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The Prince George, British Columbia, earthquake of 21 March 1986

GEOLOGICAL SURVEY OF CANADA PACIFIC GEOSCIENCE CENTER, P.O. Box 6000, SIDNEY, B.C. V8L 4B2, Canada
DEPARTMENT OF GEOPHYSICS AND ASTRONOMY UNIVERSITY OF BRITISH COLUMBIA, VANCOUVER, B.C. V6T 1W5, Canada

Abstract

On 21 March 1986 a widely felt earthquake occurred near 54°N, 122°W adjacent to the Rocky Mountains in east central British Columbia in an area of low historic seismicity. The earthquake had a body-wave magnitude of 5.4, a surface-wave magnitude of 5.2, and a seismic moment of 2.4 x 10²⁴ dyne-cm. The stress drop is estimated as 3 bars from the aftershock area. The epicentral region is sparsely populated and only minor damage occurred although the earthquake was felt strongly (Modified Mercalli intensity of V) in the nearby city of Prince George. Within 48 hours of the main shock, portable short-period seismographs began to be deployed in the epicentral region and were maintained there for eight days. Few aftershocks were recorded. They were confined to a depth range of 9 to 16 km and the largest was magnitude 2.5. No foreshocks were detected by the Canadian Seismograph Network. The focal mechanism of the earthquake was thrust faulting on a northwest-southeast plane dipping about 40° to the southwest. This is similar to the structural trends mapped in the region. The occurrence of this earthquake and several other moderate to large earthquakes in the Foreland Fold and Thrust Belt raises the question of whether there is a potential seismic hazard along the entire eastern margin of the Canadian Cordillera. The lower than expected attenuation for seismic waves observed for this earthquake also has implications for seismic hazard assessment in the Cordillera.

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