At the northern Cascadia margin, the Juan de Fuca plate is underthrusting North America at about 45 mm yr-1 (ref. 1), resulting in the potential for destructive great earthquakes2,3. The downdip extent of coupling between the two plates is difficult to determine because the most recent such earthquake (thought to have been in 1700)4 occurred before instrumental recording. Thermal and deformation studies5 indicate that, off southern Vancouver Island, the interplate interface is presently fully locked for a distance of ∼60 km downdip from the deformation front. Great thrust earthquakes on this section of the interface (with magnitudes of up to 9)4,5 have been estimated to occur at an average interval of about 590 yr (ref. 3). Further downdip there is a transition from fully locked behaviour to aseismic sliding (where high temperatures allow ductile deformation), with the deep aseismic zone exhibiting slow-slip thrust events6. Here we show that there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked, to a broad reflection band at greater depth where aseismic slip is thought to be occurring. This change in reflection character may provide a new technique to map the landward extent of rupture in great earthquakes and improve the characterization of seismic hazards in subduction zones.
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We thank M. Fisher, T. Brocher, and many other participants for their part in organizing and carrying out the 1998 SHIPS experiment. The land seismic reflection data were collected as part of the Lithoprobe project. M.R.N. is grateful to S. Carbotte for allowing him to complete this work while he was a Post Doctoral Research Scientist at the Lamont-Doherty Earth Observatory of Columbia University. We thank T. Brocher for a series of critical reviews, A. Calvert and S. Mazzotti for discussions, and the Lamont-Doherty Earth Observatory of Columbia University for financially supporting the publication of this paper.
The authors declare that they have no competing financial interests.
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Nedimović, M., Hyndman, R., Ramachandran, K. et al. Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface. Nature 424, 416–420 (2003). https://doi.org/10.1038/nature01840
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