBS locations, in cases where there are few acoustic transponder data (<3 sites). The new method consists of two steps. A global search is performed as the first step, to find nodes of the bathymetric grid that are the closest to explaining the observed direct water-wave traveltimes from airgun shots, and acoustic ranging using a transponder system. The use of precise 2D bathymetric data is most important if the bottom topography near the OBS is extremely rough. The locations of the nodes obtained by the first step are used as initial values for the second step, to avoid falling into local convergence minima. In the second step, a non-linear inverse method is executed. If the OBS internal clock shows large drift, a secondary correction for the OBS internal clock is obtained, as well as the OBS location, as final results by this method. We discuss the error and the influence of each measurement used in the determination of OBS location.