Short-timescale correlations between slow slip and tremor in Cascadia

We compare strain data from PBO borehole strainmeters with seismic data recorded at co-located or nearby stations in order to investigate the relationship between slow slip and tremor in Cascadia on timescales from less than an hour to about one day. The seismic data provide a simple proxy for the tremor rate, which we take to be the seismogram envelope, filtered to the periods of interest and normalized by the mean value in a several-day window. We use this tremor rate to place all the times with strain records into 5 to 10 groups and perform a fit to the strain data to obtain a single preferred strain rate for all the times in each group. By averaging over entire slow slip events and over multiple stations and/or events, we are able to reduce the noise in the strain data to a level where we can observe significant differences in the strain rates in different groups. Groups with high tremor rate usually also have a high strain rate (relative to the mean), indicating that, on average, increases in tremor rate are associated with increases in the moment rate of slow slip. By repeating this analysis after filtering the seismic amplitude and the strain data to the periods of interest, we show that this correlation exists on timescales from less than an hour to more than 12 hours. We also find that the implied changes in the moment rate of slow slip are quite large, even on relatively short timescales. For instance, when we consider only periods shorter than four hours, the strain rate in the group with the highest tremor rate is typically 1.5 to 3 times larger than that in the group with the lowest tremor rate. In many frequency ranges, the variation in strain rate we identify by grouping the data according to tremor is larger than the coherent variation observed at the tidal periods, where the strain rate changes by a factor of 1.5 to 2. This suggests that the intrinsic variability in the moment rate of slow slip is more important than the variability introduced by tidal forcing.