The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Volume 115, Issue 9
Displaying 1-7 of 7 articles from this issue
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Articles
  • Shallow, deep slow earthquake families associated with the subduction of the Philippine Sea plate
    Kazushige Obara
    2009 Volume 115 Issue 9 Pages 437-447
    Published: 2009
    Released on J-STAGE: March 10, 2010
    DOIhttps://doi.org/10.5575/geosoc.115.437
    JOURNAL FREE ACCESS
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    The recent development of a dense seismic observation network in Japan enables the detection of various slow earthquakes around the upper boundary of the Philippine Sea Plate, which is subducting beneath Southwest Japan at the Nankai Trough. At the deeper extension of the seismogenic zone, a narrow belt with a length of 600 km is marked by short-term, slow slip events (representing stick-slip at the plate interface with a duration of several days), deep very-low-frequency earthquakes with a predominant period of 20 seconds, and deep low-frequency tremors with a predominant frequency of around 2 Hz. These areas of deep, slow earthquakes can be divided into segments in which events occur with recurrence intervals of 3 or 6 months. Shallow, very-low-frequency earthquakes with a predominant period of 10 seconds occur within the accretionary prism, landward of the Nankai Trough. Shallow, deep slow earthquakes may be related to deformation of the accretionary prism above the subducting plate or metamorphism and dehydration of the subducting plate at depths of around 30 km.
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  • Afterslip associated with interplate earthquakes and the viscoelastic behavior of rocks
    Yusuke Kawada, Hiroyuki Nagahama, Naoki Uchida, Toru Matsuzawa
    2009 Volume 115 Issue 9 Pages 448-456
    Published: 2009
    Released on J-STAGE: March 10, 2010
    DOIhttps://doi.org/10.5575/geosoc.115.448
    JOURNAL FREE ACCESS
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    Temporal changes in afterslip displacement rate associated with interplate earthquakes are investigated using a constitutive law for the viscoelastic behavior of rocks. The law is based on irreversible thermodynamics with internal state variables and is derived as the relaxation modulus following a temporal power law that emerges as the collective dynamics of internal states with respective relaxation times at various time-scales. The constitutive law can represent the transient behavior in response to sudden changes in stress and strain rate, as well as the steady-state behavior of rocks. Analysis of afterslip (cumulative displacement) time series estimated from the seismic moments of small repeating earthquakes in the northeastern Japan subduction zone shows that the temporal change in afterslip follows a temporal power law. This finding suggests that afterslip comprises many transient responses associated with seismic events of various sizes.
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  • Stress history in the forearc region of the Nankai trough subduction zone: paleostress analysis based on faults in core samples from the Kumano Ichiura and Kihoku Miyama sites, Kii Peninsula, SW Japan
    Makoto Otsubo, Norio Shigematsu, Yuichi Kitagawa, Naoji Koizumi
    2009 Volume 115 Issue 9 Pages 457-469
    Published: 2009
    Released on J-STAGE: March 10, 2010
    DOIhttps://doi.org/10.5575/geosoc.115.457
    JOURNAL FREE ACCESS
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    We determined the stress history in the forearc region of the Nankai trough subduction zone by examinug boreholes at the Miyama and Ichiura observation wells, located on the northern and southern margins of the northern unit of the Kumano Acidic Rocks, respectively, eastern Kii Peninsula, SW Japan. To estimate paleostress, the multiple inverse method was applied to fault planes identified within the core samples; to estimate the present-day stress, the borehole breakout method was used. The results obtained using the multiple inverse method indicate that the two observation wells have similar stress histories, involving E-W compressive stress followed by NE-SW extensional stress and finally N-S compressive stress. The present-day maximum horizontal compressive stress (σhmax) at the Miyama observation well trends E-W, consistent with the present-day regional stress in SW Japan and different from the final stage of stress determined by the multiple inverse method. The present-day σhmax at the Ichiura observation well trends NNE-SSW, similar the final stage of stress determined by the multiple inverse method. These findings indicate that the boundary between the areas of E-W compressive stress (characteristic of SW Japan) and N-S compressive stress (influenced by subduction of the Philippine Sea Plate) has gradually migrated southward over time.
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  • Mud diapirs and associated intrusion structures in the Miocene Tanabe Group
    Yuichiro Miyata, Kunihiko Miyake, Kazuhiro Tanaka
    2009 Volume 115 Issue 9 Pages 470-482
    Published: 2009
    Released on J-STAGE: March 10, 2010
    DOIhttps://doi.org/10.5575/geosoc.115.470
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    We examined the material properties and intrusive structures of several mud diapirs in the Miocene Tanabe Group with the aim of determining the style and process of fluid intrusion. Tank experiments were performed to examine the factors controlling the intrusive. Four types of intrusive structures (cylinder, dome, sill, and associated dikes) were observed in ascending stratigraphic order in the Shirahama Formation, overlying the muddy Asso Formation of the Tanabe Group: (1) cylinder type: the Ichieminami mud diapir, about 20 m in diameter, intruded near-vertically into bedded sand and siltstones; (2) dome type: the dome-shaped Ichiezaki mud diapir is about 150 m in diameter and contains blocks and sand grains of the host sediments, incorporated by stoping; dikes intruding radially into the host rocks are clayey in the early stage and sandy in the later stage; (3) sill type: the Migusa mud diapir represents a lenticular laccolith of at least 200 m in length, consisting mainly of pebbly mudstone containing blocks of surrounding strata; and (4) associated dikes: these diapirs are commonly accompanied by mudstone dikes. The tank experiments produced a series of lenticular, intrusive mud-slurry bodies on the top of the vertical conduit during dome-like upheaval, transforming into a mud chamber that grew in size with stoping of the roof sediments. When the chamber collapsed, a conduit of upward-escaping fluids, and sill and dike structures, formed above the chamber. This style of intrusion is comparable with that observed in the Tanabe Group. Consequently, we interpret that mud diapirs of the Tanabe Group show a variety of intrusive structures resulting from a single event in which high-pressure fluid with a minor mud component was injected through a narrow conduit to the level where successive, large mud chambers began to form. The nature of intrusive structures is determined by the permeability contrast between layers and by the degree of solidification of the host sediments.
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Report
  • Outline of the Nankai Trough Seismogenic Zone Experiment
    Shin’ichi Kuramoto
    2009 Volume 115 Issue 9 Pages 483-487
    Published: 2009
    Released on J-STAGE: March 10, 2010
    DOIhttps://doi.org/10.5575/geosoc.115.483
    JOURNAL FREE ACCESS
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    This report introduces the scientific drilling project “NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment)” proposed for the IODP (Integrated Ocean Drilling Program). NanTroSEIZE is a large, international, multidisciplinary scientific project that required long-term planning and implementation. The project involved almost two decades of planning and may continue for another decade while carrying out drilling, coring, and long-term borehole monitoring. This large scientific project has now officially started, and we expect that the resulting scientific findings will be of international significance.
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Review
  • Overview of scientific results of the deep drilling project of the Chelungpu Fault, upon which the 1999 Chi-Chi (Taiwan) earthquake occurred: Main findings and future research
    Wonn Soh, Wataru Tanikawa, Takehiro Hirose, Weiren Lin, Masaharu Tanim ...
    2009 Volume 115 Issue 9 Pages 488-500
    Published: 2009
    Released on J-STAGE: March 10, 2010
    DOIhttps://doi.org/10.5575/geosoc.115.488
    JOURNAL FREE ACCESS
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    The Chelungpu Fault, upon which the 1999 Chi-Chi (Taiwan) earthquake (Mw 7.6) occurred, records significant differences in coseismic slip behavior between its northern and southern segments, even for a single seismic event. To understand these differences, we investigated the process of fault rupture propagation, including fault-weakening mechanisms that act to reduce the dynamic fault strength and differences in the in situ physico-chemical environment between the northern and southern segments. To this end, the Taiwan Chelungpu-Fault Drilling Project (TCDP) was begun in 2003, resulting in successful borehole logging and analysis of core samples from Hole A (2000 meter below surface (mbs)) and Hole B (1350 mbs). In this paper, we review the recent scientific results of TCDP, particularly in terms of analyses of core samples, with the aim of gaining a better understanding of the physico-chemical environment of the fault, which is likely to be strongly related to the mechanism of dynamic fault weakening and fault healing. We also propose a new model for the mechanism of fault rupture propagation and for the geochemical response of the fault rocks to dynamic fault motion during the rupture and post-rupture stages.
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