Temporal and spatial occurrence of deep non-volcanic tremor: From Washington to northern California

Regional network data reveal deep non-volcanic tremor occurs in five independent sections along the U. S. section of the Cascadia subduction zone with different recurrence intervals. Sizeable tremor episodes occur during periods with no measurable geodetic deformation as well as coincident with episodic slow slip events, and the displacement amplitudes of the tremor with and without geodetic deformation are similar. The tremor depths cover a wide range that does not coincide directly with the subducting slab interface. The tremor can migrate bidirectionally from the point of initiation, and more than one spatially distinct tremor source can be active at a given time. These results indicate that the tremor is not directly related to the slow slip event. Instead we suggest that the tremor is related to the presence and movement of fluids at depth, which can explain the locations and episodic occurrence as well as the seismic signal characteristics themselves; namely, the limited frequency content, small size, emergent onsets, and prolonged durations with and without geodetic deformation.