Mikiya Yamashita

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.

Structure and Evolution of the East Antarctic Lithosphere: Tectonic Implications for the Development and Dispersal of Gondwana

Abstract Lithospheric evolution of the Antarctic shield is one of the keystones for understanding continental growth during the Earths evolution. Architecture of the East Antarctic craton is characterized by comparison with deep structures of the other Precambrian terrains. In this paper, we review the subsurface structure of the Lower Paleozoic metamorphic complex around the Lutzow-Holm area (LHC), East Antarctica, where high-grade metamorphism occurred during the Pan-African orogenic event. LHC is considered to be one of the collision zones in the last stage of the formation of Gondwana. A geoscience program named ‘Structure and Evolution of the East Antarctic Lithosphere (SEAL)’ was carried out since 1996-1997 austral summer season as part of the Japanese Antarctic Research Expedition (JARE). Several geological and geophysical surveys were conducted including a deep seismic refraction/wide-angle reflection survey in the LHC. The main target of the SEAL seismic transect was to obtain lithospheric structure over several geological terrains from the western adjacent Achaean Napier Complex to the eastern Lower Paleozoic Yamato-Belgica Complex. The SEAL program is part of a larger deep seismic profile, LEGENDS (Lithospheric Evolution of Gondwana East iNterdisciplinary Deep Surveys) that will extend across the Pan-African belt in neighboring fragments of Gondwana.

Seismic reflection imaging of a Warm Core Ring south of Hokkaido

A multi-channel seismic reflection (MCS) survey was conducted in 2009 to explore the deep crustal structure of the Pacific Plate south of Hokkaido. The survey line happened to traverse a 250-km-wide Warm Core Ring (WCR), a current eddy that had been generated by the Kuroshio Extension. We attempted to use these MCS data to delineate the WCR fine structure. The survey line consists of two profiles: one with a shot interval of 200 m and the other with a shot interval of 50 m. Records from the denser shot point line show much higher background noise than the records from the sparser shot point line. We identified the origin of this noise as acoustic reverberations between the sea surface, seafloor and subsurface discontinuities, from previous shots. Results showed that a prestack migration technique could enhance the signal buried in this background noise efficiently, if the sound speed information acquired from concurrent temperature measurements is available. The WCR is acoustically an assemblage of concave reflectors dipping inward, with steeper slopes (~2°) on the ocean side and gentler slopes (~1°) on the coastal side. Within the WCR, we recognised a 30-km-wide lens-shaped structure with reflectors on the perimeter.

Deep Reflection Imaging beneath the Mizuho Plateau, East Antarctica, by SEAL-2002 Seismic Experiment

A seismic exploration was conducted on the Mizuho Plateau, East Antarctica, during the 2001/2002 austral summer season as the “Structure and Evolution of the East Antarctic Lithosphere (SEAL)” project by the 43rd Japanese Antarctic Research Expedition (JARE-43). The survey line of this exploration (SEAL-2002 profile) was almost perpendicular to the Mizuho inland traverse routes (JARE-41 refraction survey line; SEAL-2000) and was almost parallel to the coastal line along the Lutzow-Holm Bay. Several seismic shot records were obtained with clear arrivals of phases until a distance of 150 km in length. We have analyzed two shot data of both ends of the SEAL-2002 profile by using the conventional reflection method. Interval velocities were estimated by applying the normal-move-out (NMO) correction, then the obtained single-fold section obtained explicitly presents the horizontal reflectors originated from the middle crust, the lower crust and the Moho discontinuity. First, the reflector from the top of the middle crust was located at the depth of 23–24 km, which was corresponding to 8–9 s of two way travel time (TWT) in the single-fold section. Next, the reflector from the top of the lower crust was located at a depth of 31–34 km, corresponding to 11–12 s of TWT. The Moho reflector was observed in 13–14 s of TWT and the depth was estimated to be approximately 41–42 km.

Seismic imaging for an ocean drilling site survey and its verification in the Izu rear arc

To evaluate the crustal structure of a site proposed for International Ocean Discovery Program drilling, the Japan Agency for Marine-Earth Science and Technology carried out seismic surveys in the Izu rear arc between 2006 and 2008, using research vessels Kaiyo and Kairei. High-resolution dense grid surveys, consisting of three kinds of reflection surveys, generated clear seismic profiles, together with a seismic velocity image obtained from a seismic refraction survey. In this paper, we compare the seismic profiles with the geological column obtained from the drilling. Five volcaniclastic sedimentary units were identified in seismic reflection profiles above the 5 km/s and 6 km/s contours of P-wave velocity obtained from the velocity image from the seismic refraction survey. However, some of the unit boundaries interpreted from the seismic images were not recognised in the drilling core, highlighting the difficulties of geological target identification in volcanic regions from seismic images alone. The geological core derived from drilling consisted of seven lithological units (labelled I to VII). Units I to V were aged at 0–9 Ma, and units VI and VII, from 1320–1806.5 m below seafloor (mbsf) had ages from 9 to ~15 Ma. The strong heterogeneity of volcanic sediments beneath the drilling site U1437 was also identified from coherence, calculated using cross-spectral analysis between grid survey lines. Our results suggest that use of a dense grid configuration is important in site surveys for ocean drilling in volcanic rear-arc situations, in order to recognise heterogeneous crustal structure, such as sediments from different origins. To evaluate the crustal structure of a site proposed for ocean drilling, high-resolution dense seismic surveys were carried out in the Izu rear arc, Japan. Our results suggest that use of a dense grid configuration is important in volcanic rear-arc situations, in order to recognise heterogeneous crustal structure, such as sediments from different origins.

Structural characteristics of the Bayonnaise Knoll caldera as revealed by a high-resolution seismic reflection survey

The Bayonnaise Knoll caldera is a conical silicic caldera located on the eastern part of the back-arc rift zone of the Izu-Ogasawara arc. Many geological and geophysical surveys have shown that the Bayonnaise Knoll caldera contains hydrothermal sulfide deposits. The Japan Agency for Marine-Earth Science and Technology conducted high-resolution multi-channel seismic reflection surveys across the Bayonnaise Knoll caldera to ascertain details of the crustal structure, such as the configuration of faults around the caldera. A reflection profile of excellent quality was obtained by high-density velocity analysis at about 150-m intervals. We applied prestack depth migration by using the results of the high-density velocity analysis and further analyzed this region. The depth-migrated profile shows many faults, which correspond to bathymetric lineations, on the eastern side of the Bayonnaise Knoll caldera. The velocity structure of the Bayonnaise Knoll caldera resembles that of the Myojin Knoll caldera, which has been well surveyed and is associated with the hydrothermal deposit. The depth-migrated profile shows a clear reflective zone that is distributed asymmetrically to the Bayonnaise Knoll caldera center. These data suggest that caldera formation was controlled by back-arc rifting activity in the Izu-Ogasawara arc. The hydrothermal fluid migration path in the Bayonnaise Knoll caldera is estimated to be the result of faulting and magmatic intrusion on the eastern side of the structure. It is assumed that these fluids formed the Kuroko-type sulfide deposit in the eastern part of the caldera structure.

Physical properties and seismic structure of Izu‐Bonin‐Mariana fore‐arc crust: Results from IODP Expedition 352 and comparison with oceanic crust

Most of the well-preserved ophiolite complexes are believed to form in supra-subduction zone (SSZ) settings. We compare physical properties and seismic structure of SSZ crust at the Izu-Bonin-Mariana (IBM) fore arc with oceanic crust drilled at Holes 504B and 1256D to evaluate the similarities of SSZ and oceanic crust. Expedition 352 basement consists of fore arc basalt (FAB) and boninite lavas and dikes. P-wave sonic log velocities are substantially lower for the IBM fore arc (mean values 3.1-3.4 km/s) compared to Holes 504B and 1256D (mean values 5.0-5.2 km/s) at depths of 0-300 m below the sediment-basement interface. For similar porosities, lower P-wave sonic log velocities are observed at the IBM fore arc than at Holes 504B and 1256D. We use a theoretical asperity compression model to calculate the fractional area of asperity contact Af across cracks. Af values are 0.021-0.025 at the IBM fore arc and 0.074-0.080 at Holes 504B and 1256D for similar depth intervals (0-300 m within basement). The Af values indicate more open (but not necessarily wider) cracks in the IBM fore arc than for the oceanic crust at Holes 504B and 1256D, which is consistent with observations of fracturing and alteration at the Expedition 352 sites. Seismic refraction data constrains a crustal thickness of 10-15 km along the IBM fore arc. Implications and inferences are that crust composing ophiolites formed at SSZ settings could be thick and modified after accretion, and these processes should be considered when using ophiolites as an analog for oceanic crust. This article is protected by copyright. All rights reserved.

Distribution and migration of aftershocks of the 2010 Mw 7.4 Ogasawara Islands intraplate normal‐faulting earthquake related to a fracture zone in the Pacific plate

We describe the aftershocks of a Mw 7.4 intraplate normal-faulting earthquake that occurred 150 km east Ogasawara (Bonin) Islands, Japan, on 21 December 2010. It occurred beneath the outer trench slope of the Izu-Ogasawara trench, where the Pacific plate subducts beneath the Philippine Sea plate. Aftershock observations using ocean bottom seismographs (OBSs) began soon after the earthquake and multichannel seismic reflection surveys were conducted across the aftershock area. Aftershocks were distributed in a NW-SE belt 140 km long, oblique to the N-S trench axis. They formed three subparallel lineations along a fracture zone in the Pacific plate. The OBS observations combined with data from stations on Chichi-jima and Haha-jima Islands revealed a migration of the aftershock activity. The first hour, which likely outlines the main shock rupture, was limited to an 80 km long area in the central part of the subsequent aftershock area. The first hour activity occurred mainly around, and appears to have been influenced by, nearby large seamounts and oceanic plateau, such as the Ogasawara Plateau and the Uyeda Ridge. Over the following days, the aftershocks expanded beyond or into these seamounts and plateau. The aftershock distribution and migration suggest that crustal heterogeneities related to a fracture zone and large seamounts and oceanic plateau in the incoming Pacific plate affected the rupture of the main shock. Such preexisting structures may influence intraplate normal-faulting earthquakes in other regions of plate flexure prior to subduction.

Characteristic Seismic Waves Associated with Cryosphere Dynamics in Eastern Dronning Maud Land, East Antarctica

Several kinds of natural source signals are recorded by seismic exploration stations on the continental ice sheet in Eastern Dronning Maud Land, East Antarctica, during 2002 austral summer. They include not only tectonic earthquakes, but also ice-related phenomena possibly involving recent global climate change. The recorded signals are classified into (1) teleseismic events, (2) local ice quakes, and (3) unidentified events (X-phases). The teleseismic waves show the high signal-to-noise ratio in spite of the small magnitude of the event; this indicates that it is highly feasible to study not only the local shallow structure but also the deep structure of the earth by using teleseismic events. Frequency spectra of the all waveforms represent discordances along the observation seismic profile. The abrupt change of topography in the valley along the seismic profile might cause both the anomalous frequency content and travel times. Finally, an origin of the X-phases is speculated as the intraplate earthquakes or possibly large ice-quakes (glacial earthquakes) around Antarctica, involving global warming appeared in polar region.