The Western Canada Deformation Array (WCDA) is a permanent Global Positioning System (GPS) Tracker Network established by the Geological Survey of Canada as part of the Canadian National Earthquake Hazards Program. Specifically the array is used to investigate crustal deformation in south western British Columbia as part of a comprehensive multi-disciplinary study of seismic hazard in the densely populated area of the Lower Mainland and Vancouver Island.

WCDA station map larger image [JPEG, 457.1 kb, 1277 X 706, notice]

Southwestern British Columbia (lower mainland and Vancouver Island) is located on the western edge of the North American Plate on top of the actively subducting Juan de Fuca Plate (Cascadia subduction zone). Central Vancouver Island has experienced two large (Magnitude>7) earthquakes this century, west of Courtenay 1946 M 7.3 and south of Gold River 1918 M 7.0. It is also well established that the Cascadia Region has experienced giant thrust earthquakes of magnitude greater than 8 every few centuries. During the present interseismic period (i.e. the period between the giant thrust earthquakes) this region is experiencing gradual build up of crustal strain evidenced by the deformation of the earth's crust.

As part of the Canadian National Earthquake Hazards program the Geological Survey of Canada in cooperation with the Geodetic Survey of Canada embarked on a program of crustal deformation measurements in 1981 in order to monitor present horizontal strain in the northern end of the Cascadia Subduction zone. Measurements of accumulating horizontal strain have been obtained through repeated laser-ranging and/or Global Positioning System (GPS) surveys of four geodetic control networks. Early results from these surveys indicated shear-strain rates ranging from 0.05 to 0.23 ppm per year within the networks, results consistent with models of a locked subduction zone. However these estimates are spatially and temporally isolated since they lack a continuous and precise common fiducial framework. To address this problem, a continuous automated network of regional GPS tracking stations called the Western Canada Deformation Array (WCDA) has been established. The network spans the most seismically active and most densely populated region in western Canada. The first site to be installed was DRAO (Penticton, B.C.) in 1991; since then an additional 11 sites have been brought on line to address several crustal deformation and global change problems in western and central Canada.

Each WCDA site is equipped with a dual frequency, geodetic quality GPS receiver, atomic frequency standard, high speed data communication and an uninterruptible power supply. To ensure long-term reference point stability each site is carefully monumented with a forced centered concrete pier solidly anchored in bedrock. The GPS antenna is mounted on top of the pier using a specially constructed aluminum base which permits antenna alignment.

Dual-frequency pseudorange and phase data, sampled at 30-sec intervals, are collected daily by an automated process running on a UNIX work-station. As part of this process quality checks are performed routinely by three programs (GIMP8, GPSPACE, QC) which generate statistical summaries and plots of the past day's data for each site. Baseline data processing is performed using the CGPS22 software utilizing precise satellite ephemerides, earth rotation parameters and ionospheric and tropospheric modelling. An interactive final stage in the processing permits remnant cycle slip detection and repair in order to achieve a precision of several millimetres in the relative positions of the WCDA GPS tracker sites.

Data from select WCDA sites are forwarded automatically (upon validation) to the Canadian Active Control System (CACS) and to the International GPS Service for Geodynamics (IGS).