Coulomb stressing rate changes and seismicity dynamics associated with slow slip events in Southcentral Alaska

Two long-term slow slip events (SSEs) in Lower Cook Inlet, Alaska were identified by Li SS et al. (2016). The earlier SSE lasted at least 9 years with Mw~7.8 and had an average slip rate of ~82 mm/yr. The latter SSE, occurring in a similar area, lasted about 2 years with Mw~7.2 and an average slip rate of ~91 mm/yr. To test whether these SSEs triggered earthquakes near the slow slip area, we calculated the Coulomb stressing rate changes on receiver faults using two fault geometry definitions: nodal planes of focal mechanism solutions of past earthquakes, and optimally oriented fault planes. Regions in the shallow slab (30 – 60 km) that experienced significant increase in Coulomb stressing rate due to SSEs slip show an increase in seismicity rates during SSE periods. No correlation was found in the volumes which experienced significant increase in Coulomb stressing rate during the SSE within the crust and the intermediate slab. We modeled variations in seismicity rates using a combination of Coulomb stress transfer model and the framework of rate-and-state friction. Our model indicated that the SSEs increased Coulomb stress changes on adjacent faults, thereby increasing the seismicity rates though the ratio of the SSE stressing rate to the background stressing rate is small. Each long-term SSE in Alaska brought the megathrust updip of the SSE areas closer to failure by up to 0.1-0.15 MPa. The volumes of significant Coulomb stress changes caused by the Upper and Lower Cook Inlet SSEs did not overlap.