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Geodetic strain measurements in central Vancouver Island Lisowski, Michael


A 1982 trilateration network and a 1947 triangulation network located in central Vancouver Island were studied to determine measurement precision and horizontal strain accumulation. The 1982 trilateration network was comprised of 23 distance measurements (average length 24 km), which covered a 30 km by 50 km area near the town of Gold River, east of Nootka Sound. Atmospheric refractivity corrections to the Rangemaster III measured distances were derived from end-point air pressures and aircraft-flown temperature and humidity profiles taken at the time of rangings. The standard error in a distance L (m) was estimated to be σ = (a² + b²L²)[sup ½]; where a = 0.0057m and b = 0.2x10⁻⁶ for distances reduced using the USGS probe temperatures and humidities or b « 0.26 x 10-6 using the CGS probe. There were 54 angle measurements common to the 1947 and 1982 networks. The standard error in a 1947 angle measurement was 2.3", and the estimated standard error in a 1947 to 1982 angle change was 2.4". Assuming uniform strain, the average rate of shear strain accumulation between 1947 to 1982 was 0.23±0.12 μrad/yr with the axis of maximum contaction bearing N56°E±12°. The accumulation of strain in the Gold River area was found to be similar in orientation and average rate to that observed in western Washington. The northeast orientation of the maximum compressive strain found in the geodetic networks could be reproduced with a two-dimensional, elastic dislocation model of the Cascadia subduction zone by locking the shallow interface to a depth of about 20 km. The model could not account for some details of the vertical deformation and it was not consistent with the north-south compressive stress indicated by shallow earthquake focal mechanisms. Although the possiblity of a large, shallow, thrust earthquake is inferred from the strain data, the uncertainty in the strain accumulation and the tectonic complexity of the area make such a conclusion speculative.

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