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Chadwell, CD, Spiess FN.  2008.  Plate motion at the ridge-transform boundary of the south Cleft segment of the Juan de Fuca Ridge from GPS-Acoustic data. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb004936   AbstractWebsite

We measure the present-day plate velocity of the Juan de Fuca Ridge 25 km off-axis to be 63.6 +/- 3.6 mm/a at S67.2 degrees E +/- 7.9 degrees degrees (1-sigma) relative to the Pacific plate (PA). This velocity was derived from GPS-Acoustic (GPSA) measurements in 2000, 2001, 2002, and 2003 that observed the position of a seafloor array (44 degrees 43'N, 130 degrees 03'W, 2900 m depth) with a repeatability of +/- 4-6 mm. Three transient events at the Juan de Fuca Ridge and Blanco Transform account for similar to 10% of this motion in viscoelastic modeling, suggesting that the observed GPSA-PA velocity is due primarily to steady state plate dynamics. Subtracting the modeled transient motion gives a velocity of 57.3 +/- 3.9 mm/a at S72.9 degrees E +/- 12.1 degrees degrees (1-sigma), which is consistent at the 95% confidence level with the velocity calculated from the Wilson (1993) 0-0.78 Ma Euler pole. Therefore this site is interpreted to be in a region of continuous, full-rate plate motion, a robust result of this study which holds with and without correcting for transient motions. These results provide direct geodetic evidence that spreading occurs predominantly within 25 km of the axis at this intermediate spreading-rate ridge. Previously reported geodetic monitoring across the 1-km-wide axial valley from 1994-1999 and 2000-2003 shows no significant extension (Chadwell et al., 1999; Hildebrand et al., 1999; Chadwick and Stapp, 2002; W. W. Chadwick, personal communication, 2006) and seismic monitoring shows no activity. This suggests the crust between 0.5 and 25 km off-axis accommodates similar to 26 mm of aseismic deformation each year through some combination of near-axis fault motion and elastic strain accumulation.

Spiess, FN, Chadwell CD, Hildebrand JA, Young LE, Purcell GH, Dragert H.  1998.  Precise GPS/Acoustic positioning of seafloor reference points for tectonic studies. Physics of the Earth and Planetary Interiors. 108:101-112.   10.1016/s0031-9201(98)00089-2   AbstractWebsite

Global networks for crustal strain measurement provide important constraints for studies of tectonic plate motion and deformation. To date, crustal strain measurements have been possible only in terrestrial settings: on continental plates and island sites within oceanic plates. We report the development of technology for horizontal crustal motion determination at seafloor sites, allowing oceanic plates to be monitored where islands are not available. Seafloor crustal monitoring is an important component of global strain measurement because about 70% of the Earth's surface is covered by water, and this region contains most of the tectonic plate boundaries and zones of crustal deformation. Using the Global Positioning System (GPS) satellites and underwater acoustics, we have established a geodetic reference site on the Juan de Fuca plate at 2.6 km depth, approximately 150 km off the northwest coast of North America. We measure the baselines between this site and two terrestrial GPS stations on Vancouver Island, British Columbia. The Juan de Fuca plate site is an appropriate setting to develop seafloor observation methods, since it is a well studied area, easily accessible from west coast Canadian and United States ports. Determination of seafloor motion at this site addresses questions related to convergence between the Juan de Fuca and North American plates across the Cascadia Subduction Zone. At the Juan de Fuca seafloor geodetic reference site, we installed precision acoustic transponders on the seafloor, and measured ranges to them from a sound source at a surface platform (ship or buoy), The platform is equipped with a set of three GPS antennas allowing determination of the sound source position at times of signal transmission and reception. Merging the satellite and acoustic data allows determination of the transponder network location in global reference frame coordinates. Data processing to date suggests repeatabilities of +/-0.8 cm north and +/- 3.9 cm east in the seafloor transponder network position relative to reference points on Vancouver Island. (C) 1998 Elsevier Science B.V. All rights reserved.