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      Principal horizontal stress orientations prior to the 2011 Mw9.0 Tohoku-Oki, Japan, earthquake in its source area : STRESS BEFORE THE TOHOKU-OKI EARTHQUAKE

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          Coseismic and postseismic slip of the 2011 magnitude-9 Tohoku-Oki earthquake.

          Most large earthquakes occur along an oceanic trench, where an oceanic plate subducts beneath a continental plate. Massive earthquakes with a moment magnitude, M(w), of nine have been known to occur in only a few areas, including Chile, Alaska, Kamchatka and Sumatra. No historical records exist of a M(w) = 9 earthquake along the Japan trench, where the Pacific plate subducts beneath the Okhotsk plate, with the possible exception of the ad 869 Jogan earthquake, the magnitude of which has not been well constrained. However, the strain accumulation rate estimated there from recent geodetic observations is much higher than the average strain rate released in previous interplate earthquakes. This finding raises the question of how such areas release the accumulated strain. A megathrust earthquake with M(w) = 9.0 (hereafter referred to as the Tohoku-Oki earthquake) occurred on 11 March 2011, rupturing the plate boundary off the Pacific coast of northeastern Japan. Here we report the distributions of the coseismic slip and postseismic slip as determined from ground displacement detected using a network based on the Global Positioning System. The coseismic slip area extends approximately 400 km along the Japan trench, matching the area of the pre-seismic locked zone. The afterslip has begun to overlap the coseismic slip area and extends into the surrounding region. In particular, the afterslip area reached a depth of approximately 100 km, with M(w) = 8.3, on 25 March 2011. Because the Tohoku-Oki earthquake released the strain accumulated for several hundred years, the paradox of the strain budget imbalance may be partly resolved. This earthquake reminds us of the potential for M(w) ≈ 9 earthquakes to occur along other trench systems, even if no past evidence of such events exists. Therefore, it is imperative that strain accumulation be monitored using a space geodetic technique to assess earthquake potential.
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            Shallow dynamic overshoot and energetic deep rupture in the 2011 Mw 9.0 Tohoku-Oki earthquake.

            Strong spatial variation of rupture characteristics in the moment magnitude (M(w)) 9.0 Tohoku-Oki megathrust earthquake controlled both the strength of shaking and the size of the tsunami that followed. Finite-source imaging reveals that the rupture consisted of a small initial phase, deep rupture for up to 40 seconds, extensive shallow rupture at 60 to 70 seconds, and continuing deep rupture lasting more than 100 seconds. A combination of a shallow dipping fault and a compliant hanging wall may have enabled large shallow slip near the trench. Normal faulting aftershocks in the area of high slip suggest dynamic overshoot on the fault. Despite prodigious total slip, shallower parts of the rupture weakly radiated at high frequencies, whereas deeper parts of the rupture radiated strongly at high frequencies.
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              Displacement above the hypocenter of the 2011 Tohoku-Oki earthquake.

              The moment magnitude (M(w)) = 9.0 2011 Tohoku-Oki mega-thrust earthquake occurred off the coast of northeastern Japan. Combining Global Positioning System (GPS) and acoustic data, we detected very large sea-floor movements associated with this event directly above the focal region. An area with more than 20 meters of horizontal displacement, that is, four times larger than those detected on land, stretches several tens of kilometers long along the trench; the largest amount reaches about 24 meters toward east-southeast just above the hypocenter. Furthermore, nearly 3 meters of vertical uplift occurred, contrary to observed terrestrial subsidence.
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                Author and article information

                Journal
                Geophysical Research Letters
                Geophys. Res. Lett.
                American Geophysical Union (AGU)
                00948276
                April 2011
                April 2011
                September 15 2011
                : 38
                : 7
                : n/a
                Affiliations
                [1 ]Kochi Institute for Core Sample Research; Japan Agency for Marine-Earth Science and Technology; Nankoku Japan
                [2 ]Geology Course, Graduate School of Arts and Sciences; Kochi University; Kochi Japan
                [3 ]Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education; China University of Geosciences; Wuhan China
                [4 ]Institute for Research on Earth Evolution; Japan Agency for Marine-Earth Science and Technology; Yokosuka Japan
                [5 ]Center for Deep Earth Exploration; Japan Agency for Marine-Earth Science and Technology; Yokohama Japan
                [6 ]Department of Applied Science; Kochi University; Kochi Japan
                Article
                10.1029/2011GL049097
                e5df3df1-1c5d-4131-a306-4d3c61f69e95
                © 2011

                http://doi.wiley.com/10.1002/tdm_license_1.1

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