Toshiya Fujiwara

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.

A slump in the trench: Tracking the impact of the 2011 Tohoku-Oki earthquake

We present differential bathymetry and sediment core data from the Japan Trench, sampled after the 2011 Tohoku-Oki (offshore Japan) earthquake to document that prominent bathymetric and structural changes along the trench axis relate to a large (∼27.7 km 2 ) slump in the trench. Transient geochemical signals in the slump deposit and analysis of diffusive re-equilibration of disturbed SO 4 2– profiles over time constrain the triggering of the slump to the 2011 earthquake. We propose a causal link between earthquake slip to the trench and rotational slumping above a subducting horst structure. We conclude that the earthquake-triggered slump is a leading agent for accretion of trench sediments into the forearc and hypothesize that forward growth of the prism and seaward advance of the deformation front by more than 2 km can occur, episodically, during a single-event, large mega-thrust earthquake.

Bathymetry and magnetic anomalies in the Havre Trough and southern Lau Basin: from rifting to spreading in back-arc basins

Abstract The Havre Trough and Lau Basin are active back-arc basins related to Pacific-Australian plate convergence. These back-arc basins provide insight into the evolution of rifting through various stages, including a final stage of oceanic spreading. We carried out a swath bathymetry and magnetic anomaly survey focused on the less-studied Havre Trough and southern Lau Basin. The western portion of the whole Havre Trough is deeper and more thickly sedimented whereas shallow bathymetry and volcanic edifices are dominant in the east. We suggest that crustal rifting progresses asymmetrically with respect to the trough axis. The active tectono-magmatic zone migrates trenchward during ongoing widening. Rifting along normal faults trending in the longitudinal trough strike occurs in the southern Havre Trough. Volcanism is restricted to a line along the normal faults. Circular or elongated knolls in the northern Havre Trough indicate that en echelon type segmented rifting of short strike length and successive volcanic intrusion takes place. Volcanism in this area is dominated by diapir-like magma intrusion into an extended and thinned arc crust. The southernmost tip of the spreading center of Lau Basin is defined by the deep graben, striking at N20°E, located at 24°00′S, 177°10′W.

Magnetic structure of the southern Boso Peninsula, Honshu, Japan, and its implications for the formation of the Mineoka Ophiolite Belt

We conducted onshore and offshore magnetic surveys on and around the southern Boso Peninsula, Honshu, Japan, and observed prominent large amplitude anomalies along the Mineoka Ophiolite Belt, and long wavelength low anomalies to the south of the belt containing short wavelength isolated anomalies. The magnetic structure was modeled by using three-dimensional magnetic prisms and basement with about 1 A/m of magnetization. At the Mineoka Belt, the top of the magnetic prisms is located at the ground surface, and these bodies are elongate in the vertical direction, with high angle magnetic inclinations. Magnetic basement exists at shallow depth beneath the belt. The magnetic basement traces the bottom surfaces of the magnetic prisms and forms a graben structure. In the south of the Mineoka Belt, thin sheet-like magnetic prisms with low magnetic inclinations are assumed at 1–3 km depth. The magnetic structure implies the tectonic process of the formation of the Mineoka Ophiolite Belt. The belt could be fragmented pieces of an oceanic plate emplaced at a paleo-plate boundary, which originated in low latitude and was transported by obduction to the present place via northward drift.

Evaluation of spatial resolution and estimation error of seafloor displacement observation from vessel-based bathymetric survey by use of AUV-based bathymetric data

A repeated bathymetric survey reveals seafloor displacement between before and after geodynamic events. We evaluated the less-known spatial resolution and estimation error of the seafloor displacement observation from a vessel-based multi-narrow beam bathymetric survey. In this evaluation, bathymetric data from vessel-based and near-seafloor high-resolution autonomous underwater vehicle (AUV)-based surveys in the same area were used. Simulated vessel-based bathymetric “before and after” data of the seafloor displacement were made using AUV-based bathymetric data. The displacement was verified by comparing these simulated data using the analysis conditions that no locational errors of beam sounding points exist, a footprint effect is uniform, depth accuracy is constant in the analysis area, and there are no depth offset between two data. As a result, we found that the smallest vertical seafloor displacement that can be detected occurs when the horizontal extent of the deformation is larger than several times the size of the footprint (area of the narrow sounding beam projected onto the seafloor) of the used vessel’s multi-narrow beam echo sounder, and in the situation that the amplitude of the depth difference is greater than the accuracy of the vessel-based depth measurement (standard deviation of measuring error). When local slopes of the bathymetry are gentler than those of the artificial variation appeared in the depth differences between two data, the horizontal seafloor displacement seems to be difficult to resolve accurately. The local slope of the artificial depth variation is derived from the wavelength and the amplitude which are equivalent to ~1–3 times of the footprint size and the accuracy of the depth measurement, respectively.

Friction properties of the plate boundary megathrust beneath the frontal wedge near the Japan Trench: an inference from topographic variation

The 2011 Tohoku-Oki earthquake (Mw 9.0) produced a fault rupture that extended to the toe of the Japan Trench. The deformation and frictional properties beneath the forearc are keys that can help to elucidate this unusual event. In the present study, to investigate the frictional properties of the shallow part of the plate boundary, we applied the critically tapered Coulomb wedge theory to the Japan Trench and obtained the effective coefficient of basal friction μb′ and Hubbert-Rubey pore fluid pressure ratio (λ) of the wedge beneath the lower slope. We extracted the surface slope angle and décollement dip angle (which are the necessary topographic parameters for applying the critical taper theory) from seismic reflection and refraction survey data at 12 sites in the frontal wedges of the Japan Trench. We found that the angle between the décollement and back-stop interface generally decreases toward the north. The measured taper angle and inferred effective friction coefficient were remarkably high at three locations. The southernmost area, which had the highest coefficient of basal friction, coincides with the area where the seamount is colliding offshore of Fukushima. The second area with a high effective coefficient of basal friction coincides with the maximum slip location during the 2011 Tohoku-Oki earthquake. The area of the 2011 earthquake rupture was topographically unique from other forearc regions in the Japan Trench. The strain energy accumulation near the trench axis may have proceeded because of the relatively high friction, and later this caused a large slip and collapse of the wedge. The location off Sanriku, where there are neither seamount collisions nor rupture propagation, also has a high coefficient of basal friction. The characteristics of the taper angle, effective coefficient of basal friction, and pore fluid pressure ratio along the Japan Trench presented herein may contribute to the understanding of the relationship between the geometry of the prism and the potential for generating seismo-tsunamigenic slips.

Learning from crustal deformation associated with the M9 2011 Tohoku-oki earthquake

Numerous observations pertaining to the magnitude 9.

Advent of Continents: A New Hypothesis

The straightforward but unexpected relationship presented here relates crustal thickness to magma type in the Izu-Ogasawara (Bonin) and Aleutian oceanic arcs. Volcanoes along the southern segment of the Izu-Ogasawara arc and the western Aleutian arc (west of Adak) are underlain by thin crust (10–20 km). In contrast those along the northern segment of the Izu-Ogasawara arc and eastern Aleutian arc are underlain by crust ~35 km thick. Interestingly, andesite magmas dominate eruptive products from the former volcanoes and mostly basaltic lavas erupt from the latter. According to the hypothesis presented here, rising mantle diapirs stall near the base of the oceanic crust at depths controlled by the thickness of the overlying crust. Where the crust is thin, melting occurs at relatively low pressures in the mantle wedge producing andesitic magmas. Where the crust is thick, melting pressures are higher and only basaltic magmas tend to be produced. The implications of this hypothesis are: (1) the rate of continental crust accumulation, which is andesitic in composition, would have been greatest soon after subduction initiated on Earth, when most crust was thin; and (2) most andesite magmas erupted on continental crust could be recycled from “primary” andesite originally produced in oceanic arcs.

Evidence for Mass Transport Deposits at the IODP JFAST-Site in the Japan Trench

Several studies indicate that the 2011 Tohoku-Oki earthquake (Mw 9.0) off the Pacific coast of Japan has induced slip to the trench and triggered landslides in the Japan Trench. In order to better understand these processes, detailed mapping and shallow-coring landslides at the trench as well as Integrated Ocean Drilling Program (IODP) deep drilling to recover the plate boundary decollement (Japan Trench Fast Earthquake Drilling Project, JFAST) have been conducted. In this study we report sediment core data from the rapid response R/V SONNE cruise (SO219A) to the Japan Trench, evidencing a Mass Transport Deposit (MTD) in the uppermost section later drilled at this JFAST-site during IODP Expedition 343. A 8.7 m long gravity core (GeoB16423-1) recovered from ∼7,000 m water depth reveals a 8 m sequence of semi-consolidated mud clast breccias embedded in a distorted chaotic sediment matrix. The MTD is covered by a thin veneer of 50 cm hemipelagic, bioturbated diatomaceous mud. This stratigraphic boundary can be clearly distinguished by using physical properties data from Multi Sensor Core Logging and from fall-cone penetrometer shear strength measurements. The geochemical analysis of the pore-water shows undisturbed linear profiles measured from the seafloor downcore across the stratigraphic contact between overlying younger background-sediment and MTD below. This indicates that the investigated section has not been affected by a recent sediment destabilization in the course of the giant Tohoku-Oki earthquake event. Instead, we report an older landslide which occurred between 700 and 10,000 years ago, implying that submarine mass movements are dominant processes along the Japan Trench. However, they occur on local sites and not during each megathrust earthquake.

Gravity and magnetic constraints on the crustal structure and evolution of the Horeki seamount in the Izu-Ogasawara (Bonin) arc

Data on the crustal structure, bulk composition, and eruption ages of an arc seamount were obtained in an investigation aimed at studying the spatial and temporal variations in the magma composition of an intra-oceanic arc. We conducted gravity and magnetic surveys of the Horeki seamount and evaluated the density and magnetization structure using inversion and forward modeling. The seamount is located on the back-arc side of the Izu-Ogasawara arc. This seamount has an elliptical shape, a flat summit, and satellite ridges and cones on its northern and southern flanks. The flanks are consistent with a higher density anomaly, with an estimated mean density corresponding to dense basaltic rocks. A low-density anomaly is distributed in the seamount top, indicating that the top likely consists of porous basalts or differentiated rocks. The prominent circular low Bouguer gravity anomaly, which appeared in the northern part of the flat-topped summit, indicates that a light-density material fills the summit. The main body of seamount is normally magnetized. Combined with the age of the rocks, the volcanism constructing the main body may be most robust in the Gauss chron. The deeper part of seamount may consist of intrusive rocks, with induced magnetization over remanence. The eastern part of the northern ridges is reversely magnetized, while the western part is normally magnetized. These features and the ages of the sampled rocks suggest that these ridges were constructed in the Matuyama and Brunhes chrons, respectively. The southern part of the seamount flanks shows weak normal magnetization, probably caused by the small cones with different polarities of remanent magnetization.