Aseismic moment release associated with rapid tremor reversals in Cascadia

Abstract:

We identify variations in slow slip moment rate associated with rapid tremor reversals (RTRs) beneath Vancouver Island, in Cascadia. The RTRs were identified by Royer et. al., 2015 with their high-quality tremor catalog based on low frequency earthquakes. In most of them, tremor propagates a few tens of kilometers over a few hours.

We use PBO borehole strain data to search for aseismic moment rate variations associated with the reversals. We isolate components of strain that have high signal to noise ratios by avoiding components that have a strong response to atmospheric pressure. In the corrected strain data, the strain rate is systematically higher during the RTRs than during the 4 days surrounding them. On average, the strain rate increases by a factor of 2 during the reversals.

This factor of 2 increase in strain rate can be roughly interpreted as a factor of 2 increase in moment rate. The location of slip moves by just a few tens of kilometers during the 4-day periods of interest, so the Green’s functions do not change dramatically. If we scale this moment rate by the moment rate of the slow slip event, and account for the RTR durations, we estimate that each reversal releases a moment similar to that in a M 5 earthquake. If the along-dip width of the reversals is comparable to the width of the whole slow slip event, the estimated moment implies a stress drop of order 1 kPa. This is comparable to the tidal stresses and less than 10% of the slow slip stress drop. If the along-dip width of the RTRs is smaller---say 20 km---the aseismic moment implies a stress drop of order 5 kPa, only a factor of a few smaller than the slow slip stress drop.