Identifying the aseismic moment of rapid tremor reversals in Cascadia
Wednesday, 24 February 2016: 3:25 PM
Jessica C Hawthorne1, Michael G Bostock2, Alexandra A Royer2, Amanda Thomas3 and Genevieve Savard2, (1)University of Leeds, School of Earth and Environment, Leeds, United Kingdom, (2)University of British Columbia, Vancouver, BC, Canada, (3)University of Oregon, Eugene, OR, United States
Abstract:
We identify and analyze 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. Most of them last a few hours and propagate a few tens of kilometers.We search for changes in the slow slip moment rate using PBO borehole strain data. To identify the small strain signals associated with the reversals, we isolate components of strain that have reduced hydrologic and atmospheric noise. These are components that have a minimal response to atmospheric pressure and lower noise outside the slow slip events. On average, we find that the strain rate increases by about a factor of 2 in each RTR. This factor of 2 increase in strain rate can be roughly interpreted as a factor of 2 increase in moment rate. Such a change in moment rate over a timescale of a few hours implies a moment release per RTR similar to that released in a M 5 earthquake. If the along-dip width of the reversals is comparable to the width of the whole slow slip event, such a moment implies a stress drop of order 1 kPa: 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 stress drop in the main event.
