Time filter

Source Type

Nishi-Tokyo-shi, Japan

Chigira M.,Kyoto University | Tsou C.-Y.,Kyoto University | Matsushi Y.,Kyoto University | Hiraishi N.,Fukada Geological Institute | Matsuzawa M.,Public works research institute
Geomorphology | Year: 2013

Typhoon Talas crossed the Japanese Islands between 2 and 5 September 2011, causing more than 70 deep-seated catastrophic landslides in a Jurassic to Paleogene-lower Miocene accretion complex. Detailed examination of the topographic features of 10 large landslides before the event, recorded on 1-m DEMs based on airborne laser scanner surveys, showed that all landslides had small scarps near their future crowns prior to the slide, and one landslide had linear depressions along its future crown as precursor topographic features. These scarps and linear depressions were caused by gravitational slope deformation that preceded the catastrophic failure. Although the scarps may have been enlarged by degradation, their sizes relative to the whole slopes suggest that minimal slope deformation had occurred in the period immediately before the catastrophic failure. The scarp ratio, defined as the ratio of length of a scarp to that of the whole slope both measured along the slope line, ranged from 5% to 21%. Careful examination of aerial photographs from another four large landslides, for which no high-resolution DEMs were available, suggested that they also developed scarps at their heads beforehand. Twelve of the 14 landslides we surveyed in the field had sliding surfaces with wedge-shaped discontinuities that consisted of faults and bedding, suggesting that the buildup of pore pressure occurs readily on wedge-shaped discontinuities in a gravitationally deformed rock body. Most of the faults were undulatory and were probably thrust faults that formed during accretion. Other types of gravitational deformation were also active; e.g., flexural toppling and buckling were observed to have preceded one landslide. © 2013 Elsevier B.V. Source

Kawamura K.,Fukada Geological Institute | Kawamura K.,Japan Agency for Marine - Earth Science and Technology | Sasaki T.,University of Tokyo | Sasaki T.,Ocean Engineering and Development Co. | And 3 more authors.
Geophysical Research Letters | Year: 2012

We describe in detail possible large submarine landslides, several tens of kilometers in length and width, on the trench landward slope of the Japan Trench on the basis of high-resolution topographic surveys and detailed seafloor observations. These slides stopped at the toe of the trench slope. After initial movement of the toe along a basal decollement or thrust of the trench landward slope wedge during an earthquake, the basal frictional condition(s) might change drastically from static to dynamic, thus reducing the frictional strength. As a result, rapid submarine landsliding push downward on the toe, generating large horizontal displacements for tsunamis. This hypothesis should explain suitably the relation between large displacement of the thrust fault and tsunami generation by the 2011 Tohoku earthquake as well as tsunami generation by the 1896 Tohoku earthquake. Copyright 2012 by the American Geophysical Union. Source

Haraguchi S.,University of Tokyo | Ishii T.,Fukada Geological Institute | Kimura J.-I.,Japan Agency for Marine - Earth Science and Technology | Kato Y.,University of Tokyo
Contributions to Mineralogy and Petrology | Year: 2012

The northern Kyushu-Palau Ridge (KPR), remnant conjugate arc of the Izu-Ogasawara (Bonin)-Mariana (IBM) active arc, is dominated by basalt-andesite except for the Komahashi-Daini Seamount where acidic plutonic rocks of 38 Ma were recovered. These mafic to intermediate volcanics are produced by the rifting volcanism in the proto-IBM arc associated with spreading of the Shikoku Basin. The HFSE and HREE contents and ratios of these volcanics indicate enriched source mantle composition compared to recent volcanic front. The LILE ratios exhibit similar characteristics to reararc volcanism of the recent Izu arc, and some enriched volcanics exhibit high abundance of sediment melt inputs. Based on these observations and compilations of the published data set, the replacement event of the wedge mantle under the IBM arc occurred two times. The first event occurred between 45 and 38 Ma, with Pacific type mantle being replaced by depleted Indian type mantle. The second event occurred between 36 and 25 Ma, enriched mantle flowed from reararc side. The slab component during the proto-IBM arc rifting was a similar characteristic to recent reararc volcanism of the Izu arc, and sediment melt added in a local area. © 2011 Springer-Verlag. Source

Kitahashi T.,University of Tokyo | Kawamura K.,Fukada Geological Institute | Kawamura K.,Yamaguchi University | Kojima S.,University of Tokyo | Shimanaga M.,Kumamoto University
Deep-Sea Research Part I: Oceanographic Research Papers | Year: 2013

We investigated the genus diversity and community composition of harpacticoid copepods and their relationship with environmental factors around the Kuril Trench (490-7090. m). Harpacticoid densities did not decrease with water depth and were highest at 1000. m water depth. Diversity values based on the number of genera, Shannon diversity and the expected number of genera (rarefaction) indicated unimodal patterns with water depth, with peaks at intermediate depth; genus evenness increased with water depth and slightly decreased at hadal depths. This result suggested that the general relationship between water depth and diversity described for macrofauna and megafauna could be extended to meiofauna across all depth ranges. However, the regulating factor that affects harpacticoid diversity could not be identified. The community composition of harpacticoids gradually changed with water depth (from bathyal to hadal depths). In addition, comparison of assemblages between the trench slope, trench floor and abyssal plain suggested that the community found at hadal depth was largely different from those found on the trench slope and abyssal plain. Multivariate analyses suggested that water depth, or certain factors associated with water depth, affects harpacticoid assemblages around the Kuril Trench. © 2013 Elsevier Ltd. Source

Kamemura K.,Fukada Geological Institute
ISRM International Symposium - 8th Asian Rock Mechanics Symposium, ARMS 2014 | Year: 2014

In Japan, a lot of underground structures have been constructed in spite of complex and poor geological conditions and are now in use. On the other hand, an aseismic design to prevent the severe accident of structures is very important in Japan. Though, an aseismic design of structures constructed in rock has not been well discussed up to now. This is because we have had a tacit understanding that a seismic vibration is small enough in the deep underground and underground structures do not respond like structures which have been constructed on the ground surface. However, in the recent earthquakes which had caused a lot of structural damage, some cases where tunnel structures had been damaged had been also reported. So, the aseismic design for the structures constructed in rock mass should be discussed as an important and urgent problem, especially for the new structures which are going to utilise various properties of rock mass. They are high-level radio-active waste disposal facility, and underground storage facility for oil and gas. If these underground facilities are damaged and lose their functions by great earthquake, a social influence is too serious. In this paper, the present situation of static and aseismic design for underground structures ranging from cut-and-cover tunnel in a shallow ground to mountain tunnel in the deep rock mass is reviewed. And problems that should be discussed to establish a proper method of aseismic design are presented. © 2014 by Japanese Committee for Rock Mechanics. Source

Discover hidden collaborations