STATIC STRESS CHANGES AND FAULT INTERACTION RELATED TO THE 1985 NAHANNI EARTHQUAKES, WESTERN CANADA

The 1985 Nahanni earthquakes (05/10/1985, Mw=6.6; 23/12/1985, Mw=6.8) were the largest observed events of the past 100 years not only in the Nahanni region of Canada but in the NE Cordillera. Their short interevent period (about two months) between the two mainshocks and their shallow depth (about 8 km) make them interesting since those characters for seismicity has rarely been reported. I compute the changes in static stresses along optimally oriented planes, caused by these events using the slip models of Hartzell et al. [1994] for the mainshock sources to investigate the possibility of dip-slip interaction and compare the relation between the patterns of aftershock seismicity and static stress. Thus, I then combine our coseismic stress changes with the regional stress to determine the induced stress changes for explaning short-term field-recorded aftershock seismicity nearly in one month and medium-term seismicity (1985-2002). The static stress change generated by the 5 October event onto the fault plane of December event are calculated and concluded that the occurrence of the 23 December Nahanni event seems triggered since the shallow major asperity along the fault plane is appeared to be induced by stress due to October earthquake Subsequently, in 1988, a large event (Mw=6.2) which is located at 62.11 N and 124.209 W (Boore and Atkinson 1989) and its aftershocks, occurred in a region where the static stress level was increased by the 1985 Nahanni earthquakes.