Subduction zones produce the largest earthquakes. Over the past two decades, space geodesy has revolutionized our view of crustal deformation between consecutive earthquakes. The short time span of modern measurements necessitates comparative studies of subduction zones that are at different stages of the deformation cycle. Piecing together geodetic 'snapshots' from different subduction zones leads to a unifying picture in which the deformation is controlled by both the short-term (years) and long-term (decades and centuries) viscous behaviour of the mantle. Traditional views based on elastic models, such as coseismic deformation being a mirror image of interseismic deformation, are being thoroughly revised.

After a large subduction earthquake, crustal deformation continues to occur, with a complex pattern of evolution. This postseismic deformation is due primarily to viscoelastic relaxation of stresses induced by the earthquake rupture and continuing slip (afterslip) or relocking of different parts of the fault. When postseismic geodetic observations are used to study Earth's rheology and fault behaviour, it is commonly assumed that short-term (a few years) deformation near the rupture zone is caused mainly by afterslip, and that viscoelasticity is important only for longer-term deformation. However, it is difficult to test the validity of this assumption against conventional geodetic data. Here we show that new seafloor GPS (Global Positioning System) observations immediately after the great Tohoku-oki earthquake provide unambiguous evidence for the dominant role of viscoelastic relaxation in short-term postseismic deformation. These data reveal fast landward motion of the trench area, opposing the seaward motion of GPS sites on land. Using numerical models of transient viscoelastic mantle rheology, we demonstrate that the landward motion is a consequence of relaxation of stresses induced by the asymmetric rupture of the thrust earthquake, a process previously unknown because of the lack of near-field observations. Our findings indicate that previous models assuming an elastic Earth will have substantially overestimated afterslip downdip of the rupture zone, and underestimated afterslip updip of the rupture zone; our knowledge of fault friction based on these estimates therefore needs to be revised.

[1] The shallow part of the interface between the subducting slab and the overriding mantle wedge is evidently weakened by the presence of hydrous minerals and high fluid pressure. We use a two-dimensional finite element model, with a thin layer of uniform viscosity along the slab surface to represent the strength of the interface and a dislocation-creep rheology for the mantle wedge, to investigate the effect of this interface ''decoupling.'' Decoupling occurs when the temperature-dependent viscous strength of the mantle wedge is greater than that of the interface layer. We find that the maximum depth of decoupling is the key to most primary thermal and petrological processes in subduction zone forearcs. The forearc mantle wedge above a weakened subduction interface always becomes stagnant (<0.2% slab velocity), providing a stable thermal environment for the formation of serpentinite. The degree of mantle wedge serpentinization depends on the availability of aqueous fluids from slab dehydration. A very young and warm slab releases most of its bound H 2 O in the forearc, leading to a high degree of mantle wedge serpentinization. A very old and cold slab retains most of its H 2 O until farther landward, leading to a lower degree of serpentinization. Our preferred model for northern Cascadia has a maximum decoupling depth of about 70-80 km, which provides a good fit to surface heat flow data, predicts conditions for a high degree of serpentinization of the forearc mantle wedge, and is consistent with the observed shallow intraslab seismicity and low volume of arc volcanism.Citation: Wada, I., K. Wang, J. He, and R. D. Hyndman (2008), Weakening of the subduction interface and its effects on surface heat flow, slab dehydration, and mantle wedge serpentinization,