Spatial Comparisons of Tremor and Slow Slip as a Constraint on Fault Strength in the Northern Cascadia Subduction Zone

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.