Initiation and Propagation Phases of Northern Cascadia Episodic Tremor and Slip Events

Abstract:

Large northern Cascadia episodic tremor and slip (ETS) events appear to have distinct initiation and propagation phases, based on tremor energy rates determined for the four large ETS events, from 2007-2010, using the very quiet stations of the Cascadia Array for Earthscope (CAFE) and Array of Arrays experiments. During the 8-day initiation phase, there is a roughly linear increase in tremor amplitude, corresponding to a linear increase in the areal distribution of tremor. These episodes typically initiate down dip, and after approximately 8 days have organized and migrated to fill the up-dip/down-dip width of the tremorgenic zone. Because the increase in tremor amplitude during the initiation phase is proportional to the increase in tremoring area, the source amplitude per unit area is approximately constant, indicating a constant radiated energy rate per unit area associated with tremor. This is consistent with a spatial dimension growing as the square root of time, indicating a diffusion process with a diffusion coefficient of (2-5)*10³ m²/s. Ide [2010, Nature] and Ando et al., [2012, JGR] also inferred a diffusive behavior for tremor in Japan with similar diffusion coefficients. There is little tidal modulation of tremor amplitudes during this diffusive, physically unbounded initiation phase, similar to Ide’s conclusion that diffusive behavior was most clear when tidal modulation was weak. In contrast, during the propagation phase, tremor amplitudes vary wildly and are strongly modulated by tidal stresses, as the tremor propagates along strike in one or both directions at roughly 8 km/day, continuing for 3-5 weeks. During this phase the fault slip is bounded by its width in the dip direction. Tremor swarms between the major ETS events can begin similarly, but do not reach their maximum area and amplitude, and instead terminate in less than 5-10 days.