Abstract
Deep, episodic slow slip on the Cascadia subduction megathrust of western North America is accompanied by low-frequency tremor in a zone of high fluid pressure between 30 and 40 km depth. Tremor density (tremor epicenters per square kilometer) varies along strike, and lower tremor density statistically correlates with upper plate faults that accommodate northward motion and rotation of forearc blocks. Upper plate earthquakes occur to 35 km depth beneath the faults. We suggest that the faults extend to the overpressured megathrust, where they provide fracture pathways for fluid escape into the upper plate. This locally reduces megathrust fluid pressure and tremor occurrence beneath the faults. Damping of tremor and related slow slip caused by fluid escape could affect fault properties of the megathrust, possibly influencing the behavior of great earthquakes.
Highlights
Episodic slip on the Cascadia subduction megathrust of the northwest United States and adjacent Canada occurs 30–40 km deep in the transition zone between the shallow locked fault and free slip at depth (Dragert et al, 2001; Ghosh et al, 2012). These slow slip events typically are marked by low-frequency tremor, and together the two phenomena are known as episodic tremor and slip (ETS; Rogers and Dragert, 2003) (Fig. 1A)
TREMOR AND UPPER PLATE FAULTS Here we examine the variability of tremor density and tremor propagation along strike and compare it to the structure of the overlying plate determined from geological and geophysical surveys
They are resolvable to depths of 25–30 km with seismic tomography (Calvert et al, 2011), and to >35 km depth as subvertical zones of seismicity beneath lowland faults (McCrory et al, 2012; Fig. DR4)
Summary
Episodic slip on the Cascadia subduction megathrust of the northwest United States and adjacent Canada occurs 30–40 km deep in the transition zone between the shallow locked fault and free slip at depth (Dragert et al, 2001; Ghosh et al, 2012). Along-strike variations in tremor may reflect structural segmentation of the upper plate by northward motion and clockwise rotation of forearc blocks (Bassett and Watts, 2015; Wells et al, 1998, 2003; McCaffrey et al, 2013). We consider the possibility that these patterns in tremor periodicity and lateral migration are related to the fundamental crustal structure of the overlying plate, and that tremor density may provide clues to potential rupture mode variability on the Cascadia megathrust.
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