Abstract

The 29 May and 5 June 1940 Richardson Mountains earthquakes are the two largest events (MS = 6.2 and 6.5, respectively) known to have occurred in the northern Yukon Territory, near the boundary of the Canadian Cordillera and the stable continental craton. The earthquakes were located ∼ 20 km apart at 66.87°N, 135.33°W and 66.85°N, 135.80°W, respectively. For the May event, mB = 6.3, MS = 6.2 and M0 = 3.5 × 1025 dyne-cm, and for the June event mB = 6.4, MS = 6.5 and M0 = 7.5 × 1025 dyne-cm. Modeling of teleseismic and regional body waves for these earthquakes indicates a complex rupture sequence for each, consisting of two to three subevents. Focal depths ranged from 14.5 to 11 km for the May rupture sequence and 10 to 7 km for the June sequence. The focal mechanisms determined from modeling body waves are primarily strike-slip along either N-S or E-W trending planes. The former is preferred, based on mapped surface faults in this region, and the relative locations determined for the subevents. The mechanisms are similar to those of other earthquakes in the Richardson Mountains, but different from the pure thrust events that are observed in the Mackenzie Mountains, ∼200 km to the southwest. The alignment of the near-horizontal pressure axes for the Richardson Mountains earthquakes suggests reactivation of pre-existing zones of weakness within the crust by the contemporary stress field. Stress drops for these earthquakes (22 to 28 bars) are low for intraplate events, but similar to those estimated for other earthquakes in the eastern Canadian Cordillera.

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