Abstract

On 16 March 1999 at 12:50 UT, a moderate earthquake of mN 5.1 occurred in the Lower St. Lawrence Seismic Zone (LSZ), 60 km south of Sept-Îles, Québec. Due to its location under the St. Lawrence River, more than 50 km away from inhabited areas, the earthquake was named the Côte-Nord earthquake (the French name for the Quebec North Shore region along the St. Lawrence River). In this zone, approximately 60 events are located yearly; historically, only one earthquake was nearly as large as the Côte-Nord event. Felt up to 300 km distant, the mainshock did not cause any damage. Peak ground accelerations of 0.22 g and 0.13 g were measured at epicentral distances of 130 km and 360 km by digital accelerographs. Three different determination methods each suggest a depth of approximately 20 km, within the Precambrian basement. A mixed strike-slip/reverse-faulting focal mechanism was obtained using regional data. Surface-wave inversion suggests a similar solution but with slightly different nodal planes. No foreshock was recorded by the regional network, which has a mN 2.0 detection threshold. The number of aftershocks was anomalously high for a mN 5.1 mainshock; more than 75 aftershocks with mN ranging from 0.9 to 3.5 were recorded in the 30 days following the mainshock. Characteristic parameters of aftershock decay were normal. Within these aftershocks, five doublets and one triplet were found, representing 13 out of 77 events. We interpret the northwest-southeast nodal plane to be the fault plane for the mainshock from the preferred alignment of aftershocks and from the orientation of nearby magnetic and gravimetric lineaments. This orientation is, however, different from the general assumption that northeast-southwest faults are those reactivated in the current local stress field.

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