EPISODIC TREMOR AND SLIP COMPARED WITH SEISMICITY AND SUBDUCTION ZONE STRUCTURE: INTEGRATING THE NEW KID INTO THE EXISTING FRAMEWORK

Tectonic plate boundaries can generate great earthquakes when there is a sudden release of elastic strain on the locked zone of the plate interface. Recent geodetic observations reveal that at depth, where increasing temperatures and pressure as well as changing petrology and fluid content affect frictional behavior, faults also release strain through slow slip. Periods of slow slip have been correlated with weak, non-volcanic tremor, forming episodic tremor and slip (ETS). We have developed a tremor scanning technique for mean amplitudes of envelope seismograms and a hyperbolic tangent fitting algorithm for GPS position time series to identify slow slip. We further analyze tremor waveforms with a semi-automated process for detecting prominent tremor bursts, and analyst-refined relative arrival times are inverted for source locations. Employing these techniques across Cascadia, we find 3 broad, coherent zones with different ETS recurrence intervals where interval duration is inversely proportional to surface topography and spatial extent correlates with geologic terranes. The zones are further divided as the initiation and termination points of tremor activity are repeatable features, and these patches appear to be immediately down-dip from fore-arc basins interpreted as manifestations of megathrust asperities. Tremor epicenters occur within a narrow band between the 30- and 40-km contours of the subducting plate interface. Tremor activity is spatially anticorrelated with seismicity, suggesting the two processes are mutually exclusive. Modulation of tremor signals by tides and passing surface waves imply very low effective stress is a necessary condition for ETS, supported by frictional modeling and the presence of ultralow seismic velocities and high Vp/Vs ratios that indicate high pore fluid pressures. Globally, ETS is not tied to particular pressure-temperature conditions, but slow slip occurs within the geodetically defined transition zone, while tremor may be restricted to cases where high conductivity is consistent with fluid release. We propose the transition in frictional behavior and high pore fluid pressures favor tremor generation instead of earthquakes, and frequent ETS produce a relaxation of strain that further inhibit seismogenesis surrounding the ETS source region.