PRECISE GPS ACOUSTIC POSITIONING OF SEA-FLOOR REFERENCE POINTS FOR TECTONIC STUDIES/

Global networks for crustal strain measurement provide important const raints for studies of tectonic plate motion and deformation. To date, crustal strain measurements have been possible only in terrestrial set tings: on continental plates and island sites within oceanic plates. W e report the development of technology for horizontal crustal motion d etermination at seafloor sites, allowing oceanic plates to be monitore d where islands are not available. Seafloor crustal monitoring is an i mportant component of global strain measurement because about 70% of t he Earth's surface is covered by water, and this region contains most of the tectonic plate boundaries and zones of crustal deformation. Usi ng the Global Positioning System (GPS) satellites and underwater acous tics, we have established a geodetic reference site on the Juan de Fuc a 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 t errestrial GPS stations on Vancouver Island, British Columbia. The Jua n de Fuca plate site is an appropriate setting to develop seafloor obs ervation methods, since it is a well studied area, easily accessible f rom west coast Canadian and United States ports. Determination of seaf loor motion at this site addresses questions related to convergence be tween the Juan de Fuca and North American plates across the Cascadia S ubduction Zone. At the Juan de Fuca seafloor geodetic reference site, we installed precision acoustic transponders on the seafloor, and meas ured 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 allo wing determination of the sound source position at times of signal tra nsmission and reception. Merging the satellite and acoustic data allow s determination of the transponder network location in global referenc e frame coordinates. Data processing to date suggests repeatabilities of +/-0.8 cm north and +/- 3.9 cm east in the seafloor transponder net work position relative to reference points on Vancouver Island. (C) 19 98 Elsevier Science B.V. All rights reserved.