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.
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We thank X. Zhou for discussion. H.L. was supported by a James A. & Laurette Agnew Memorial Award, a University of Victoria Graduate Award, Graduate Support from Ocean Networks Canada, and Discovery Grant RGPIN-2016-03738 to K.W. from the Natural Sciences and Engineering Research Council of Canada. This is Geological Survey of Canada contribution 20200524.
The authors declare no competing interests.
Peer review information Nature Geoscience thanks Sylvain Barbot, Onno Oncken and Fred Pollitz for their contribution to the peer review of this work. Primary Handling Editors: Tamara Goldin; Stefan Lachowycz.
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Extended Data Fig. 1 Global synthesis of postseismic uplift following subduction earthquakes observed in the forearc near the volcanic arc.
Events are labelled with index numbers (circled) as listed in Extended Data Fig. 8, year of occurrence, moment magnitude, and age of the subducting plate at trench. Black-solid line boxes approximately outline the map areas of Fig. 4c and Extended Data Figs. 2, 4, and 6. Plate boundaries (grey lines) are from ref. 74. Locations of volcanoes are from Aster Volcano Archive (NASA, METI; https://gbank.gsj.jp/vsidb/image/).
Extended Data Fig. 2 Map view of observed and modelled postseismic deformation 5 years after the 2011 Mw 9 Tohoku-oki earthquake.
a, Horizontal component. Coseismic slip distribution (Extended Data Fig. 9) is contoured at 10 m interval (coppery lines). Cumulative afterslip, contoured at 1 m interval (grey lines), is slightly modified from ref. 30. b, Vertical component. The black solid line shows the location of the corridor in Fig. 3. Land GNSS data are based on GNSS daily solutions from the Geospatial Information Authority of Japan (GSI)31,75, and seafloor GNSS data are from Japan Coast Guard32. Time series of the labelled land and seafloor GNSS sites are shown in Extended Data Fig. 3. In these and other map view figures in Fig. 4 and Extended Data, plate interface depth is contoured using dashed lines, and locations of volcanoes from Aster Volcano Archive (NASA, METI; https://gbank.gsj.jp/vsidb/image/) are indicated using red triangles.
Extended Data Fig. 3 Observed (colour symbols) and modelled (black curves) time series of GNSS sites after the 2011 Mw 9 Tohoku-oki earthquake.
From left to right: east, north, and vertical components. Site locations are shown in Extended Data Fig. 2. a, Land sites. b, Seafloor sites.
Extended Data Fig. 4 Map view of observed and modelled postseismic deformation 5 years after the 2010 Mw 8.8 Maule earthquake.
a, Horizontal component. Coseismic slip distribution (Extended Data Fig. 9) is contoured at 4 m interval (coppery lines). Cumulative afterslip, contoured at 0.3 m interval (grey lines), is modelled using the method of ref. 19. b, Vertical component. The black line shows the location of the corridor in Fig. 3. GNSS data are from ref. 33. Time series of the labelled GNSS sites are shown in Extended Data Fig. 5.
Extended Data Fig. 5 Observed (colour symbols) and modelled (black curves) time series of GNSS sites after the 2010 Mw 8.8 Maule earthquake.
From left to right: east, north, and vertical components. Site locations are shown in Extended Data Fig. 4.
Extended Data Fig. 6 Map view of observed and modelled postseismic deformation 5 years after the 2007 Mw 8.4 Bengkulu earthquake.
a, Horizontal component. Coseismic slip distribution is contoured at 1 m interval (coppery lines; ref. 76). Cumulative afterslip, contoured at 0.4 m interval (grey lines), is modelled using the method of ref. 19. b, Vertical component. The black line shows the location of the corridor in Fig. 3. GNSS data are from ref. 35. Time series of the labelled GNSS sites are shown in Extended Data Fig. 7.
Extended Data Fig. 7 Observed (colour symbols) and modelled (black curves) time series of GNSS sites after the 2007 Mw 8.4 Bengkulu earthquake.
From left to right: east, north, and vertical components. Site locations are shown in Extended Data Fig. 6.
Extended Data Fig. 8 Summary of postseismic forearc uplift observations following subduction earthquakes.
Locations of the earthquakes are shown in Extended Data Fig. 1.
Extended Data Fig. 10 2-D Models of postseismic deformation to test the effects of upper plate thickness.
The model setup is identical to that in Fig. 2 (Methods), except for the use of different upper plate thicknesses and/or coseismic slip depths in different simulations. Shown cumulative postseismic displacements are 5 years after an earthquake, normalized by peak coseismic slip. a, Model tests showing how the upper plate thickness affects postseismic forearc uplift with or without the elastic cold nose. Minor uplift in the area of interest may occur without the cold nose if the upper plate in the warm arc and backarc area is unreasonably thick (that is, 50 km). b, Same as a except for deeper coseismic slip. c, Model structure and parameters. Inset shows coseismic slip distribution along the megathrust for the models in a (Reference) and b (Deeper).
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Luo, H., Wang, K. Postseismic geodetic signature of cold forearc mantle in subduction zones. Nat. Geosci. 14, 104–109 (2021). https://doi.org/10.1038/s41561-020-00679-9
Nature Geoscience (2021)