OBSERVING COSEISMIC AND INTERSEISMIC DEFORMATION: WHAT MODELERS KNOW AND DON'T KNOW

Seismological and geodetic observations have enabled us to define a general deformation pattern throughout a subduction zone earthquake cycle. It has long been recognized that the deformation pattern and mechanism depend on timescale. Viscoelastic behaviour of the earth causes the deformation to be more localized around the rupture zone at the time of the earthquake but more spread out after the fault has been locked for some time. Therefore coseismic and interseismic deformations are not mirror images of each other. Because interseismic deformation evolves with time, data collected at a given time after a great earthquake only provide a snapshot for that specific time. If we were to use an elastic dislocation model to infer the state of locking using data collected at another time, we would draw a different conclusion on the size of the locked zone and the degree of locking.

Coseismic and other fast, transient deformation of the timescale of seconds to days can be rather accurately described using elastic models; the cause of the deformation is predominantly fault slip. Uncertainties in the assumptions about the slip distribution overshadow errors in material properties, such that in practice the model of a homogeneous half space usually suffices. Vertical deformation inferred from coastal paleoseismic observations is particularly useful in constraining these models, provided that the potential effect of post-seismic motion is accounted for. Long-term interseismic deformation of the timescale of decades to centuries can be modeled using the simplest viscoelastic model; details of the fault motion are less important than the viscous behaviour of the rocks in the upper mantle and lower crust. Modeling post-seismic deformation of the timescale of days to years presents great challenges, because fast fault motion (afterslip) and viscoelastic stress relaxation are both important, and because of complications in rock’s viscoelastic behaviour in short-term deformation.