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Imanishi K.,Geological Survey of Japan | Kuwahara Y.,Geological Survey of Japan | Takeda T.,Japan National Research Institute for Earth Science and Disaster Prevention | Mizuno T.,Geological Survey of Japan | And 8 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2011

Focal mechanisms have been determined from P wave polarity data as well as body wave amplitudes for 154 microearthquakes that occurred around the Atotsugawa fault in central Japan between 2002 and 2004. While we found many microearthquakes with a pure strike-slip mechanism that is similar to the faulting style of the Atotsugawa fault, a considerable number of microearthquakes with reverse faulting components are also occurring. Most of the P axes are horizontal and oriented in the WNW-ESE direction, which conforms to the general tectonic trend in this area. In contrast, the T axes have a wide range of plunge, suggesting that reverse-faulting-type earthquakes as well as strike-slip ones are occurring. The most conspicuous feature in the focal mechanism distribution is the depth dependence, where shallow earthquakes are primarily reverse faulting and strike-slip earthquakes become predominant with depth. A stress tensor inversion reveals that the shallower part is characterized by a mixture of reverse and strike-slip faulting regimes and that a pure strike-slip faulting regime appears only around the bottom of the seismogenic zone. Together with other geophysical observational evidence for the fault, we suggest that the existence of a localized aseismic deformation below the Atotsugawa fault is the simplest scenario that can explain the observed stress fields. This scenario provides a stress accumulation mechanism of disastrous shallow inland earthquakes, in which the localized aseismic deformation accumulates stress onto the fault plane in the seismogenic zone during an earthquake cycle and the main shock would occur when the failure stress is reached on the fault. Copyright 2011 by the American Geophysical Union.

Tajikara M.,Association for the Development of Earthquake Prediction | Yasue K.,Japan Atomic Energy Agency | Yanagida M.,Hanshin Consultants Co. | Furusawa A.,Geological Survey of Japan | And 3 more authors.
Geographical Review of Japan | Year: 2011

Many papers reported that Quaternary climate and sea-level fluctuations controlled riverbed elevation in the river basins in northeastern Japan. However, in the southern part of the Chubu region of Japan, such climatic-controlled riverbed fluctuations have not been reported, except in a few papers based on uncertain chronological data. In this research, we investigated fluvial terraces along the Tokigawa (Shonaigawa) River that flows through the low-relief mountainous areas in the northernmost part of the Mikawa Highlands, in the southern part of the Chubu region of Japan, and examined whether riverbed fluctuations similar to those in rivers in northeastern Japan occurred in the Tokigawa River basin. We mapped fluvial terraces based on air photo analysis and inferred the age and climate at the time of formation of these terraces based on 14C dating, tephra analysis, and pollen analysis. Fluvial terraces in the Tokigawa River basin were classified into 10 treads (H1-4 surfaces, M1-3 surfaces, L1-3 surfaces). We estimated that the L2 terrace was a depositional terrace formed during marine oxygen isotope stage (MIS) 2, based on 14C age, marker tephra (Kikai Akahoya tephra) in the soil layer covering terrace deposits, distribution pattern and morphology of terrace surfaces, and thickness of terrace deposits. The M2 terrace was correlated to MIS 6 based on the relationship between the marker tephra (Aso-4 tephra, Kikai Tozurahara tephra) and terrace deposits, existence of red weathered soil, and distribution pattern of the M2 terrace. Based on these results, we concluded that the fluvial terraces in the Tokigawa River basin were formed as a consequence of riverbed fluctuations linked to climate change. We estimated uplift rates in the Tokigawa River basin at 0.11-0.16 mm/year based on the elevation difference between M2 and L1 terrace surfaces.

Yamada S.,Nagoya University | Noda T.,Nagoya University | Asaoka A.,Association for the Development of Earthquake Prediction | Shina T.,Penta Ocean Construction Co.
Geotechnical Engineering | Year: 2013

In this study, the influence of brittle property of geomaterials on the failure behavior of the ground in an undrained bearing capacity problem was investigated numerically from the standpoint of taking the brittle behavior of cement treated soil as softening behavior of the soil element. The numerical analyses were performed using the soil-water coupled finite deformation analysis code GEOASIA mounted with the SYS Cam-clay model, which describes the soil skeleton structure at work. Cement treated ground and naturally deposited clay ground were modelled and compared, and it was found that they showed widely differing failure processes depending on differing initial conditions. Especially, it was found that when progressive failure in which strain localization region develops due to propagation of material failure occurs, even though the ground is composed of brittle materials such as cement treated soil, those brittle properties do not directly manifest in the load-settlement relationship. Additionally, the investigation revealed that, since every soil element on the slip lines does not reach its peak strength simultaneously when progressive failure occurs, post-peak material properties, i.e. the ratio of residual strength to peak strength and softening rate from peak to residual state, affect the bearing capacity of the ground.

Kimura T.,Japan National Research Institute for Earth Science and Disaster Prevention | Murakami H.,Association for the Development of Earthquake Prediction | Matsumoto T.,Japan National Research Institute for Earth Science and Disaster Prevention
Earth, Planets and Space | Year: 2015

Background: Broadband seismometer data are essential for the development of seismological studies such as those investigating earthquake sources and the Earth's structure. However, previous studies have revealed that the metadata describing these data can possibly be contaminated by instrumentation response errors that are often difficult to recognize from visual waveform checks. Herein, we report on the development of a systematic method of assessing seismometer conditions when recording ground motions at a period range of 50 to 200 s in observation networks whose station intervals are smaller than 200 km. Findings: The method is based on comparisons between teleseismic surface wave records at a target station and those at multiple surrounding reference stations, from which we calculate three index parameters and evaluate in situ instrumentation conditions, including amplitude and phase responses against input ground motions. In our experiments, we applied the proposed method to F-net broadband seismometers covering the Japanese Islands, where station intervals are approximately 100 km. This allows us, through calculations of the index parameters, to evaluate instrumentation health at each station at least once every 60 days. Using our proposed method, we found that approximately 75% of the evaluated index parameters distributed well around the standard values, and for most examined broadband seismometers, response anomalies are not detected at the period range of 50 to 200 s. However, instrumentation errors, such as gain decrease over the evaluated periods and gradual changes in amplitude and phase frequency responses (sometimes covering several years) were identified at a few stations. Additionally, overdamping errors at the STS-1 seismometers, which experience significant amplitude and phase frequency response variations around the 360-s corner, appear to have been common at several stations. In contrast, STS-2 seismometers appear to have functioned more reliably than STS-1 seismometers. Conclusions: We developed a method to evaluate broadband seismometer instrument conditions by comparing teleseismic surface waves observed at a target station with those at multiple surrounding stations. It is believed that the systematic evaluation of instrumentation health using our method will enhance the operation of seismic networks, and allow researchers to eliminate contaminated data before conducting various data analyses. © 2015 Kimura et al.; licensee Springer.

Heidarzadeh M.,Port and Airport Research Institute | Heidarzadeh M.,University of Tokyo | Harada T.,University of Tokyo | Satake K.,University of Tokyo | And 2 more authors.
Geophysical Research Letters | Year: 2016

The July 2015 Mw 7.0 Solomon Islands tsunamigenic earthquake occurred ~40 km north of the February 2013 Mw 8.0 Santa Cruz earthquake. The proximity of the two epicenters provided unique opportunities for a comparative study of their source mechanisms and tsunami generation. The 2013 earthquake was an interplate event having a thrust focal mechanism at a depth of 30 km while the 2015 event was a normal-fault earthquake occurring at a shallow depth of 10 km in the overriding Pacific Plate. A combined use of tsunami and teleseismic data from the 2015 event revealed the north dipping fault plane and a rupture velocity of 3.6 km/s. Stress transfer analysis revealed that the 2015 earthquake occurred in a region with increased Coulomb stress following the 2013 earthquake. Spectral deconvolution, assuming the 2015 tsunami as empirical Green's function, indicated the source periods of the 2013 Santa Cruz tsunami as 10 and 22 min. © 2016. American Geophysical Union. All Rights Reserved.

Ichimura T.,University of Tokyo | Tanaka S.,University of Tsukuba | Hori M.,University of Tsukuba | Yamamoto Y.,University of Tokyo | And 4 more authors.
Journal of Earthquake and Tsunami | Year: 2016

To enhance the reliability of estimates of seismic behavior, with a special emphasis on quality assurance of numerical simulations, this paper presents a full three-dimensional (3D) seismic response analysis of a large underground structure with a complex cross section. We conduct a full 3D seismic-response analysis using a high-fidelity model with quality assurance and a high-performance computing technique in a supercomputer environment. Due to the large computational costs of such analyses, we propose a two-step numerical simulation method based on multi-scale analysis, image-based modeling, and sophisticated approximation techniques. We provide numerical examples showing that the method can successfully reproduce the dynamic behavior of a full 3D model, but with a considerable reduction in computational costs. Our method is expected to support practical seismic safety inspections and the design of underground structures with complex large-cross-sectional configurations, as it has already been applied to the practical seismic safety inspection of an actual underground highway. © 2016 World Scientific Publishing Company

Kurita K.,Tokyo Metropolitan College of Industrial Technology | Kinugasa Y.,Association for the Development of Earthquake Prediction | Deguchi T.,Nittetsu Mining Consultants Co. | Rimando R.E.,Institute of Volcanology and Seismology
Advances in Civil Engineering and Building Materials - Selected Peer Reviewed Papers from 2012 2nd International Conference on Civil Engineering and Building Materials, CEBM 2012 | Year: 2012

Buildings and pavements are damaged by ground vertical displacement along the creeping segment of the Marikina Valley fault system (MVFS) on the southern part of Metro Manila, Philippines. To clarify fault creep trigger (e.g., groundwater withdrawal, tectonic), deformation is monitored through repeated leveling surveys. Any short term link between fault displacement and rainfall is also monitored through a continuous creep measurement device installed across one of the sites used for leveling survey. From repeated leveling surveys, the average slip rate since September 1999 to January 2010 ranges from 1.77 cm/y to 2.45 cm/y. Creep acceleration since 2004 is observed in some leveling sites. Continuous creep measurement obtained an average slip rate of 0.01 mm/day (3.65 mm/y) until January 2009. However fault creep at NPC B appears to have stopped even with the coming of the rainy season in May 2009. © 2013 Taylor & Francis Group, London.

Noda T.,Nagoya University | Xu B.,Nagoya University | Asaoka A.,Association for the Development of Earthquake Prediction
Soils and Foundations | Year: 2013

Based on a soil-water coupled finite deformation analysis, theoretical considerations and numerical calculations were carried out under the undrained plane strain condition in order to reproduce a uniform deformation field. Rather than the "quasi-static" equation of motion, which does not include inertia forces, a dynamic equation of motion which includes inertia forces was used. At first, a theoretical consideration was carried out to realize uniform deformation for a saturated soil that satisfied the element-wise undrained/constant-volume condition. This presents an "infinitely slow loading" case without ignoring the inertia term based on the u-p formulation. In other words, it can be seen that under general slow loading that is not infinitely slow, a gradient in the pore water pressure will always be produced, resulting in the migration of pore water and loss/collapse of uniformity. This first conclusion is useful for verifying numerical analysis code made in the finite deformation regime. Next, the uniform deformation of a plane strain rectangular soil specimen was measured under constant cell pressure and undrained boundary conditions using a dynamic soil-water coupled analysis in which the SYS Cam-clay model was employed as the elasto-plastic constitutive model for the soil skeleton. In addition, the effects of the loading rates as well as loading applications, with/without inertia forces, on the loss of uniformity in deformation were shown to have a significant influence on the inertia term even though the loss itself was extremely small. © 2013 The Japanese Geotechnical Society.

Noda T.,Nagoya University | Xu B.,Nagoya University | Asaoka A.,Association for the Development of Earthquake Prediction
Soils and Foundations | Year: 2013

In the conventional bifurcation and strain localization analyses of geomaterials, the inertia forces are generally ignored, based on the quasistatic equilibrium equation. Even though a great deal of literature exists on dynamic strain localization analyses, information on acceleration generation during the formation of shear bands has not been emphasized. Inspired by the acoustic emission phenomenon in laboratory tests and the seismic acceleration related to the slippage of faults, a dynamic soil-water coupled strain localization analysis is performed in the present paper on a saturated rectangular clay specimen subjected to constant cell pressure under plane strain conditions, employing the SYS Cam-clay model as the elasto-plastic constitutive model for the soil skeleton. An initial geometrical imperfection was introduced to the specimen to trigger one single shear band, and the following results were found: (1) Two types of oscillation occurred within the specimen during acceleration when the specimen was subjected to compression deformation at a constant rate, namely, (a) one caused by the sudden external compression and (b) the second induced by the formation of strain localization/a shear band. With the occurrence of the shear band, if, for example, the vertical rate was equivalent to about 10 cm/s, the accelerations that occurred within the specimen were in the order of several thousand gal, which is similar to those measured during earthquakes; (2) The effects of the time increment, the mesh division, the initial confining pressure, the OCR and the stress-control loading on the generated acceleration in (b) were investigated in detail. It was found that under stress control, even though the formation of the shear band was similar to that under displacement control, the induced acceleration behaved quite differently. & 2013 The Japanese Geotechnical Society. Production and hosting by Elsevier B.V. All rights reserved.

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