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The 1989 (M_S 6.3) Ungava, Quebec, earthquake: a complex intraplate event

Geological Survey of Canada, Ottawa Ontario K1A 0Y3 Canada

Abstract

On 25 December 1989 the largest earthquake (M_S 6.3) in northern Quebec in at least 60 yr occurred in the Ungava Peninsula. This earthquake was unique in that it marked the first time surface faulting could be confirmed for a historical eastern North American earthquake. Field observations indicated a combination of thrust and strike-slip motion. Waveforms modeled in this study show that the earthquake consisted of two subevents—a thrust subevent on a NE-SW striking plane followed by a larger strike-slip subevent on a NNE-SSW striking plane. The surface waves give an equivalent double-couple moment of 1.3 x 10²⁵ dyne cm. The sum of the subevent moments is greater than the simple source moment by almost a factor of 2. Both subevents occurred at a depth of 3 km. They were separated by 0.9 sec in time with the second subevent occurring 5.2 km southwest of the first. The source parameters determined by modeling and those inferred from the ground observations generally agree, but there are some differences with respect to the subevent offset. These may be reconciled if the strike-slip motion was distributed among the several small faults at the southwestern end of the rupture or if the complete surface rupture was not found. The stress orientation (NW compression) inferred from the fault plane solution is consistent with the focal mechanisms of many other recent northeastern Canadian events. The Ungava earthquake and many intraplate events from several continents share a number of characteristics, including shallow hypocenters, source complexity, and occurrence on faults that showed no evidence for previous ruptures.

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