About a decade ago, observations using the global-positioning satellite (GPS) system led to the discovery of "slow-slip" behavior along the deepest extent of faults. These transient events have now been documented along most of the Earth's subduction zones, including the Cascadia margin of the Pacific Northwest, where the University of Oregon has been cataloging their behavior, and in other geologic settings such as the San Andreas and Hawaii. As their name suggests, slow-slip events occur at sliding rates that are much slower than those during earthquakes. They nevertheless are capable of releasing large amounts of the strain energy that accumulates during the time intervals (typically months to years) that separate them and they play an important role in determining the conditions along faults just prior to the infrequent, large earthquakes that pose some of the greatest risks to populations and infrastructure in these settings. Our efforts focus on unraveling the geological conditions that enable slow-slip to occur. We are developing a predictive understanding for how the pressure of pore fluids (i.e. water) along the subduction interface is controlled by the compaction of sediments and dehydration reactions that release water from mineral particles. We are also developing mathematical models to determine whether sliding on brittle faults embedded in a more compliant matrix in a "subduction channel" can cause slip behavior that is consistent with observations. This research effort is part of the training for a promising new graduate student, it will involve an undergraduate in our broader research effort, and it will enable us to more effectively disseminate the University of Oregon catalogue of slow-slip source parameters to the broader scientific community.
Further progress in understanding slow slip requires that improved geologic constraints be included in the design and implementation of predictive slow-slip models. Of particular note are the high pore pressures that seem to be required for slow-slip to occur and the structural features in exhumed subduction zone rocks that suggest coexistence of brittle and ductile deformation mechanisms. To explore how high pore pressures develop, we consider consolidation and fluid flow in porous underthrust sediments and incorporate the effects of chemical dehydration reactions to develop a synoptic quantification of pore pressure evolution that follows a subducted volume from the trench to the region of slow slip. Constitutive behavior will be constrained using measured changes in sediment permeability and void ratio during uniaxial deformation experiments on hemipelagic mudstones under elevated pressure and temperature conditions, potentially including the influence of the smectite-illite transition. A complementary modeling effort examines the deformation produced by frictional sliding on faults embedded within a ductile shear zone. Preliminary efforts focus on a two-dimensional treatment of a subduction channel of fixed width that contains a single fault in a compliant matrix; subsequent modeling phases will be geared towards more realistic descriptions that incorporate variations in the fraction of rigid, competent material within the compliant matrix and include the effects of roughness along the direction of fault slip, consistent with observations of exhumed m elange structures. Our new modeling framework is constrained by an expanding catalog of slow-slip events and scaling relationships that have been inferred from geodetic observations along the Cascadia margin. With a combined theoretical, observational, and experimental approach, this targeted research effort will improve our understanding of the dynamics of slow slip and the tectonic behavior and hazards along subduction margins.
