Cascadia Slow Slip Models Constrained by Tremor-Derived Slip Histories

Abstract:

We explore new ways to constrain the kinematic slip distributions for slow slip using constraints from tremor. Source time functions are traditionally used to describe the moment release as a function of time for a faulting event. Making a direct measure of the source time function for an ETS event is problematic for several reasons. Geodetic observations can constrain the cumulative source time function of the entire event, but do not provide the spatial or temporal resolution to reveal the subtleties of how rupture evolves with individual rupture fronts. Tremor provides detailed information about where and when the fault is slipping, but an individual tremor burst does not capture the full moment release in space, time, or frequency content. However, fluctuations in tremor amplitude for a tremor sequence do provide constraints on how slip evolves, assuming that the tremor is directly driven by slip. Recent work (Houston, 2015) inferred a crude representative stress time history at an average point using the tidal stress history, the static stress drop, and the timing of the evolution of tidal sensitivity of tremor over several days of slip. To convert the stress time history into a slip time history, we use dislocation theory to explore the stressing history of a small locked patch due to an approaching rupture front. We assume that the locked patch releases strain through a series of tremor bursts whose amplitude is related to the stressing history. Our objective here is not to directly constrain the magnitude of slip, but rather to constrain the plausible functional form of the slip history using the inferred stressing history. To test whether the functional form is reasonable, we create a hypothetical slip model using the slip time history to create a propagating rupture front that extends across the fault. The width of the rupture front, which is allowed to vary spatially, is constrained by existing tremor catalogs in Cascadia. The slip amplitude is scaled appropriately to match the total magnitude of the candidate event. Through a forward simulation, we evaluate the ability of the tremor-derived slip history to accurately predict surface displacements observed by GPS.