Takeshi Sato

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.

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.

Detailed structural image around splay‐fault branching in the Nankai subduction seismogenic zone: Results from a high‐density ocean bottom seismic survey

[1]xa0To investigate megathrust earthquake and tsunami generation in a subduction seismogenic zone, it is important to know the detailed structure around the plate boundary and active splay-fault system. The Nankai Trough, southwestern Japan, is among the best studied subduction zones with splay faults in the world. This paper presents a detailed structural image around the splay fault in the coseismic rupture zone of the 1944 Tonankai earthquake in the central Nankai Trough, based on results from a wide-angle, high-density ocean bottom seismograph (OBS) survey. Our seismic image clearly shows for the first time the subduction structure along with the splay-fault reflections imaged on a previously obtained seismic reflection profile. The correspondence between the splay-fault reflections imaged on the seismic reflection profile and our reflectivity image was confirmed by converting the time-migrated section to a depth section by using a velocity model estimated from the wide-angle OBS data in this study. The high-resolution seismic image obtained by the high-density OBS survey demonstrates a zone of low-velocity gradient just above and along the splay fault. We focused specifically on the detailed structure around the splay fault. A simple amplitude analysis of the wide-angle reflections showed that it is likely that a thin low-velocity layer with velocities 0.5–1.5 km/s slower than those in the surrounding rock exists above the splay fault. The presence of this layer suggests elevated fluid pressure in the fault zone, which cut through the relatively homogeneous rock of the accretionary prism and generated past megathrust earthquakes.

Systematic changes in the incoming plate structure at the Kuril trench

[1]xa0Recent seismic structural studies in trench-outer rise regions have shown that Vp within the incoming oceanic plate systematically decreases toward the trench, probably owing to bending and fracturing of the plate. To understand the mechanisms acting to reduce Vp, Vs is critical because the Vp/Vs ratio is a sensitive indicator of lithology, porosity, and the presence of fluid. In the outer rise region of the Kuril trench, we conducted an extensive seismic refraction and reflection survey that revealed systematic changes in Vp, Vs, and Vp/Vs. Our results suggest that water content within the incoming oceanic plate increases toward the trench accompanied by the development of bending-related fractures at the top of the oceanic crust, consistent with the seawater percolation. Our results support the idea that plate bending and fracturing during the bending in the outer rise of the trench play an important role in the water cycle of subduction zones.

Crustal structure of the continental margin of Korea in the East Sea (Japan Sea) from deep seismic sounding data : evidence for rifting affected by the hotter than normal mantle

Abstract Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.

Structural variations of arc crusts and rifted margins in the southern Izu‐Ogasawara arc–back arc system

[1]xa0We carried out a reflection/refraction seismic survey across the southern Izu-Ogasawara arc–back arc system, covering three arcs with different crustal ages. The oldest Eocene arc has middle and lower crust with high velocities of 6.4–6.6 and 6.8–7.4 km/s, respectively, suggesting denser crustal materials. The current volcanic arc has middle and lower crust with lower velocities of 5.7–6.5 and 6.7–7.1 km/s, suggesting advanced crustal differentiation. The crust-mantle transition layer, with a velocity of 7.5–8.0 km/s, is distributed beneath the current volcanic arc and the rear arc, suggesting a pool of dense materials emanating from the crust through the crustal growth. These structural differences between the Eocene arc and current arc indicate a difference of crustal growth based on basaltic and andesitic magmas according to known petrologic studies. Commonly, rifted crusts have lower crusts with high velocities of over 7.0 km/s, and the arc–back arc transition zone also has a thinner more reflective crust that may have been affected by postrifting magmatism.

Seismic imaging of a possible paleoarc in the Izu-Bonin intraoceanic arc and its implications for arc evolution processes

Crustal evolution processes in intraoceanic arcs, including crustal accretion and rifting, have been long discussed. To examine crustal evolution in the Izu-Bonin intraoceanic island arc, we conducted an active-source wide-angle seismic study along a north-south profile (500 km long) within a possible paleoarc in the rear arc (i.e., the Nishi-shichito ridge) about 150 km west of the present-day volcanic front. In this study, the seismic velocity and reflectivity images are obtained using the wide-angle seismic data. For the seismic velocity imaging, we applied refraction tomography in which 93,535 picks were used. The overall root-mean-square (rms) misfit calculated from the initial model of the refraction tomography was 483.1 ms, and those calculated from the final model were reduced to 66.7 ms. The resultant seismic image shows marked variations of crustal thickness along the seismic profile: thin crust (10–15 km thick) in the northern part, three discrete thick crustal segments (20–25 km thick) in the central part, and a moderately thick crust (∼15 km thick) in the southern part. These variations are mainly attributed to thickness variations of the middle crust having seismic velocity of 6.0–6.8 km/s. This variation of crustal thickness does not correlate with seafloor topography, which is characterized by post-Miocene across-arc seamount chains. It does correlate well with crustal variations observed along the present-day volcanic front of the Izu-Bonin arc. These findings suggest that the magmatic activity that created the across-arc seamount chains had little effect on the rear-arc crust and that the main part of the rear-arc crust was created before the rear arc separated from the volcanic front. By correlating the structural variations along the rear arc (i.e., the variation of the average seismic velocity as well as the thickness of the middle crust) and those along the present-day volcanic front, we found that the direction of rifting to separate the rear arc (paleoarc) from the present-day volcanic front was north-northeast.

Geochemical variations in Japan Sea back-arc basin basalts formed by high-temperature adiabatic melting of mantle metasomatized by sediment subduction components

The Yamato Basin in the Japan Sea is a back-arc basin characterized by basaltic oceanic crust that is twice as thick as typical oceanic crust. Two types of ocean floor basalts, formed during the opening of the Japan Sea in the Middle Miocene, were recovered from the Yamato Basin during Ocean Drilling Program Legs 127/128. These can be considered as depleted (D-type) and enriched (E-type) basalts based on their incompatible trace element and Sr-Nd-Pb-Hf isotopic compositions. Both types of basalts plot along a common mixing array drawn between depleted mantle and slab sediment represented by a sand-rich turbidite on the Pacific Plate in the NE Japan fore arc. The depleted nature of the D-type basalts suggests that the slab sediment component is nil to minor relative to the dominant mantle component, whereas the enrichment of all incompatible elements in the E-type basalts was likely caused by a large contribution of bulk slab sediment in the source. The results of forward model calculations using adiabatic melting of a hydrous mantle with sediment flux indicate that the melting conditions of the source mantle for the D-type basalts are deeper and hotter than those for the E-type basalts, which appear to have formed under conditions hotter than those of normal mid-oceanic ridge basalts (MORB). These results suggest that the thicker oceanic crust was formed by greater degrees of melting of a hydrous metasomatized mantle source at unusually high mantle potential temperature during the opening of the Japan Sea.

Last stage of the Japan Sea back‐arc opening deduced from the seismic velocity structure using wide‐angle data

The Japan Sea is one of the most well studied back-arc basins in the northwestern Pacific. The seismic crustal model, however, has been inadequate to elucidate the detailed opening model of the Japan Sea. In 2002, to clarify the late stage of the formation style of the Japan Sea opening, a seismic experiment using 35 ocean bottom seismographs (OBSs), an air gun array, and a multichannel hydrophone streamer was undertaken in the areas from the southwestern Yamato Basin, the Oki Ridge, and the southwestern Oki Trough to the coast of the southwestern Japan Island Arc. The crusts beneath the southwestern Yamato Basin and the Oki Ridge are estimated as having approximately 13 km and 19.5 km, respectively. The upper and lower crusts of the southwestern Yamato Basin are approximately 3.2 km and 8 km thick, respectively. Those of the Oki Ridge are approximately 8.2 km and 10.5 km thick, respectively. The upper crust of the Oki Ridge thickens more steeply than that of the southwestern Yamato Basin; however, the lower crust thickens more gently. The crustal structure of the southwestern Yamato Basin shows the extended continental crust accompanied with the opening of the Japan Sea. A remarkable structural characteristic, the upper crust being thinner than the lower crust, caused by listric or complicated normal faults developed in the upper crust of the southwestern Yamato Basin. This deformed upper crust is a common structural characteristic in the southern Japan Sea, which includes the Yamato Basin. The southern Yamato Basin, including the southwestern Yamato Basin, has the thinnest upper and lower crusts in the Japan Sea. For that reason, it is suggested that the southern Yamato Basin had the strongest deformation by a back-arc opening and that the period of the opening in the southern Yamato Basin had been longest in the southern Japan Sea. The formation process of the southern Yamato Basin is inferred to have two stages: rifting and extension of continental crust separating the northeastern and southwestern Japan Island Arcs from the Asian continent and, further, the extension affected by the rotation of the southwestern Japan Island Arc.

Recent Progress in Silica Aerogel Cherenkov Radiator

Abstract In this paper, we present recent progress in the development of hydrophobic silica aerogel as a Cherenkov radiator. In addition to the conventional method, the recently developed pin-drying method for producing high-refractive-index aerogels with high transparency was studied in detail. Optical qualities and large tile handling for crack-free aerogels were investigated. Sufficient photons were detected from high-performance aerogels in a beam test.