HR: 14:20h
AN: G43A-03 INVITED     [Abstracts]
TI: Investigating the Relationship of Episodic Tremor and Slip to Seismic Activity Along the Cascadia Margin
AU: * Dragert, H
EM: hdragert@NRCan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Rd., Sidney, BC V8L 4B2 Canada
AU: Kao, H
EM: hkao@NRCan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Rd., Sidney, BC V8L 4B2 Canada
AU: Rogers, G C
EM: grogers@NRCan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Rd., Sidney, BC V8L 4B2 Canada
AU: Cassidy, J F
EM: jcassidy@NRCan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Rd., Sidney, BC V8L 4B2 Canada
AB: Based on seismic and GPS observations along the northern Cascadia Margin, "Episodic Tremor and Slip" (ETS) is currently defined empirically as repeated, transient ground motions at a plate margin, roughly opposite to longer-term elastic deformation motion, accompanied by distinct, emergent seismic signals in the 0.5 to 5 Hz band. Although the processes involved in ETS are not fully understood, observations to date suggest spatial and possibly temporal relationships between ETS and the three types of regional earthquake activity that characterize Cascadia seismic hazard. Spatially, ETS and regional seismicity tend to be mutually exclusive. Although the last Cascadia subduction thrust earthquake pre-dates written historical records, instrumental observations of ETS in the Nankai subduction zone of southern Japan (Obara, 2002) suggest that the ETS zone may mark the landward limit for megathrust rupture. Furthermore, the northern Cascadia ETS zone appears to be confined to a region overlying the descending Juan de Fuca plate where few in-slab earthquakes occur. Detailed observations of the March 2003 and July 2004 tremor episodes, which established an extended depth distribution for tremor sources, also suggest tremors occur in the fluid-rich crust overlying the slip zone where fewer crustal earthquakes are found. Temporal relationships are more conjectural. If ETS episodes are the direct result of slip on the deeper plate interface, it is conceivable that a slip event could develop into a trigger for rupture on the locked portion of the thrust fault lying directly up-dip of the slip region. Commensurately, the cumulative weekly conditional probability for a megathrust event is increased at the time of an ETS event (Mazzotti and Adams, 2004). A search for changes of rates of in-slab earthquakes coincident with ETS events has produced ambiguous results so far, showing marginal changes in seismicity rates for some of the ETS events. No correlation has yet been found between ETS occurrence and low-magnitude crustal seismicity, although Coulomb stress estimates predict a correlation if crustal stresses are close to critical for rupture. Interestingly, the July 2004 M6.4 earthquake that occurred 40 km offshore from central Vancouver Is. correlates with an "out-of-sequence" ETS event in northern Vancouver Is., suggesting stress interaction may exist for large earthquakes.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Geodesy [G]
MN: Fall Meeting 2005