The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 118, Issue 5
Displaying 1-6 of 6 articles from this issue
SPECIAL ISSUE The 2011 off the Pacific coast of Tohoku Earthquake: Towards an integrated understanding
Review
  • Why did the magnitude 9 event occur?
    Yoshihisa Iio, Toru Matsuzawa
    2012 Volume 118 Issue 5 Pages 248-277
    Published: May 15, 2012
    Released on J-STAGE: October 05, 2012
    JOURNAL FREE ACCESS
    In order to understand why this event occurred, we review the results of various studies conducted to date, and suggest avenues for future research programs. The first area of research concerns the state in and around the plate boundary fault off Tohoku before the earthquake occurred, with particular focus on the spatio-temporal distribution of aseismic slip on this structure. Second, we investigate preseismic, coseismic, and postseismic slip distributions. Finally, we review several numerical models that have been proposed to explain the generation of the M 9 earthquake on the basis of slip distributions. We conclude that the most significant factor that contributed to the M 9 event was the plate boundary fault near the trench axis had been locked for a long period and/or that dynamic weakening occurred along the fault between the hypocenter and the trench axis. The process that generated the M 9 Tohoku earthquake needs further investigation.
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Articles
  • Takuya Nishimura
    2012 Volume 118 Issue 5 Pages 278-293
    Published: May 15, 2012
    Released on J-STAGE: October 05, 2012
    JOURNAL FREE ACCESS
    We accumulated geodetic triangulation, trilateration, leveling, tide-gauge, and continuous GPS data to clarify the nature and temporal variation of crustal deformation in northeastern Japan for the 120-year period preceding the 2011 Mw 9.0 Tohoku-Oki earthquake. Horizontal, regional strain is characterized by north–south extension and east–west compression for ∼90 and 10 years, respectively. Large compressional strain in an east–west direction was observed in the deformation zone in the central axis of northeastern Japan and along the eastern margin of the Japan Sea. Vertical deformation is the result of subsidence along the Pacific coast. The subsidence rate was roughly constant for 120 years, except for the Pacific side of the Fukushima prefecture where uplift was observed for ∼20 years beginning from 1939. Continuous GPS observations were collected from 1994 and show deceleration of east–west compressional velocity in the Fukushima prefecture around 2000. Interplate coupling estimates suggest that deceleration was caused by weakening of the coupling offshore of Fukushima and Ibaraki prefectures. This implies that the plate interface in part of the source region to the Tohoku-Oki earthquake was unlocked, contributing to the seismic rupture. There is a well-known discrepancy between geodetic deformation and geological deformation; there is also a large temporal variation even within geodetic strain rate. The giant Tohoku-Oki earthquake event did not resolve the discrepancy between strain rates. Ongoing post-seismic deformation may be the key to resolving this discrepancy.
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Review
  • Yasutaka Ikeda, Shinsuke Okada, Masayoshi Tajikara
    2012 Volume 118 Issue 5 Pages 294-312
    Published: May 15, 2012
    Released on J-STAGE: October 05, 2012
    JOURNAL FREE ACCESS
    In this paper, we review the process of strain buildup and release in the Northeast Japan (NEJ) arc on a geologic time scale. There has been a discrepancy between long-term (geologic) and short-term (geodetic) strain observations in both horizontal and vertical directions over the NEJ arc. Since the Pliocene, the NEJ arc has been subjected to east–west compression due to the westward subduction of the Pacific plate at the strongly coupled Kuril-Japan trench. Geodetic observations in the past ∼100 years have revealed strain accumulation over the NEJ arc at a rate as high as 10−7 strain/yr, whereas geologically observed strain rates are one order of magnitude slower. A similar discrepancy exists also in vertical movement; tide gauge records along the Pacific coast have indicated subsidence at rates as high as ∼10 mm/yr over the last ~80 years, despite the fact that late Quaternary marine terraces along the Pacific coast indicate long-term uplift at 0.1–0.3 mm/yr. The ongoing rapid subsidence of the Pacific coast is due to dragging by the subducting Pacific plate beneath the NEJ arc. Thus, most of the strain that has accumulated in the last 100 years at abnormally high rates is elastic, and is to be released by slip on the coupled plate interface. Only a fraction (∼10%) of plate convergence is accommodated within the NEJ arc as long-term (inelastic) deformation. However, large (Mw 7–8) subduction earthquakes that have occurred in the past ∼100 years had nothing to do with strain release or coastal uplift. The 2011 earthquake of Mw 9.0, whose rupture surface encompassed those of previously occurred Mw 7–8 subduction earthquakes, is likely to be such a decoupling event that effectively releases the elastic strain due to plate coupling. The pattern of interseismic subsidence indicates that, at 50–100 km depths down-dip of the 2011 rupture, there still exists a coupled part of plate interface, on which a large amount of aseismic after slip may occur in the coming decades. The NEJ subduction zone is unique in that its decoupling process is two-fold: the decoupling occurs seismically on the shallower interface (at 0–50 km depths) and aseismically on the deeper interface (at 50–100 km depths). A global survey of other subduction zones that produced gigantic (Mw ≥ 9.0) earthquakes suggests no such deep coupling in interseismic periods. A possible cause for the deep coupling is thermal: the oceanic lithosphere of the western Pacific is very old and therefore cold, and it has subducted beneath the NEJ–Kuril arc.
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  • Mitsuhiro Matsu'ura
    2012 Volume 118 Issue 5 Pages 313-322
    Published: May 15, 2012
    Released on J-STAGE: October 05, 2012
    JOURNAL FREE ACCESS
    Earthquake occurrence is the sudden release of tectonically accumulated stress by faulting. In the case of interplate earthquakes, the stress accumulation is caused by the interseismic gradual increase of slip deficits in potential source regions, and so earthquake occurrence can be regarded as the sudden recovery of the slip deficits. Since the crustal deformation due to interseismic slip-deficit increase is detectable by GPS observations, as well as that due to coseismic slip-deficit recovery, we can now monitor the slip-deficit and -recovery processes at plate interfaces through the inversion analysis of GPS array data. On March 11th of 2011, an Mw 9.0 mega-thrust earthquake occurred at the North American–Pacific plate interface off the Tohoku region, Japan. The inversion analysis of GPS data for an interseismic period (June 1996–May 2000) prior to this event has shown that five remarkable slip-deficit zones are distributed on the plate interface along the southern Kuril–Japan trench. On the other hand, the inversion analysis of coseismic GPS data (March 10–11, 2011) shows that the fault slip of the 2011 mega-thrust earthquake has a bimodal distribution with a northern main peak of 25 m and a southern sub peak of 6 m, corresponding to the Miyagi-oki and Fukushima-oki slip-deficit zones, respectively. In the Miyagi-oki slip-deficit zone, M 7.5-class earthquakes with about 3 m of coseismic slip have repeated every 40 years over the past two centuries. The occurrence of extraordinarily large earthquake with 25 m of coseismic slip in the same slip-deficit zone suggests the possibility of scale-dependent multiple earthquake generation cycles, and leads to the conclusion that the so-called asperity is not a physical substance but a concept representing the spatial irregularity in frictional properties of faults.
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Articles
  • Jun Muto, Mako Ohzono
    2012 Volume 118 Issue 5 Pages 323-333
    Published: May 15, 2012
    Released on J-STAGE: October 05, 2012
    JOURNAL FREE ACCESS
    A two-dimensional rheological profile crossing the northeastern Japan arc was created to evaluate and distinguish various styles of post-seismic deformation relating to the 2011 Tohoku-Oki earthquake. The calculated profile is based on recent rock-mechanics studies and geophysical observations that explain observed patterns of geodetic strain accumulation and shallow seismicity prior to the earthquake. Viscosities were then calculated assuming stress-change magnitudes commonly associated with interplate earthquakes. Recently derived flow laws for various rocks and minerals predict the presence of weak zones that developed via processes likely operating in the lithosphere (e.g., partial melting and shear zone development). Strain localization into weak zones explains low viscosity estimates (1018—1020 Pas) from post-seismic creep after inland earthquake events. Our calculations reveal significant lateral variations in strengths and viscosities across the northeastern Japan arc. These viscoelastic lithospheric structures should be taken into account in order to differentiate viscous relaxation from other post-seismic deformation processes.
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