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Speaker: Kelin Wang, Research Scientist, Pacific Geoscience Centre, Geological Survey of Canada

Host: Thorsten Becker

Title: Earthquake deformation in a viscoelastic Earth

Abstract: The viscoelastic rheology of the upper mantle controls the evolution of crustal deformation in earthquake cycles. In this seminar, I will explain how it works by discussing four points. (1) After a great subduction earthquake, the upper plate exhibits reversal of motion direction progressively away from the rupture zone: from the initial seaward motion, which opposes the postseismic motion of the near-trench area, to eventual wholesale landward motion. The larger the earthquake, the longer it takes to complete the motion reversal. For example, the reversal process following the 1960 M=9.5 Chile earthquake is still not completed today. (2) Geodetically observed deviation from this “standard” pattern raises new questions about the mechanics of the megathrust fault. For example, a premature and rapid reversal of motion direction in the early 21^st century in the southern part of the 1960 Chile earthquake area suggests enhanced locking of the deeper megathrust for which the mechanism is not yet understood. (3) Postseismic viscoelastic deformation may allow “geodetic imaging” of subsurface structure, contributing to understanding the thermo-petrologic field. For example, short-term vertical postseismic motion shows diagnostic signature of a cold and elastic forearc mantle wedge. (4) Because of viscoelasticity, deformation rate decreases with time when the fault is locked, raising questions about its relevance to assessing earthquake potential. To focus on the fundamental process, we revisit the simplest possible viscoelastic earthquake cycle model – the model of a strike-slip fault with periodic earthquake cycles. The results re-deliver the commonly forgotten message that, if the recurrence interval is long, most of the elastic strain energy that propels the next earthquake is accrued early in the interseismic period. Deformation rate slows down to a minimum just before the next earthquake, such that geodetically observed slow deformation by no means indicates low earthquake potential.