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Earthquake swarm on the Queen Charlotte Islands fracture zone Wetmiller, Robert Joseph

Abstract

An earthquake swarm occurring on a segment of the ocean rise system in the northeast Pacific Ocean is considered. The data are in the form of standard seismograph records from the western portion of British Columbia, covering the period August 27 to September 1, 1967. The study follows two lines of investigation. Five seismograph stations in western British Columbia are considered as an array and 19 of the larger magnitude events located with respect to this array. The closest station to the swarm is on the northern end of Vancouver Island about 180 km from the source area. This station recorded 217 distinct arrivals in the 5-day period. Statistical properties of the swarm plus the magnitude-frequency relationship can thus be estimated. The Pn and Sn arrivals in western British Columbia show the following time-distance relationships: T[subscript Pn] = (5.51±0.38) + Δ/(7.69±0.03) T[subscript Sn] = (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 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 Island Fracture Zone. The events in the swarm show a non-random distribution in time and the polarity of the first arrival varies with station and with event. These facts suggest a generating process involving strain release by movement on a fault or faults coupled with some sort of triggering mechanism. The magnitude-frequency relationship is determined as Log₁₀N = (4.07±0.50) - (1.10±0.20)M[subscript L] for 2.0 < M[subscript L] < 4.0. The value for the slope (-1.10) is characteristic of earthquake swarms generated as a result of volcanic activity. The apparent paradox concerning the source mechanism is reconciled in the idea of "The New Global Tectonics" that extension across ridge crests (involving generation of new crustal material by diapiric intrusions) and transform faulting of the off-setting fracture zones will occur together.

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