Controls on upper plate deformation through the earthquake cycle in subduction zones

The recent Mw 9.0 Tohoku event and other great subduction zone earthquakes over the past decade provide observations of the range of behaviors of upper plate deformation through the seismic cycle for megathrust events. This set of great subduction zone earthquakes, including the 2004 Sumatra event, the Kurile Islands doublet (Mw 8.2 and 8.0), the Solomon Islands event (Mw 8.1), the Samoa-Tonga events (complex, contemporaneous normal and thrust events) and Chile (Mw 8.8), show a diversity of deformational behavior co-seismically implying diverse inter-earthquake strain accumulation behavior. What controls this range of behavior is not clear but has important consequences for our ability to use observations of pre- co- and post-seismic displacements and deformation to infer the nature, pattern and magnitude of moment accumulation on the megathrust plate boundary. Typically, the assumption is made that the upper plate is essentially a passive recorder of the slab interface stress/strain evolution, rather than another component (along with the slab itself) of the deformational system; each part of which plays an important role in terms of both our observations and also the details of earthquake rupture during great earthquakes. The recent Tohoku earthquake provides observations (made on the upper plate) of both strain accumulation and release that allow us to explore the roles of variability of lithospheric mechanical properties, megathrust coupling, and rupture progress during the megathrust earthquake to improve our capability of linking surface observations to deformational processes along eh megathrust. We are exploring this issue with a two-pronged approach. From a theoretical side we are systematically testing via numerical modeling the effects of variations in rheology (e.g. elastic parameters, viscosity etc.) in controlling the partitioning of deformation during the earthquake cycle. Here, we find stark differences in the overriding plate deformation field depending on whether it is mechanically weak or strong relative to the subducting lithosphere. If relatively strong, pre-seismic surface deformation may give the false impression of the rate at which the subduction interface is accumulating slip deficit. We are also using observation from these recent great earthquakes to assess how slip and moment release on the megathrust is mapped into observed co-seismic displacements; essentially determining the way in which the upper plate filters the observations of plate boundary slip.