Long-term strain buildup in the Northeast Japan arc-trench system and its implications for gigantic strain-release events

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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.

Long-term strain buildup in the Northeast Japan arc-trench system and its implications for gigantic strain-release events

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