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      Expedition 343/343T summary

      Expedition 343/343T Scientists

      Proceedings of the IODP

      Integrated Ocean Drilling Program

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          Abstract

          The main science goal of the Japan Trench Fast Drilling Project (JFAST) is to understand the physical mechanisms and dynamics of large slip earthquakes, which is fundamental to understanding the huge tsunami that caused extensive damage during the 2011 Tohoku-oki earthquake. Specifically, the level of frictional stress during the earthquake rupture and the physical characteristics of the fault zone are investigated through drilling. The objectives of JFAST include locating the fault that ruptured during the Tohoku-oki event using logging while drilling (LWD); characterizing the composition, architecture, and fundamental mechanisms of dynamic frictional slip and healing processes along the fault by taking core samples; and estimating the frictional heat and stress within and around the fault zone by placing a temperature measurement observatory across the fault.

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          Geologically current plate motions

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            The 2011 magnitude 9.0 Tohoku-Oki earthquake: mosaicking the megathrust from seconds to centuries.

            Geophysical observations from the 2011 moment magnitude (M(w)) 9.0 Tohoku-Oki, Japan earthquake allow exploration of a rare large event along a subduction megathrust. Models for this event indicate that the distribution of coseismic fault slip exceeded 50 meters in places. Sources of high-frequency seismic waves delineate the edges of the deepest portions of coseismic slip and do not simply correlate with the locations of peak slip. Relative to the M(w) 8.8 2010 Maule, Chile earthquake, the Tohoku-Oki earthquake was deficient in high-frequency seismic radiation--a difference that we attribute to its relatively shallow depth. Estimates of total fault slip and surface secular strain accumulation on millennial time scales suggest the need to consider the potential for a future large earthquake just south of this event.
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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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                Author and article information

                Journal
                10.2204/iodp.proc.343343T.2013
                Proceedings of the IODP
                Integrated Ocean Drilling Program
                1930-1014
                19 July 2013
                10.2204/iodp.proc.343343T.101.2013

                This work is licensed under a Creative Commons Attribution 4.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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