Constraints on Slow Slip Behavior in Cascadia Through the Integration of PBO Borehole Strainmeters, GPS Time Series, and Tremor Locations

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Slow slip events and non-volcanic tremor provide new insight into the mode of strain release along the deep extension of faults. These events are noteworthy for their long durations (~days) and large magnitudes (~M6). We have yet to fully understand what properties or conditions on the plate interface act to prevent the rapid slip that is typically observed with earthquakes. Slow slip and tremor also constrain the lower edge of the seismogenic zone, a variable that is useful for seismic hazard studies, and event histories are an input into studies that explore the time-dependent loading of stress on the seismogenic zone. Most of the existing geodetic studies of slow slip have focused on observations made by GPS. In this work, we integrate borehole strainmeters with GPS data to more fully characterize the spectrum of slow slip behavior on the Cascadia subduction zone. Time-dependent inversions of GPS and strainmeter time series are performed using the Extended Network Inversion Filter. By incorporating the strainmeters into our analysis, we resolve smaller slow slip events and extract greater information about the propagation characteristics than by using GPS alone. From our expanded catalog of slow slip events, we are extracting the source parameters and testing whether the smaller slow slip events (~M5+) obey the same scaling relationships as has been inferred for the larger ones (~M6+). The strainmeter data also allow us to better compare the spatial and temporal correlation of slow slip and the location of tremor. We are testing the idea of whether tremor marks the leading edge of slow slip or whether tremor is embedded within the slipping zone. Lastly, we are evaluating the up-dip taper of slow slip and seeing if the surface observations are consistent with an abrupt up-dip edge, as suggested by the tremor. Our analysis of the strainmeter data allows us to better evaluate the calibration and noise characteristic of the strainmeter time series. We will specifically use the GPS-derived slip models to independently validate the strainmeter calibrations.