HR: 09:00h
AN: T11F-05    [Abstracts]
TI: Observing Episodic Slow Slip with PBO Borehole Strainmeters along the Northern Cascadia Margin
AU: * Dragert, H
EM: hdragert@nrcan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada
AU: Wang, K
EM: kwang@nrcan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada
AU: Kao, H
EM: hkao@nrcan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada
AB: Episodic non-volcanic low-frequency seismic tremor and slow slip (ETS) downdip of the megathrust seismogenic zone are now routinely observed at the Nankai and Cascadia subduction zones. These phenomena yield important information about the mechanism of the downdip termination of the seismogenic zone and raise intriguing questions regarding the process of strain accumulation and release during time periods between great megathrust earthquakes. Borehole strainmeters (BSM"s), newly installed along the Cascadia margin under the U.S. Plate Boundary Observatory (PBO) initiative, have enabled us to study ETS with much improved detection sensitivity. Here we report preliminary results to demonstrate the value of the BSM's in helping us define the temporal character of prolonged ETS events and the spatial character of shorter-lived ETS events. In the region from Puget Sound to southern Vancouver Island, rather regularly occurring, prolonged (> 6 days) ETS events affect large areas, spanning up to a few hundred kilometres along the strike of the subduction zone. The ETS event of September 2005, the first one to be monitored with BSM observations as well as GPS and tremor observations, started from northern Puget Sound, migrated northwest along the margin, and stopped in southern Vancouver Island. BSM's in north-western Washington detected significant changes in strain components during this event and their temporal character is consistent with a dislocation model of a slow slip on the deeper plate interface propagating northwest at a constant rate of about 7 km/day. Between these regularly occurring prolonged ETS events, we have also observed sporadic tremor activity with shorter durations. Whether these small tremor events are also accompanied by slow slip and the characteristics of such slip, if present, are largely unknown because of their small geodetic signal. A 4-day episode of seismic tremor in central Vancouver Island in November 2006 resulted in ill-defined surface displacements at the closest continuous GPS stations. However, strain data from a borehole strain meter (B012) at Ucluelet, located on the west coast of central Vancouver Island, are consistent with slip on a small patch of the deeper subducting plate interface to the northeast of this site, the region where the tremors were observed. Simple elastic dislocation models with 3 to 5 cm of slip constrained to the subducting plate interface can replicate the observed strain signal as well as the limited surface displacements.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 8150 Plate boundary: general (3040)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting