Kelley Hall

We explore the evolution of slow slip on the Cascadia megathrust during two large episodic tremor and slip events and compare stress changes to the spatial evolution of tremor from Pacific Northwest Seismic Network tremor locations. We used displacement time series from ~72 GPS stations, along with the Extended Network Inversion Filter to solve for the time‐dependent fault slip. The 2010 (Mw 6.8) and 2012 (Mw 6.8) events propagated northward and southward, respectively, allowing us to assess… Show more

We explore the evolution of slow slip on the Cascadia megathrust during two large episodic tremor and slip events and compare stress changes to the spatial evolution of tremor from Pacific Northwest Seismic Network tremor locations. We used displacement time series from ~72 GPS stations, along with the Extended Network Inversion Filter to solve for the time‐dependent fault slip. The 2010 (Mw 6.8) and 2012 (Mw 6.8) events propagated northward and southward, respectively, allowing us to assess directional effects on slip behavior. We observed that tremor occurs on the leading edge of propagating slipping regions, well ahead of the highest slip rates, independent of the along‐strike propagation direction. Resolution tests using the actual tremor distributions to generate synthetic data show that our result of peak tremor rates leading peak slip rates is not due to biases introduced by temporal smoothing. Calculated stress changes due to the time‐dependent slip distributions imply that tremor is sensitive to kilopascals of stress, consistent with studies of tidally triggered tremor. Within the resolution of our model, our results are consistent with the hypothesis that significant tremor is triggered by stresses ahead of the highest slip rates. We also observe ongoing slip continuing several days after tremor has passed. Our observations are consistent with some numerical models of tremor patches that suggest that this behavior can be explained by densely packed asperities resulting in somewhat crack‐like propagation rather than a slip pulse that is as concentrated as the tremor activity. Show less

We measured displacement vectors from horizontal components of 80 Global Positioning System stations to analyze six major episodic tremor and slip (ETS) events from 2007 to 2016 in northern Cascadia and inverted for slip on a realistic plate interface. Our results indicate that slow slip of up to 2 cm extends updip of tremor by about 15 km beneath the Olympic Peninsula. In these ETSs, slow slip extends from the downdip portion of the tremorgenic region beyond the updip extent of tremor… Show more

We measured displacement vectors from horizontal components of 80 Global Positioning System stations to analyze six major episodic tremor and slip (ETS) events from 2007 to 2016 in northern Cascadia and inverted for slip on a realistic plate interface. Our results indicate that slow slip of up to 2 cm extends updip of tremor by about 15 km beneath the Olympic Peninsula. In these ETSs, slow slip extends from the downdip portion of the tremorgenic region beyond the updip extent of tremor, although still downdip of the inferred locked megathrust. Slip updip of tremor is a persistent feature of all six ETS events. Inversions that restrict slip to occur only in regions that generated tremor produced slip distributions with unphysical characteristics, such as 8‐cm slip concentrated at the updip part of the tremor footprint. Updip slow slip without tremor may suggest that the gap between stress and strength widens updip above the observed limit of tremor. In these ETSs, the region updip of tremor may have undergone only limited ductile failure surrounding potentially tremorgenic patches. A widening gap between stress and strength in the updip direction is consistent with an observed along‐dip dependence of low‐frequency earthquake occurrence and numerical simulations of slow slip. Alternatively, rheological properties in the region updip of tremor may favor stable slip and not permit seismic slip (i.e., tremor). Additionally, we find that along‐strike variations in the amount of slow slip updip of tremor correspond to changes in lithology of the overlying crust. Show less