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The Early Cretaceous Ontong Java Plateau (OJP) in the southwestern Pacific Ocean is the largest oceanic plateau by volume on Earth, and a broad range of observations has been conducted to reveal its formation and evolution. However, because seafloor seismic observations of the OJP and surrounding areas have been insufficient so far, such experiments are capable of generating additional information regarding the crustal and mantle structure of the OJP. To image seismic velocity discontinuities from the crust to the uppermost mantle, we applied receiver function analysis to seismic records acquired by 17 broadband ocean bottom seismometers deployed across the region in and around the OJP and 3 broadband stations located on ocean islands in Micronesia (one: permanent, two: temporary). The results revealed midcrustal discontinuities and the Moho at depths of 10–20 and 30–40 km (from the top of the basement), respectively, in the central OJP. Moreover, a mantle discontinuity was also imaged at the depth of 55–60 km (from the top of the basement) in the central OJP. These boundaries were not imaged outside the OJP, implying that they are characteristic features of the OJP. In addition, receiver function images showed Moho signals at the depth of 20 km in the eastern OJP, where few previous seismic exploration surveys have been conducted. This depth is comparable with that found in the Manihiki and Hikurangi Plateaus that were potentially separated from the OJP. Key Points: We produced receiver function images of crustal and uppermost mantle structure beneath the Ontong Java Plateau (OJP)Multiple seismic discontinuities of the midcrust (10–20‐km depth) and Moho (30–40‐km depth) were imaged in the central OJPA horizontal seismic discontinuity was imaged in the mantle 57–63 km below sea level in the central OJP [ABSTRACT FROM AUTHOR]

Barotropic tidal currents over bottom topography force density surfaces to oscillate vertically and thereby to act as quasi‐stationary internal tide sources. Deploying a seafloor pressure gauge array with an aperture of 30 km for a year (2014–2015), we detected the low‐mode semidiurnal internal tidal waves propagating with a horizontal phase speed of ~1 m/s in the onshore and offshore directions over the array along the eastern slope of Aogashima Island, south of Japan. The amplitudes of the offshore propagating waves were greater than those of the onshore propagating waves, and both were positively correlated with the amplitudes of the local semidiurnal tide, which peaked in September and March. A tide‐resolving ocean circulation model (JCOPE‐T) well reproduced the observed onshore and offshore internal tidal wave propagation. The model indicated a standing wave region on the slope, where offshore propagating waves interact with standing waves locally pinned to the slope. Along the same profile over a distance of 100 km, we conducted seismic‐oceanographic analysis of the legacy multichannel seismic reflection data to retrieve vertical cross sections of the reflecting layers, which indicated sharp temperature changes in the ocean. Many of the slant reflecting layers were subparallel to the contour lines of the semidiurnal internal‐tide‐associated temperature anomalies in the JCOPE‐T model, suggesting a causal link between the fine reflection layering structure and the semidiurnal low‐mode internal tidal field. Plain Language Summary: Internal tidal waves are generated in the ocean by interactions between surface tides and the bottom topography. Their amplitudes are less than a few centimeters at the sea surface but can be more than a few tens of meters within the ocean. These waves play important roles in tidal dissipation, ocean mixing, hydroacoustic transmission, and submersible technology. Thus far, observations of internal tides have mainly been made from above by satellite altimetry and through ocean columns by cable mooring. In this report, we show that a pressure gauge array at the bottom of the ocean is a promising tool for detecting internal tides and constraining their generation mechanisms. We incorporated a tide‐resolving ocean circulation model that could simulate bottom pressure variations at the same locations in the same period as that during which the observations were made so that the same analyses could be performed for both the observed and simulated data. Remarkable agreement was found between the observed and simulated internal tidal signals. Concurrent simulation of ocean circulation with tides is useful for interpreting not only the array records of ocean bottom pressure gauges but also vertical cross‐sectional images of ocean acoustic reflections obtained using a seismic–oceanographic technique. Key Points: We detected partly standing semidiurnal internal tides on the eastern slope of the Izu‐Bonin ridge using a seafloor pressure gauge arrayA tide‐resolving ocean circulation model (JCOPE‐T) well reproduced the observed nature of internal tides, including seasonal variationsThe temperature anomaly contours of the modeled internal tides tended to be subparallel to the acoustically detected reflecting layers [ABSTRACT FROM AUTHOR]

This paper evaluates the importance of frequency dispersion in the propagation of recent trans-Pacific tsunamis. Frequency dispersion induces a time delay for the most energetic waves, which increases for long propagation distances and short source dimensions. To calculate this time delay, propagation of tsunamis is simulated and analysed from spectrograms of time-series at specific gauges in the Pacific Ocean. 1-D and 2-D simulations are performed by solving either shallow-water (SW) or Boussinesq equations and by considering realistic seismic sources. 1-D sensitivity tests are first performed in a constant-depth channel to study the influence of the source width. 2-D tests are then performed in a simulated Pacific Ocean with a 4000-m constant depth and by considering tectonic sources of 2010 and 2015 Chilean earthquakes. For these sources, both the azimuth and the distance play a major role in the frequency dispersion of tsunamis. Finally, simulations are performed considering the real bathymetry of the Pacific Ocean. Multiple reflections, refractions as well as shoaling of waves result in much more complex time-series for which the effects of the frequency dispersion are hardly discernible. The main point of this study is to evaluate frequency dispersion in terms of traveltime delays by calculating spectrograms for a time window of 6 hr after the arrival of the first wave. Results of the spectral analysis show that the wave packets recorded by pressure and tide sensors in the Pacific Ocean seem to be better reproduced by the Boussinesq model than the SW model and approximately follow the theoretical dispersion relationship linking wave arrival times and frequencies. Additionally, a traveltime delay is determined above which effects of frequency dispersion are considered to be significant in terms of maximum surface elevations. [ABSTRACT FROM AUTHOR]

Abstract: An array of 10 absolute pressure gauges (APGs) deployed in deep water 50 km east of Aogashima, an island in southern Japan, observed several isolated signals in the infragravity wave (IGW) frequency band (0.002–0.03 Hz) during boreal summer, whereas relatively high IGW energy persisted during boreal winter. The isolated IGW shows dispersion with a delay time of 4–5 days as a function of frequency. Here we estimate the excitation locations of IGWs for the two seasons with estimated incoming direction of IGW, calculation of transoceanic IGW trajectories and propagation times, and spatiotemporal variations of significant wave heights from WAVEWATCH III. In boreal summer, the isolated IGWs are primarily caused by IGW energies excited at the shoreline of South America, based on the following three observations: IGWs observed at the array originated from the east: the easterly ray path from the array reaches South America: and an event‐like IGWs were observed at the array when a storm approaches eastward to the shoreline of South America, in which the observed delay time of 4–5 days was also supported by the frequency‐dependent calculation of IGW propagation times. In boreal winter, the incessant IGWs consist of transoceanic IGW energies leaked from the shoreline, primarily from North America, and secondly from South America and the western Aleutian Islands. [ABSTRACT FROM AUTHOR]

In order to prevent hydrofoil colliding with cetaceans, the underwater speaker (UWS) has been installed to repel cetaceans. Yamada et al. (2012) analyzed and devised the UWS sound as it fits the cetaceans' acoustic properties to prevent the collision furthermore. The new UWS sound was devised and synthesized by Yamada et al. (2015) with expectation of avoiding collision with large cetaceans (Patent applied for, JP2014-171411). In this research project, the new UWS sound was investigated by the playback experiment on humpback whale (Megaptera novaeangliae) and by sighting survey in the actual hydrofoil shipping service route. As a result, a physiological and behavioral change of the humpback whale was observed in the playback experiment of the new UWS sound, and the chance of hydrofoil encountering cetaceans of the new UWS sound was smaller than that of the previous UWS sound. Therefore, the improvement of the new UWS sound was confirmed. Lastly, we wish this research project would contribute toward the safer cruise of hydrofoil in the future. [ABSTRACT FROM AUTHOR]

Shear wave splitting measurements provide significant information about subduction zone mantle flow, which is closely tied to plate motions, lithospheric deformation, arc volcanism, and backarc spreading processes. We analyse the shear wave splitting of local S waves recorded by a large 2003–2004 deployment consisting of 58 ocean-bottom seismographs (OBSs) and 20 land stations and by nine OBSs from a smaller 2001–2002 deployment. We employ several methods and data processing schemes, including spatial averaging methods, to obtain stable and consistent shear wave splitting patterns throughout the arc–backarc system. Observed fast orientation solutions are dependent on event location and depth, suggesting that anisotropic fabric in the mantle wedge is highly heterogeneous. Shear waves sampling beneath the northern island arc (latitudes 17.5°–19°N) and between the arc and backarc spreading centre show arc-parallel fast orientations for events shallower than 250 km depth; whereas, fast orientations at the same stations are somewhat different for deeper events. Waves sampling beneath the central island arc stations (latitudes 15.5°–17.5°) show fast orientations subparallel to both the arc and absolute plate motion (APM) for events <250 km depth and APM-parallel for deeper events. Ray paths sampling west of the spreading centre show fast orientations ranging from arc-perpendicular to APM-parallel. Arc-parallel fast orientations characterize the southern part of the arc with variable orientations surrounding Guam. These results suggest that the typical interpretation of mantle wedge flow strongly coupled to the downgoing slab is valid only at depths greater than ∼250 km and at large distances from the trench. We conclude that the arc-parallel fast orientations are likely the result of physical arc-parallel mantle flow and are not due to recently proposed alternative lattice preferred orientation mechanisms and fabrics. This flow pattern may result from along-strike pressure gradients in the mantle wedge, possibly due to changes in slab dip and/or convergence angles. [ABSTRACT FROM AUTHOR]

We obtained phase velocities of fundamental Rayleigh waves in the northwestern Pacific ocean and the northern Philippine sea from seismograms retrieved from a long-term broad-band ocean bottom seismometer (LT-OBS) array deployed in 1999–2000 and from those recorded in Japan, Guam and Ponape islands. This array observation was one of the first attempts to record continuous seismograms on the seafloor using broad-band seismographs on a long-term basis. The geographical distribution of the LT-OBS stations and the surrounding land stations enabled us to determine pure-path phase velocities in a frequency range from 0.01 to 0.05 Hz across the Mariana trough, the Parece Vela basin, the Shikoku basin and the Minami Daito basin, which were found to be systematically lower than those of the Pacific seafloor at comparable ages. This result, along with other geophysical and geological observations in the Philippine sea, may be explained by an uppermost mantle that is relatively Fe rich. [ABSTRACT FROM AUTHOR]

Copyright of Archives of Orthopaedic & Traumatic Surgery / Archiv für Orthopädie, Mechanotherapie und Unfallchirurgie is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Archives of Orthopaedic & Traumatic Surgery / Archiv für Orthopädie, Mechanotherapie und Unfallchirurgie is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: We analyzed seismic waveforms recorded by the broadband ocean bottom seismographs deployed in the South Pacific superswell to determine the depths of the mantle discontinuities using a receiver function method. We estimated the thickness of the mantle transition zone (MTZ) to be 245km on average beneath the superswell region, which is close to the global average. The MTZ is found to be thinned locally beneath the Society hot spot by 30km. Temperature anomalies computed from the thinned transition zone and the Clapeyron slope of the olivine phase transformations are 150–200K beneath the Society hot spot. Previous studies, using land-based data, suggested the presence of a hot MTZ beneath the Pitcairn hot spot. The locally hot transition zone beneath the hot spots is attributed to narrow mantle plumes probably rising from the lower mantle. The normal average thickness obtained from the present study indicates that there is no broad upwelling across the transition zone beneath the entire superswell region at present. [Copyright &y& Elsevier]

Abstract: Three-dimensional P-wave velocity structure of the mantle beneath the South Pacific Superswell is determined through passive broadband seismic experiments on the ocean floor and islands between 2003 and 2005. We collected approximately 1500 relative times of long-period teleseismic P-waves by using a waveform cross-correlation. We analyzed this data set with relative time tomography to depths of 2000km. The resultant structure shows lateral heterogeneity of approximately ±2%, in which a distinct low velocity region is found beneath the center of the Superswell at a depth of 1600km. At 1200km depth, an elongated low velocity region is found beneath the Society to Pitcairn hotspots. At 800km depth, two linear low velocity regions are located beneath Tuamotu and Austral islands. Isolated low velocity regions are identified beneath the Society, Marquesas, and Macdonald hotspots at 400km depth. Our new tomographic images reveal that the large low velocity region rooted in the deep lower mantle is split into two sheets at 1200km depth and these terminate at approximately 800km depth. This feature appears to be consistent with the characteristics of a thermo-chemical pile or dome. [Copyright &y& Elsevier]

Abstract: A shear-wave splitting analysis was performed in the Mariana trench on data recorded by an array of semi broad-band Ocean Bottom Seismometers (OBS), with the aim of studying the Mariana back-arc spreading system and clarifying the nature of the back-arc process. We look for possible mechanisms relating anisotropy and mantle flow. Direct sampling of ray paths through the slab suggests that anisotropy in the slab is relatively small compared with that of the mantle wedge. Fast polarizations at most OBS are consistent with the direction of relative motion between the subducting Pacific and overriding Philippine plates. Exceptions are seismic stations located on the ridge spreading axis which show fast polarizations perpendicular to the ridge curvature. From simple synthetic modeling we estimate an upper limit for the depth of the layer of mantle flow due to the spreading ridge, but as the data show, its influence wanes quickly with distance from the axis. These results suggest that the “sea-anchor” or “extrusion” models are the most favorable scenarios of back-arc opening among other geodynamic models. [Copyright &y& Elsevier]