Simultaneous measurements of compressional wave and shear wave velocities, Poisson's ratio, and Vp/Vs under deep crustal pressure and temperature conditions: Example of silicified pelitic schist from Ryoke Belt, Southwest Japan

The ultrasonic technique for measuring travel times of compressional and shear waves using dual-mode transducers was adapted to a piston cylinder apparatus, allowing simultaneous measurements of travel times of compressional and shear waves of island arc samples under the high pressure and temperature conditions of island arcs. This method enables us to determine elastic properties and their pressure and temperature derivatives simultaneously. Furthermore, Vp/Vs can be directly determined from travel times of compressional and shear waves independently of length change due to compression or thermal expansion of rock samples under deep crustal conditions, providing more accurate Vp/Vs values than those determined from individual measurements of travel times of both elastic wave types using single-mode transducers. Experimental techniques and results are demonstrated using data on silicified pelitic schist from the Ryoke Belt to 0.6 GPa. The simultaneous measurement gives Vp  = 5.60 km/s, ∂ Vp /∂ P  = 0.090 (km/s)/GPa, Vs  = 3.37 km/s, ∂ Vs /∂ P  = 0.05 (km/s)/GPa, σ  = 0.216, and Vp / Vs  = 1.66 at ambient conditions. The temperature derivatives were constrained from fitting using linear functions of temperature, yielding ∂ Vp /∂ T  = −0.518 × 10⁻³ (km/s)/K and ∂ Vs /∂ T  = −0.182 × 10⁻³ (km/s)/K. Performing simultaneous measurements of travel times of compressional and shear waves using dual-mode transducers, it is possible to accurately determine Vp / Vs and Poisson's ratio of crustal minerals and rocks at deep crustal conditions to study the composition of the crustal interior, e.g. rock types and fluids below the hypocentral region of earthquakes or around bright spots.     

Silicification of pelitic schist in the Ryoke low-pressure/temperature metamorphic belt, Southwest Japan: Origin of competent layers in the middle crust

We propose that competent layers composed of silicified rocks in the Cretaceous Ryoke low-pressure/temperature (low- P/T ) metamorphic belt in the Iwakuni–Yanai area, Southwest Japan, may be an exhumed equivalent of a seismically-reflective bright-layer in the middle crust. Silicified rocks crop out as layers or long lenticular bodies several to fifteen meters in thickness, and they are restricted to the area of greenschist facies conditions within a structural thickness of about one kilometer. The silicified rock layers exhibit a sharp contact with subjacent biotite schist but a gradational contact with overlying pelitic schist. Silicified rock is mainly composed of fine-grained quartz and minor muscovite and biotite, and some colored minerals have been variably decolorized by alteration. This implies that silicification turned the color of pelitic schists to pale gray or milky white. Quartz veins cutting schistosity at high angles were preferentially developed in the silicified rocks, while schistosity-parallel quartz veins, which underwent viscous flow, were observed in the underlying biotite schists. En echelon quartz veins and fishnet-like quartz veins are characteristic of silicified rocks. The modes of occurrence of quartz veins indicate that silicified rocks were competent relative to underlying biotite schist. The combination of contact with high-competence contrast adjacent to low-permeability silicified rocks may be a good reflector of seismic waves. If the silicified rocks were distributed in the middle crust to a considerable extent, they may result in seismic bright-layer reflections.     

Subhorizontal tectonic framework of the Horoman peridotite complex and enveloping crustal rocks,south‐central Hokkaido,Japan

Geological observations in the Horoman area, south‐central Hokkaido, show that the Horoman peridotite complex of the Hidaka metamorphic belt is a tectonic slice about 1200 m thick. The peridotite slab is intercalated into a gently east‐dipping structure. The underlying unit is a Cretaceous–Paleogene accretionary complex. Riedel shear planes in the sedimentary layers of the accretionary complex near the structural bottom of the peridotite slab indicate top‐to‐the‐west (thrust) displacement. The overlying unit is composed of felsic–pelitic gneisses and mafic–felsic intrusive rocks (the Hidaka metamorphic rocks). The boundary surface between the peridotite complex and metamorphic rocks forms a domal structure. Microstructures of sheared metamorphic rocks near the structural top of the peridotite slab indicate top‐to‐the‐east (normal) displacement. The results combined with previous studies suggest that the Horoman peridotite complex was emplaced onto the Asian margin (Northeast Japan) during the collision between the Asian margin and the Hidaka crustal block.     

Geologic evidence for late Quaternary repetitive surface faulting on the Isurugi fault along the northwestern margin of the Tonami Plain,north‐central Japan

Our detailed field investigation, paleoseismic trenching, and airborne light detection and ranging (LiDAR)‐derived topographic data provides the first direct evidence for late Quaternary repetitive surface faulting on the northeast‐striking Isurugi fault along the northwestern margin of the Tonami Plain in the Hokuriku region of north‐central Japan. This fault has been interpreted previously by different researchers as both inactive and active, owing to a lack of geologic evidence and a failure to identify fault‐related geomorphic features. Our mapping of LiDAR topography revealed a series of northeast‐trending warped fluvial terraces, about 1.5 km long and 170 m wide, with an age of ≤ 29 ka. We interpreted these geomorphologic features to represent an active pop‐up structure bounded to the southeast by the northwest‐dipping main thrust of the Isurugi fault and to the northwest by a southeast‐dipping backthrust that splays off the main thrust in the shallow subsurface. Paleoseismic trenching across the northwestern part of an elongate terrace exposed a series of southeast‐dipping backthrusts and associated northwest‐verging monoclines. The deformation and depositional age of the strata provide evidence for repetitive surface rupturing on the backthrusts since the latest Pleistocene; the latest of these events occurred in the Holocene between about 4.0 and 0.9 ka. Despite the poor preservation of the surface expression of the Isurugi fault, repetitive scarp‐forming faulting in the late Quaternary and the proximity of the Oyabe River and its tributaries to the fault trace suggest that there may be an extension of the Isurugi fault to the northeast and southwest beneath the Tonami Plain that makes the fault long enough to generate a large earthquake (Mw ≥ 6.8) accompanied by surface rupture.     

On the response of yielding seismically isolated structures

This paper investigates the seismic response of yielding isolated structures. To establish a general understanding of the nonlinear response of seismically isolated structures, this study first investigates the nonlinear response of isolated structures subjected to steady‐state harmonic motion and nonlinear transient ground excitation. The response of both viscously damped and hysteretically damped isolation systems is investigated in three phases. Initially, basic insights are gained through simple nonlinear two degrees of freedom (2‐DOF) models subjected to harmonic motion of varying frequencies. Next, the transient response analysis of the nonlinear 2‐DOF model is investigated for a wide range of isolation system and superstructure properties. The results obtained from both approaches indicate that the yielding behavior of a structure on an isolation system is significantly different from that of the comparable fixed‐base structure. Finally, the response of the nonlinear 2‐DOF system model is compared with that of a 15‐story, three‐dimensional model. Based on the results of these analytical investigations, some important considerations for the design of seismically isolated structures are presented. Copyright © 2007 John Wiley & Sons, Ltd.     

Seed supply for coastal brackishwater shrimp farming: environmental impacts and sustainability

Globally, shrimp farming has been a significant agro-based economic activity since the early 1970s. Because it offered a huge immediate economic return, shrimp farming showed a booming expansion and soon became a multimillion dollar industry. However, it has been under extreme criticism because of its devastating ecological and socio-economic impacts. Because seed is the primary input, the impact from farming has started from the source of seed supply, so that not only are natural stocks of shrimp seeds now overexploited worldwide but seed collection activities also significantly reduce stocks of other living resources. Although hatcheries were developed as potential alternative and have replaced the natural seed source to a great extent, large-scale hatchery productions provide a potential source of coastal pollution. However, this area is still poorly studied. The present paper provides a review of the environmental impacts of the wild shrimp seed fishery as well as the possibility of environmental degradation from artificial shrimp seed production in hatcheries.     

New pop-up type Ocean Bottom Seismometer

We designed a new pop-up type Ocean Bottom Seismometer (OBS) in order to study micro-earthquakes in off-shore areas. With a 57 cm O.D. sphere of high tension aluminium alloy, the OBS system, including one vertical and one horizontal geophone, can safely operate on ocean floors of up to 6000 m depth for seismic observations. The amplified seismic data and the time code are directly recorded on the four-channel cassette deck for periods of up to one month. The frequency response curve throughout the recording and play-back system is flat for the range, 1–15 Hz (–3 dB). The anchor release and the geophone clamp are operated by an acoustic command signal.So far, we have deployed our OBS's 42 times in the ocean. All of the OBS's deployed have been recovered safely. Seismic data has provided seismological evidence for a number of processes associated with tectonism along subduction zones and spreading ridges (e.g., Eguchi et al., 1986).     

A simple two-way coupling method of BEM and VOF model for random wave calculations

A numerical method, which combines the boundary element method (BEM) and the volume of the fluid method (VOF method), has been presented to solve wave–structure interactions; the intense wave motion at the proximity of the structure is modeled by the VOF method and the rest of the fluid region is modeled by the BEM. The combined method can considerably reduce the time-consuming VOF domain, and thus practically makes it possible to apply the VOF method for random wave calculations, in which long time computations are usually required to obtain statistically meaningful results, and therefore the use of the single-VOF model often becomes prohibitive in terms of computational time and storage memories. A VOF model CADMAS-SURF, which is based on SMAC scheme and had been constructed by a number of VOF researchers in coastal engineering in Japan, is used in the combined BEM–VOF model. The two-way coupling treatment, which enables us to deal with bidirectional wave propagations, which was originally given for the SOLA-VOF model by Yan et al. (2003a) and later improved by Kim et al. (2007), was modified for the SMAC scheme. The coupling treatments are described in detail in the paper. The validity of the combined BEM–VOF model was investigated by comparing the numerical results with the theoretical results for the propagations of Stokes 5th order waves and random waves.     

Meteorological influences of eddy-resolving ocean assimilation around the cold tongue to the north of the Japanese islands during winter 2004/2005

Use of ocean data assimilation in meteorological applications is expected to reveal the influence of cloud-covered oceanic mesoscale processes on wintertime weather and climate in coastal areas. In particular, eddy-resolving Ocean Circulation Model (OCM) data assimilation that reproduces seasonally persistent oceanic mesoscale eddies is useful when simulating coastal precipitation. In the present study, the OCM-assimilation sea surface temperature (SST) is applied to a long-term atmospheric simulation over the Japan/East Sea area in the 2004/2005 winter season (December–February, DJF), to investigate seasonal and daily influences of oceanic mesoscale eddies on precipitation. The simulated winter precipitation is improved by the OCM assimilation via the DJF evaporation around a cold tongue. The strong intrusion of the southeast-directed cold tongue reduces the degree of overestimation by coastal precipitation simulations in December and January. In contrast, the ocean assimilation barely improves the simulation results in February because of weak intrusion of the cold tongue. In December and January, an abruptly large anomaly of northwesterly surface wind (> 1 m s^?1) resulting from the OCM assimilation often influences 3-hour precipitation in the downstream area of the cold tongue. In contrast, the slowly-varying anomaly of evaporation does not necessarily lead to daily precipitation anomalies, although the DJF evaporation anomaly is important in the DJF precipitation.