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Seismicity along the continental margin of Vancouver Island

University of British Columbia

The oceanic Juan de Fuca and Explorer plates are subducting beneath the continental North America plate west of Vancouver Island at rates of 47 mm/yr and 21 mm/yr, respectively. The Nootka fault zone, which is the boundary between the two subducting plates, experiences left-lateral shear due to their different rate of subduction. The Juan de Fuca/North America interaction vector is normal to the boundary beneath central-south Vancouver Island and oblique beneath western Washington. During the last 15 years, seismograph station coverage in western Canada has been adequate to record a good number of small magnitude earthquakes lying mainly in three regions: the Nootka fault zone, the Georgia Strait-Puget Sound region, and along the west coast of Vancouver Island and immediate offshore region. A detailed study of seismicity data in the Vancouver Island and offshore region, within the depth range of 25-55 km and recorded in the Canadian Earthquake Catalogue during the period 1976-1989, has been undertaken. The objectives of this study are to determine the frequency of occurrence and b-value, to improve the hypocentral locations and to interpret them in terms of both structure and the subductions process, and to examine the focal mechanisms to gain understanding of the stress field. A total of 127 earthquakes satisfied the depth range criteria and were analyzed in detail. Earthquake depths, recalculated with a recently determined velocity structure from refraction-reflection data and using the best station distribution and the more reliable arrival times, delineate a zone of seismicity beneath central-south Vancouver Island. The hypocenters show a distinct band of activity which extends ~60 km down-dip at a distance range of 35 to 95 km from the shelf edge and at a depth of 25 to 45 km. The structure is about 9 km thick and dips at an angle of 14° at N29°E. Most of these deep earthquakes are located in the subducting oceanic crust, a conclusion consistent with the position of the oceanic plate from other geophysical studies. The b-value of the magnitude-frequency distribution calculated as 0.79 also is consistent with the conclusion of the seismicity occurring within the slab, on the basis of similar studies in western Washington and in other subduction zones. Areally, the new results confirm previous suggestions that the subducting plate is arched below the Georgia Strait-Puget Sound region. Composite focal mechanism solutions constructed for five groups of earthquakes show strike-slip and normal faulting mechanisms, but no thrust events. This important result confirms, at the small magnitude level, previous observations of the absence of thrust faulting earthquakes. Normal faulting may be associated with plate bending, an expected result, but the strike-slip mechanisms which suggest lateral shearing within the subducting plate, are an unexpected result. The composite of P (pressure) and T (tension) axes does not show a preferred orientation; the only noticeable feature is that the T axes indicate in-plane tensions. The diversity in the orientation of the P and T axes suggests a more complex state of stresses in the subducted slab than expected from the application of a normal subduction model. Such a complexity may be related to the arched plate geometry, to stresses generated by phase changes in the plate and to plate interactions at their boundaries.

Text

2010-11-05

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http://hdl.handle.net/2429/29828

Geophysics

University of British Columbia

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