A comparative study of low frequency earthquake templates in northern Cascadia

Using network correlation methods, we generate low frequency earthquake templates for a set of 4 composite arrays on Vancouver Island and Washington state that employ data from EarthScope (Transportable and Flexible Arrays, Plate Boundary Observatory), POLARIS and permanent network (Canadian National Seismograph Network, Pacific Northwest Seismic Network) sources. On the basis of empirical and semi-analytical arguments, the templates can be viewed as Greenʹs function sections corresponding to moment tensor point sources with step-function time dependence in displacement. Low frequency earthquake hypocentres follow the general epicentral distribution of tremor and occur along tightly defined surfaces in depth with Washington locations averaging slightly deeper than those on Vancouver Island. We invert template waveforms for moment tensor mechanisms and find that data are well modelled by double couple sources. For southern Vancouver Island, with the highest quality templates, the majority of mechanisms are consistent with shallow thrusting in the direction of plate motion. The three other data sets with lower signal to noise levels show predominantly thrust mechanisms with more variable orientations. Taken together with other constraints, our observations support the hypothesis that low frequency earthquakes manifest shear slip on a relatively thin plate boundary.