EPISODIC TREMOR AND SLIP (ETS) ON THE CASCADIA SUBDUCTION ZONE: THE CHATTER OF SILENT SLIP

The Cascadia subduction zone is a region that has repeatedly ruptured in great thrust earthquakes of moment magnitude (Mw) greater than 8. Recently, the occurrence of discrete slip events on the deeper part of the northern Cascadia subduction zone interface has been recognized by observing transient surface deformation on a network of continuously recording Global Positioning System (GPS) sites. For the plate interface underlying southern Vancouver Is., these slip events have occurred down-dip from the currently locked, seismogenic portion of the subduction zone, and appear to repeat at 13 to 16 month intervals. These past slips were not accompanied by earthquakes and were thought to be seismically "silent". However, using records from the regional digital seismic network, unique non-earthquake signals have now been identified to accompany the occurrence of slip. These pulsating, tremor-like seismic signals appear similar to those reported in the fore-arc region of Japan, but the signals observed in Cascadia correlate temporally and spatially with the six deep slip events observed over the past seven years. At other times, this tremor activity is minor or non-existent. These tremors have a lower frequency content than nearby earthquakes, and they appear uncorrelated with the deep or shallow earthquake patterns in the region. They have been observed only near the subduction zone interface and specifically in the same region as the deep slip events. These observations establish that Cascadia transient slip has a characteristic seismic signature, and we refer to this associated tremor and slip phenomenon as Episodic Tremor and Slip (ETS). We propose that ETS activity may be used to map the maximum depth extent of long-term stress accumulation on the subducting plate interface, and to indicate, in real time, stress loading of the Cascadia megathrust earthquake zone.