UBC Theses and Dissertations

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UBC Theses and Dissertations

A snapshot of the Queen Charlotte fault zone obtained from P-wave refraction data Horn, James R.


In 1979 a refraction seismic study was conducted across the Queen Charlotte fault zone, a transform boundary between the Pacific and North American plates along the northwestern coast of British Columbia. Two explosion profiles, one parallel to and one perpendicular to the strike of this fault zone were recorded on three ocean bottom seismographs (OBS's) and one land-based seismograph. The parallel profile recorded at one of the OBS's passed entirely over oceanic crust. Assuming lateral homogeneity along this profile, one dimensional amplitude modelling of the P-wave data with the WKBJ alorithm produced a velocity model with the characteristics of standard oceanic crust. The partially reversed perpendicular profile recorded at 2 OBS's and the land station, passed over the fault zone, which in topographic cross-section shows two linear fault escarpments separated by a flat terrace. An initial velocity structure provided by time term analysis of the complete data set was modified by ray tracing until the travel time data for the three stations were satisfied. The final velocity model shows a terrace crustal unit consisting of a 2 km thick layer with an average velocity of 3.9 km/s and an underlying 9 km thick layer with an average velocity of 6.4 km/s, which have lower velocities than corresponding layers in the oceanic crust to the west (4.5 and 6.9 km/s) and in the upper layer of the continental crust to the east (4.4 km/s). A mantle with a velocity of 7.9 km/s dips 20° beneath the terrace. The 15° difference between the fault trace and the relative plate motion vector requires approximately 1.5 cm/yr of convergence. Underthrusting of oceanic crust could accommodate this convergence and could lead to the extensive crustal shearing responsible for the velocity reductions in the terrace crust.

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