Triggered Tremor as a Slow Slip Meter?

A growing number of observations show that seismic waves trigger tremor. These observations are providing key constraints on mechanisms of tremor generation. Triggered tremor refers to bursts of seismic energy with the same characteristics as ambient tremor but are temporally modulated by the triggering seismic waves. We hypothesize that triggered tremor serves as an indicator of slow slip. If correct, triggered tremor would be a useful tool for detecting slow slip in the many regions with triggered seismic stations but lacking geodetic and continuous seismic monitoring. This hypothesis is premised on the growing observations of correlated rates of ambient tremor activity and slow slip. Thus, if the probability of triggering tremor depends on the ambient tremor rate, we can infer that it also depends on the amplitude or rate of slow slip. We test the assumption that the probability of triggering tremor depends on the ambient tremor rate by employing ideas used to study earthquake triggering. We measure tremor rates for Cascadia using a new tremor catalog spanning 2007 and 2008, a period containing two episodes of slow slip and increased tremor activity. We also document the characteristics of large teleseismic waves that passed through the region during this period, and which of these triggered tremor. Data used come from the Pacific Northwest Seismic Network and USArray. Preliminary qualitative results suggest the probability of triggering tremor correlates with the ambient tremor rate. To test further the hypothesis that triggered tremor serves as an indicator of slow slip, we perform a blind test, proposing that we should observe triggered tremor where and when slow slip is occurring. We attempt this in the upper Cook Inlet of Alaska, where a Mw7.2 slow slip event occurred between 1998 and 2001, coincident spatially and temporally with the BEAAR broadband seismic deployment. An initial examination reveals no triggered tremor, but the lack of constraint on the evolution of the slow slip warrants a higher resolution temporal sampling of the BEAAR data over the entire time period, which is the focus of ongoing work.