An earthquake swarm on the Queen Charlotte Islands Fracture Zone

Abstract

abstract An earthquake swarm occurring on a segment of the ocean ridge system in the northeast Pacific Ocean is investigated by means of standard seismograph records from the western portion of British Columbia, Canada, covering the period from August 27 to September 1, 1967. Five seismograph stations (PHC, ALB, VIC, FSJ and PNT), all located within regional distances, are used as an array to locate 19 of the larger magnitude events; PHC, on the northern end of Vancouver Island less than 200 km from the source area, recorded 217 distinct arrivals in the 5-day period so that statistical properties of the swarm and the magnitude-frequency relationship can be estimated. The Pn and Sn arrivals in western British Columbia show the following time-distance relationships: ( T P n ± 1.00 ) = ( 5.51 ± 0.38 ) + Δ / ( 7.69 ± 0.03 ) ( T S n ± 2.30 ) = ( 10.16 ± 1.25 ) + Δ / ( 4.45 ± 0.03 ) These travel times are consistent with a crust on Vancouver Island greater than 50 km thick. The events in the swarm show a nonrandom distribution in time and the polarity of the first arrival varies with station and with event. The magnitude-frequency relationship for 217 events is determined as ( Log 10 N ± 0 . 2 ) = ( 4.1 ± 0.5 ) − ( 1.3 ± 0.2 ) M L or 2.0 M L 4.0 . All epicenters are located with respect to a regional model of the Earth9s crust which approximates the nature of the crust of the Earth in western British Columbia and the northeast Pacific Ocean. The resulting epicenters for events in the swarm are associated with two distinct tectonic features in the northeast Pacific Ocean—the Explorer Trench and the Queen Charlotte Islands Fracture Zone.