Postseismic geodetic signature of cold forearc mantle in subduction zones

A sharp thermal contrast between the cold forearc and the hot arc and backarc is considered fundamental to various subduction-zone processes. However, direct observational evidence for this contrast is rather limited. If this contrast is present, it must cause a rheological contrast in the mantle wedge: elastic in the forearc and viscoelastic in the arc and backarc for the timescale of earthquake cycles. Here we demonstrate that postseismic deformation following large subduction earthquakes provides independent evidence for the thermally controlled rheological contrast. Specifically, we show that seaward postseismic motion is deflected upward at the edge of the cold forearc mantle wedge, causing diagnostic uplift just seaward of the volcanic arc. From numerical simulations of postseismic deformation following the 2011 moment magnitude (Mw) 9 Tohoku-oki, 2010 Mw 8.8 Maule, 2007 Mw 8.4 Bengkulu and 1960 Mw 9.5 Chile earthquakes, together with a global synthesis of postseismic uplift measurements, we find that cold forearc mantle is present irrespective of the diversity in tectonic settings. Our findings also indicate that field surveys eight years after the 1960 Chile earthquake provided some of the earliest evidence for viscoelastic postseismic deformation. We suggest that the established link between long-term thermal processes and short-term earthquake cycle deformation is important to understanding subduction-zone dynamics. Deformation after large subduction earthquakes reflects the thermal contrast between the mantle wedge and its nose, according to numerical simulations and a synthesis of postseismic uplift data from subduction zones.