In recent years continuously operating GPS sites have been established in many parts of the world, including Earthscope's Plate Boundary Observatory network in the western US. Prior to that, most GPS observations were made in survey-mode, where campaigns were used to occupy the marks on a yearly or multi-year basis. This project is to explore how to extract long-term plate kinematics and time-dependent deformation information from both continuous and survey-mode GPS data. The science target is to learn about the spatial and temporal relationship between slow slip and non-volcanic tremor throughout Cascadia. The continuous networks of PBO and PANGA are being used to test whether or not slow slip beneath Cascadia coincides spatially and temporally with non-volcanic tremor. Southern Cascadia has a very different pattern of slow-slip than northern Cascadia where slow-slip events are very regular and similar in size. Non-volcanic tremor suggests that there may be some, but different, patterns in southern Cascadia but tremor does not constrain the sizes of the slow slip events. Utilizing the time series directly in the analyses and applying new GPS processing techniques will enhance the signal-to-noise ratio of the vertical component which is critical in estimating the up-dip extent of slow slip on the Cascadia interface. One goal is to test whether or not non-volcanic tremor (NVT) coincides with slow slip. Since NVT is easier to observe than slow slip for small events, their coincidence will help delineate the portions of faults that slip slowly. The work will place important bounds on the up-dip limit of slow slip at Cascadia and hence whether or not slow slip extends into the locked portions of the megathrust. The research will further our understanding of slow-slip events and how they should be presented to the public. The simple question of whether they increase or decrease the earthquake hazard is not yet answered but is of obvious importance.
Our research utilized continuous GPS observations of the Plate Boundary Observatory from Washington and Oregon in order to characterize the movement on faults and the process of slow-slip, whereby faults move slowly to relieve pressure. The goals were to understand better the relationship of the portion of the Cascadia subduction fault that is currently stuck and increasing stress to the regions that slip freely or slip intermittently. We processed new data that were collected during the 2005 slow-slip event. The analysis of the GPS data included developing a new method that was designed to help separate out the various sources of deformation that contribute to the movement of the ground, thereby isolating the strain signal due to locking on the Cascadia thrust. We found that the shallow locked section of the fault overlaps at depth with the section where slow slip is occurring. This work suggests that the part of the fault that may rupture in the next great earthquake is somewhat larger than previously suggested, if the rupture extends into the region that is transitional between fully stuck and freely slipping. The research also included participation of an undergraduate student who is now going on to graduate school in geophysics.
