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31.
Satoshi Itaba Naoji Koizumi Norio Matsumoto Ryu Ohtani 《Pure and Applied Geophysics》2010,167(8-9):1105-1114
In 2006, we started construction of an observation network of 12 stations in and around Shikoku and the Kii Peninsula to conduct research for forecasting Tonankai and Nankai earthquakes. The purpose of the network is to clarify the mechanism of past preseismic groundwater changes and crustal deformation related to Tonankai and Nankai earthquakes. Construction of the network of 12 stations was completed in January 2009. Work on two stations, Hongu-Mikoshi (HGM) and Ichiura (ICU), was finished earlier and they began observations in 2007. These two stations detected strain changes caused by the slow-slip events on the plate boundary in June 2008, although related changes in groundwater levels were not clearly recognized. 相似文献
32.
The purpose of this paper is to present a parameter identification method to determine the force of a blast and the elastic modulus of the ground using the measurements of a dynamic elastic wave, the adjoint equation method of optimal control theory, and the finite element method. Before the excavation of rocky ground, it is important to estimate the ground properties. In this paper, the elastic modulus is determined as the performance function is minimized using a technique based on the first‐order adjoint method. The performance function is a square sum of the discrepancies between the computed and the observed values of the velocities. After the determination of the magnitude of the blasting force, we can determine the elastic modulus of the rock. As the basic equation to calculate the velocities of dynamic elastic body, elastic equilibrium equations with linear viscosity are employed. The adjoint equation method has been utilized in order to calculate the gradient of the performance function with respect to the parameters. The gradient of the performance function is calculated using the first‐order adjoint equation. The weighted gradient method is applied for minimization. In order to solve the state equations in space and time, the finite element method and the Newmark $\frac{1}{4}$ method are used. In this paper, we tested the practical application of our proposed method for determination of the elastic modulus of rock at the Ikawa tunnel located in the Tokushima prefecture, Japan. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
33.
The Dogo hot spring, situated in Matsuyama City, Ehime Prefecture, Japan, is one of the oldest and most famous hot springs
in Japan. The groundwater level or discharge at the spring decreased four times during the past eight or nine Nankai earthquakes.
These are large interplate earthquakes that have occurred repeatedly in the western part of the Nankai Trough at intervals
of 100–200 years since A.D. 684. To clarify the mechanism of these earthquake-related changes in the water level at the spring,
we analyzed groundwater-level data recorded at the spring immediately after the 1946 Nankai earthquake and over the period
from 1985 to 2006. We detected the other nine postseismic increases in groundwater level and no decreases, except for a large
decrease of 11.4 m related to the 1946 Nankai earthquake. The increases were probably caused by ground-shaking, while the
decrease was caused by a change in coseismic volumetric strain. These results lead to the following explanation of the recorded
earthquake-related changes in the groundwater level at the Dogo hot spring. Both coseismic changes in volumetric strain and
ground-shaking can lead to postseismic changes in groundwater pressure. The increase in groundwater pressure arising from
ground-shaking is generally greater than the change in pressure associated with changes in coseismic volumetric strain; however,
at the time of the Nankai earthquakes, the spring experiences a large increase in coseismic volumetric strain, leading to
a considerably larger decrease in the groundwater level than the increase associated with ground-shaking. Therefore, the groundwater
level at the Dogo hot spring usually increases at times of relatively large earthquakes, although the groundwater level or
discharge decreases in the case of the Nankai earthquakes. 相似文献
34.
Yuichi Kitagawa Naoji Koizumi Ryu Ohtani Kunihiko Watanabe Satoshi Itaba 《Pure and Applied Geophysics》2006,163(4):657-673
To understand the detailed process of fault activity, aseismic slip may play a crucial role. Aseismic slip of inland faults
in Japan is not well known, except for that related to the Atotsugawa fault. To know whether aseismic slip does not occur,
or is merely not detected, is an important question. The National Institute of Advanced Industrial Science and Technology
constructed an observation site near Yasutomi fault, a part of the Yamasaki fault system, and has collected data on the crustal
strain field, groundwater pressures, and crustal movement using GPS. In a departure from the long-term trend, a transient
change of the crustal strain field lasting a few months was recorded. It indicated the possibility of
an aseismic slip event. Furthermore, analyses of data from the extensometers at Yasutomi and Osawa observation vaults of Kyoto
University, as well as GPS data from the Geographical Survey Institute (GEONET), revealed unsteady crustal strain changes.
All data could be explained by local, left-lateral, aseismic slip of the order of 1 mm in the shallow part of the Yasutomi
fault. 相似文献
35.
Akira Ishiwatari Yuki Yanagida Yi‐Bing Li Teruaki Ishii Satoru Haraguchi Kazuto Koizumi Yuji Ichiyama Masaru Umeka 《Island Arc》2006,15(1):102-118
Abstract During the Hakuho‐Maru KH03‐3 cruise and the Tansei‐Maru KT04‐28 cruise, more than 1000 rock samples were dredged from several localities over the Hahajima Seamount, a northwest–southeast elongated, rectangular massif, 60 km × 30 km in size, with a flat top approximately 1100 m deep. The rocks included almost every lithology commonly observed among the on‐land ophiolite outcrops. Volcanic rocks included mid‐oceanic ridge basalt (MORB)‐like tholeiitic basalt and dolerite, calc‐alkaline basalt and andesite, boninite, high‐Mg adakitic andesite, dacite, and minor rhyolite. Gabbroic rocks included troctolite, olivine gabbro, olivine gabbronorite (with inverted pigeonite), gabbro, gabbronorite, norite, and hornblende gabbro, and showed both MORB‐type and island arc‐type mineralogies. Ultramafic rocks were mainly depleted mantle harzburgite (spinel Cr? 50–80) and its serpentinized varieties, with some cumulate dunite, wehrlite and pyroxenites. This rock assemblage suggests a supra‐subduction zone origin for the Hahajima Seamount. Compilation of the available dredge data indicated that the ultramafic rocks occur in the two northeast–southwest‐oriented belts on the seamount, where serpentinite breccia and gabbro breccia have also developed, but the other areas are free from ultramafic rocks. Although many conical serpentinite seamounts 10 km in size are aligned along the Izu–Ogasawara (Bonin)–Mariana forearc, the Hahajima Seamount may be better interpreted as a fault‐bounded, uplifted massif composed of ophiolitic thrust sheets, resembling the Izki block of the Oman ophiolite in its shape and size. The ubiquitous roundness of the dredged rocks and their thin Mn coating (<2 mm) suggest that the Hahajima Seamount was uplifted above sealevel and wave‐eroded, like the present Macquarie Is., a rare example of ophiolite exposure in an oceanic setting. The Ogasawara Plateau on the Pacific Plate is adjacent to the east of the Hahajima Seamount, and collision and subduction of the plateau may have caused uplift of the forearc ophiolite body. 相似文献