SOURCE PARAMETERS OF THE 29 MAY AND 5 JUNE, 1940 RICHARDSON MOUNTAINS, YUKON-TERRITORY, EARTHQUAKES

The 29 May and 5 June 1940 Richardson Mountains earthquakes are the tw o largest events (M(S) = 6.2 and 6.5, respectively) known to have occu rred in the northern Yukon Territory, near the boundary of the Canadia n Cordillera and the stable continental craton. The earthquakes were l ocated approximately 20 km apart at 66.87-degrees-N, 135.33-degrees-W and 66.85-degrees-N, 135.80-degrees-W, respectively. For the May event , MB = 6.3, M(S) 6.2 and M0 = 3.5 X 10(25) dyne-cm, and for the June e vent MB = 6.4, M(S) 6.5 and M0 = 7.5 x 10(25) dyne-cm. Modeling of tel eseismic and regional body waves for these earthquakes indicates a com plex rupture sequence for each, consisting of two to three subevents. Focal depths ranged from 14.5 to 11 km for the May rupture sequence an d 10 to 7 km for the June sequence. The focal mechanisms determined fr om modeling body waves are primarily strike-slip along either N - S or E - W trending planes. The former is preferred, based on mapped surfa ce faults in this region, and the relative locations determined for th e 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, approximately 200 km to the southwest. The alignment of the near-horizontal pressure axes for the Richardson Mountains earthquakes suggests reactivation of pre-exi sting zones of weakness within the crust by the contemporary stress fi eld. Stress drops for these earthquakes (22 to 28 bars) are low for in traplate events, but similar to those estimated for other earthquakes in the eastern Canadian Cordillera.