HR: 0800h
AN: T21A-0365    [Abstracts]
TI: Detection of Slow Slip Events Along the Cascadia Subduction Zone Using Plate Boundary Observatory Borehole Strainmeters
AU: * McCausland, W
EM: wmccausland@usgs.gov
AF: Cascades Volcano Observatory, 1300 SE Cardinal Ct Building 10, Suite 100, Vancouver, WA 98683, United States
AU: Roeloffs, E
EM: evelynr@usgs.gov
AF: Cascades Volcano Observatory, 1300 SE Cardinal Ct Building 10, Suite 100, Vancouver, WA 98683, United States
AB: In the spring of 2005, the Plate Boundary Observatory (PBO) began installing borehole strainmeters in the Pacific Northwest. Currently there are 18 borehole strainmeters operating from Vancouver Island, Canada to Southern Oregon that are favorably located to detect slow slip along the Cascadia subduction zone. While the longest (> two weeks) subduction tremor episodes are accompanied by slow slip events, there are shorter duration tremor episodes that have not been accompanied by GPS-detected slow slip events. It is possible that a slow slip event does occur, but is below the current resolution of the GPS network. Using data from these PBO strainmeters, we developed a scanning technique to detect strain transients. The strainmeter data are band-passed between 5 and 16 days to eliminate tides and long-term variations. In this band, the instrument responds primarily to atmospheric pressure. Excursions of the gauges from the atmospheric pressure variations are investigated as possible tectonic strain transients. Several features of the strainmeters aid us in distinguishing between tectonic strain transients and other variations in the data. First, tectonic transient strain signals will be recorded similarly on closely clustered strainmeters; there are 4 such clusters along the Cascadia subduction zone. Second, if the areal strain recalculated using two orthogonal gauges is not consistent with the areal strain calculated on three equally spaced gauges, then the signal is not caused by strain. Pumping or rainfall can cause strain variations that we can distinguish from tectonic events by looking at other measured quantities including fluid pressure, precipitation, etc. Thus far, in northern Washington and Canada, all the known transient slow slip events recorded on the strainmeters were also detected by the GPS network. In southern Oregon, our scanning technique found one possible tectonic event on the B035-B036 cluster near Grant's Pass during late July and early August 2007. Other researchers (Szeliga et al, 2004; McCausland et al, 2005; Brudzinski and Allen, in press) have reported recurring subduction tremor and slow slip in this area. Further corroborating our observations, cleaned GPS data from the PANGA website for the east-west component of nearby station DDSN, resembles signals from previous slip events.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1294 Instruments and techniques
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting