HR: 17:15h
AN: G22F-01 INVITED     [PDF]
TI: Episodic Tremor and Slip in the Cascadia Subduction Zone: A Story of Discovery
AU: * Dragert, H
EM: dragert@pgc.nrcan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Rd., Sidney, BC V8L 4B2 Canada
AB: For more than two decades, seismologists at the Pacific Geoscience Centre puzzled over the episodic appearance of simultaneous "noise" on seismographs from stations located over the Cascadia subduction zone in southwestern British Columbia. With the 1992 initiation of continuous GPS monitoring in Victoria, B.C., another puzzle presented itself in an inexplicable 5-mm westward displacement of the then solitary regional continuous GPS monument over a period of about a week in October 1994. These observations remained unexplained (and unrelated) until recently. Beginning in 1996, as a result of improvements in GPS orbits and regional densification of continuous GPS networks, transient aseismic crustal motions lasting from periods of a few days to over a year were starting to be recognized. For the Pacific Northwest, detailed analyses of continuous GPS data successfully resolved a spatially coherent, transient signal that occurred in August 1999. Unrelated to after-slip that can follow great thrust earthquakes or to shallow slow-slip "tsunami" earthquakes, this signal, detected at 7 contiguous GPS sites, was characterized by a change in site positions ranging from 2 to 5 mm over a period of 6 to 15 days in a direction opposite to long-term deformation motions. This brief reversal was modeled by $\sim$2 cm of slip on the plate interface, providing the first evidence for discrete "silent" slip events occurring on the deeper Cascadia subduction zone. In early 2002, researchers at Central Washington University established the surprising regularity of Cascadia slip events on the plate interface underlying southern Vancouver Is. and northwestern Washington State. Eight slip events were identified between 1992 and 2002, with a recurrence interval of 14.5 $\pm$2 months. Next, Japanese scientists discovered the episodic occurrence of unique, non-volcanic tremors at average depths of about 30 km along the Nankai Subduction Zone. The similarity of the average depth of slip and the migration velocity of slip for the GPS-determined Cascadia slip events, to the depth and migration velocity of the Japanese tremors triggered the search for seismic signatures for the Cascadia slip events. An examination of seismic records from 1996 to 2002 for sites on Vancouver Is. revealed that what had previously been deemed surface noise was signal from seismic tremors that accompanied slip events. The Cascadia tremors were found to be similar in character to the Japanese deep tremors. In addition, their source region was found to coincide with, or directly overlie, the region of the subducting slab interface where transient slip occurs. The close correlation of tremors with slip coined the naming of the phenomenon as Episodic Tremor and Slip (ETS). The physical processes which give rise to this dynamic behavior on the deeper plate interface are not yet well understood. To date, only the Nankai and Cascadia subduction zones have been observed to share aspects of this behavior, suggesting that this phenomenon may be restricted to young subduction zones. The release of fluids, contact with a hydrated mantle wedge, and episodic changes in shear strength or mechanical coupling may all play a part in governing this behavior. Possible connections of ETS with the development of "E-zone" reflector bands, basal erosion, and pulsating metamorphism await further research. In the context of seismic hazard, the ETS zone may mark the down-dip limit of coseismic rupture of the next megathrust earthquake. Also, since it is conceivable for a slip event to trigger a large subduction thrust earthquake, the onset of ETS activity could identify times of higher probability for the occurrence of megathrust earthquakes.
DE: 1200 GEODESY AND GRAVITY
DE: 1206 Crustal movements--interplate (8155)
DE: 1242 Seismic deformations (7205)
DE: 1243 Space geodetic surveys
SC: Geodesy [G]
MN: 2003 Fall Meeting