A relationship exists between the depth of the mainshock and the occurrence of aftershocks , and several Pacific Northwest earthquakes illustrate this link , like the February 1981 M5 .5 Elk Lake event in southwest Washington that was followed by more than 1,000 in the first two years . The M7 .3 1959 Hebgen Lake earthquake in Montana had a significant sequence of aftershocks , and the shallow M7 1983 Borah Peak event was followed by four aftershocks . In opposition , the intraslab events ( and crustal shocks above the subduction zone ) on the west side of the Cascades have had insignificant aftershock sequences , usually amounting to a minimal number of small aftershocks . For example , the 2001 Nisqually shock occurred nearly deep and was followed by only four small aftershocks , and there was a similar procession for the 1946 Vancouver Island earthquake , a M7 .6 crustal shock that also had a focal depth near
1983 Borah Peak event
1983 Borah Peak earthquake
1872 North Cascades earthquake