Kiyoshi Suyehiro

Volcanism in Response to Plate Flexure

Volcanism on Earth is known to occur in three tectonic settings: divergent plate boundaries (such as mid-ocean ridges), convergent plate boundaries (such as island arcs), and hot spots. We report volcanism on the 135 million-year-old Pacific Plate not belonging to any of these categories. Small alkalic volcanoes form from small percent melts and originate in the asthenosphere, as implied by their trace element geochemistry and noble gas isotopic compositions. We propose that these small volcanoes erupt along lithospheric fractures in response to plate flexure during subduction. Minor extents of asthenospheric melting and the volcanoes tectonic alignment and age progression in the direction opposite to that of plate motion provide evidence for the presence of a small percent melt in the asthenosphere.

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.

Structure and growth of the Izu-Bonin-Mariana arc crust: 1. Seismic constraint on crust and mantle structure of the Mariana arc-back-arc system

[1]xa0A high-resolution seismic velocity model is presented for the crust and upper mantle of the Mariana arc–back-arc system (MABS) based on active source seismic profiling. The major characteristics are (1) slow mantle velocity of <8 km s−1 in the uppermost mantle, especially, and deep reflectors under the Mariana arc (MA) and the West Mariana Ridge (WMR), (2) a deep reflector in the upper mantle beneath the relative thick crust of the Mariana Trough (MT) axis, (3) distribution of lower-velocity lower crusts (6.7–6.9 km s−1) beneath the volcanic front and adjacent to the MT, and (4) high-velocity lower crust (7.2–7.4 km s−1) beneath the boundary regions between the MA and MT, and between the WMR and the Parece Vela Basin (PVB), adding to structural characteristics of crust and upper mantle beneath the MABS. Of the characteristics described above, characteristic 1 suggests that the origins of the slow mantle velocity and the deep reflectors be explained by transfer of the lower crustal residues to the upper mantle across the Moho, considering that the WMR is extinct arc currently. On the other hand, characteristic 2 suggests that the origin of deep reflectors beneath the MT axis might be lower velocity materials due to the diffractive signals with strong amplitudes, characteristic 3 suggests that the lower-velocity lower crust advanced crustal growth and characteristic 4 suggests that the high-velocity lower crust beneath arc–back-arc transition zone is composed of mafic/ultramafic materials created by extensive partial melting of mantle peridotites or last stage of the arc magmatism rather than serpentinized peridotite.

Rifting to Spreading Process along the Northern Continental Margin of the South China Sea

Understanding the development from syn-rift to spreading in the South China Sea (SCS) is important in elucidating the western Pacifics tectonic evolution because the SCS is a major tectonic constituent of the many marginal seas in the region. This paper describes research examining the transition from rifting to spreading along the northern margin of the SCS, made possible by the amalgamation of newly acquired and existing geophysical data. The northernmost SCS was surveyed as part of a joint Japan-China cooperative project (JCCP) in two phases in 1993 and 1994. The purpose of the investigation was to reveal seismic and magnetic characteristics of the transitional zone between continental crust and the abyssal basin. Compilation of marine gravity and geomagnetic data of the South China Sea clarify structural characteristics of its rifted continental and convergent margins, both past and present. Total and three component magnetic data clearly indicate the magnetic lineations of the oceanic basin and the magnetic characteristics of its varied margins. The analyses of magnetic, gravity and seismic data and other geophysical and geological information from the SCS led up to the following results: (1) N-S direction seafloor spreading started from early Eocene. There were at least four separate evolutional stages. Directions and rates of the spreading are fluctuating and unstable and spreading continued from 32 to 17xa0Ma. (2) The apparent difference in the present tectonism of the eastern and western parts of Continent Ocean Boundary (COB) implies that in the east of the continental breakup is governed by a strike slip faulting. (3) The seismic high velocity layer in the lower crust seems to be underplated beneath the stretched continental crust. (4) Magnetic anomaly of the continental margin area seems to be rooted in the uppermost sediment and upper part of lower crust based on the tertiary volcanism. (5) Magnetic quiet zone (MQZ) anomaly in the continental margin area coincides with COB. (6) The non-magnetic or very weakly magnetized layer is probably responsible for MQZ. One of the causes of demagnetization of the layer is due to hydrothermal alteration while high temperature mantle materials being underplated. Another explanation is that horizontal sequences of basalt each with flip-flop magnetization polarity cancel out to the resultant magnetic field on the surface. We are currently developing a synthetic database system containing datasets of seismicity, potential field data, crustal and thermal structures, and other geophysical data to facilitate the study of past, contemporary and future changes in the deep sea environment around Japan; i.e. trench, trough, subduction zones, marginal basins and island arcs. Several special characteristics are an object-oriented approach to the collection and multi-faceted studies of global data from a variety of sources.

Micro‐tsunami from a local interplate earthquake detected by cabled offshore tsunami observation in northeastern Japan

A micro tsunami from an interplate earthquake (Mw 6.1) was observed in 1998 on ocean bottom tsunami meters (OBTMs) deployed east off the northeastern Japan. The offshore tsunami data without complex distortions due to the coastal topography enable us to estimate reliable tsunami source parameters. The observed amplitude was about 1.5 cm at epicentral distance less than 100 km. We numerically computed the tsunami waveform by solving the linear Boussinesq equations. The observed tsunami waveforms are well explained by synthetic waveforms assuming the fault width of 10–15 km. The depth of the fault is estimated as 5–10 km below the seafloor, which is in good agreement with the location of the plate boundary defined by previous seismic studies.

Distinct regional differences in crustal thickness along the axis of the Mariana Trough, inferred from gravity anomalies

We have compiled extensive gravity and bathymetry data for the whole Mariana Trough, which were collected during several Japanese scientific cruises over the last few years. This study aims to clarify the lateral distribution of the local differences in geochemical signatures, which have been observed locally in the Mariana Trough. Shipboard free-air gravity anomaly data from eight Japan Agency for Marine-Earth Science and Technology (JAMSTEC) cruises were compiled with those crossover errors of 2.85 mgal. Mantle Bouguer anomalies (MBA) were calculated by subtracting the predictable gravity signal due to the seawater/crust and crust/mantle density boundaries. The crustal thickness variation along the spreading axis was estimated from the MBA. Different features in crustal thickness, its variation, and segment length for each segment, allow us to identify four distinct regional differences in magmatic activity along the spreading axis of the Mariana Trough. Segment in region A (to the north of 20°35′N) shows the largest sectional dimensions of crust along the axis and it is probably affected by an additional supply from island arc magma sources. A variety of crustal thickness values and of along-axis crustal thickness variations in region B (between 15°38′N and 20°35′N) suggests two types of segments. One is similar to a slow spreading ridge segment that has a plume-like mantle upwelling under the spreading axis, and the other is a magma-starved segment. Region C (between 14°22′N and 15°38′N) is a less magmatic region (individual crustal thickness averages of 3.4–4.1 km). Region D (to the south of 14°22′N) has higher individual crustal thickness averages of 5.9–6.9 km, suggesting higher magmatic activity with a sheet-like mantle upwelling under the spreading axis. Different features in the MBA for off-axis areas suggest that these four regions have existed since the Mariana Trough started spreading. Moreover, comparison between our results of crustal thickness and previous geochemical results indicates that less magmatic spreading segments with thin crust, which are locally distributed in both regions B and C, probably result from mantle source depleted of water and incompatible elements. This suggests that lateral compositional variation of water and incompatible elements exists on a segment scale in the mantle source beneath the spreading axis of the Mariana Trough.

Aftershock distribution of the 26 December 2004 Sumatra-Andaman earthquake from ocean bottom seismographic observation

We deployed an OBS network in February–March 2005 in the rupture area of the Sumatra Andaman earthquake on 26 December 2004. We placed 17 short-term OBSs and two long-term OBSs, and recovered OBSs after observation for 19–22 days. The hypocenter distribution from 10-day data of 17 OBS revealed the detailed structure of aftershock seismicity offshore of Sumatra Island. Aftershock seismicity associated with the subducting slab starts 40 km inward from the Sunda trench axis; it ceases at 50 km depth beneath the Aceh Basin, approximately 240 km inward from the trench axis. Aftershocks in 120–170 km from the trench axis consist of a surface with a dip of 10–12° dominated by a dip-extension type mechanism. Beyond the southwestern edge of the Aceh Basin, the aftershock activity becomes higher, and dominated by dip-slip type earthquakes, with a slightly increased dipping angle of 15–20°. Three along-arc bands of shallow seismicity were identified at 70 km inward from the Sumatra trench, 110 km inward from the trench, and in the south of the Aceh Basin. These locations correspond to steep topographic slopes in the accretionary prism, suggesting the present evolutional activity of the accretionary prism offshore Sumatra Island.

Mantle Discontinuity Depths Beneath the West Philippine Basin from Receiver Function Analysis of Deep-Sea Borehole and Seafloor Broadband Waveforms

We analyzed broadband waveform data recorded by a deep-sea borehole observatory (WP-1) and a long-term broadband ocean-bottom seismograph (NOT1) deployed in the west Philippine basin by the Ocean Hemisphere Project. We determined the depths of the 660-km discontinuity beneath the west Philippine basin using the receiver function method. The “660” depths determined from the WP-1 and NOT1 are consistent with each other, indicating that the estimated depths are reliable. The 660 depth determined using both WP-1 and NOT1 data was 669 ± 9 km, which is deeper by 9 km than the global averages, beneath the west Philippine basin. Interpreting the 660 depth in terms of temperature, the slightly deep 660 can be translated to mean lower temperatures by about 100 K at the 660, using the Clapeyron slope of the olivine to β -spinel and the post-spinel phase change. The cold temperature is qualitatively consistent with the tomographic image. When compared with previous regional studies of the 660 beneath the Philippine Sea, our results suggest the presence of significant topography on the mantle discontinuities beneath the Philippine Sea, which may be caused by a stagnant Pacific slab in the mantle transition zone. The present study demonstrates that data from deep-sea observations provide useful information for investigating deep Earth structure.

P and S receiver function analysis of seafloor borehole broadband seismic data

[1]xa0The crustal and lithospheric structure of the normal oceanic plates is investigated using converted wave techniques (P and S receiver functions (RFs) and novel stacking analysis techniques without deconvolution) applied to the data from two seafloor borehole broadband seismic stations located in the central Philippine Sea and in the northwest Pacific ocean. We observe sufficient energy from at least two discontinuities within the error bounds, one from the crust-mantle (Moho) boundary and the other from the seismic lithosphere-asthenosphere boundary (LAB). Synthetic seismograms for seafloor stations show that the water reverberations interfere with the vertical component of seismograms but to a lesser extent with the radial part of P receiver functions. On the other hand, S receiver functions are devoid of such effects since all the multiples and converted waves are separated in time by the primary S wave in time. Waveform modeling of RFs shows that the crustal thicknesses of the western Philippine Sea plate and northwest Pacific plate are ∼7–8 km, and that depths of LAB are 76 ± 1.8 km and 82 ± 4.4 km, respectively, with an abrupt Vs drop at LAB of ∼7%–8%, as reported by Kawakatsu et al. (2009). The LAB depth for the eastern Philippine plate is found to be ∼55 km. To confirm the robustness of this observation, we further analyze vertical and radial components of the data without deconvolution for P wave backscattered reflection phases and P-to-S converted phases. The result indicates that the reflected/converted phases from Moho and LAB are observed at timings consistent with the receiver function results. The effect of seismic anisotropy for observed RFs is also investigated.

Three‐dimensional shear wave structure beneath the Philippine Sea from land and ocean bottom broadband seismograms

[1]xa0We obtained three-dimensional (3-D) shear wave speed structure beneath the Philippine Sea and the surrounding region from seismograms recorded by land-based and long-term broadband ocean bottom seismographic stations. The ocean bottom data gave us a better station coverage to obtain a higher spatial resolution (about 300–400 km) in the Philippine Sea than in previous studies. We employed a new technique of surface wave tomography, in which multimode phase speeds are measured and inverted for a 3-D shear wave speed structure by incorporating the effects of finite frequency and ray bending. There is a sharp speed contrast along the Izu-Bonin-Mariana trench, across which the Philippine Sea side has a significantly slower upper mantle than the Pacific Ocean side. In the upper 120 km, the shear wave speed structure is well correlated with the age of the provinces. At depths greater than 160 km, the pattern is dominated by fast anomalies of the subducted slabs of the Pacific plate and two slow anomalies to the south of the Daito ridge and in the southernmost part of the Philippine Sea.