The seismotectonics of southern Saskatchewan

Abstract

Nine earthquakes are known or thought to have occurred in southern Saskatchewan to the present time. This small number of known events is due in part to the fact that earthquakes less than about magnitude 4 could not be instrumentally located in the region before the mid-1960's. The largest earthquake on May 15, 1909, magnitude mb[Formula: see text], was widely felt over central Canada and the north-central United States and could have been located in southern Saskatchewan, northeastern Montana, or northwestern North Dakota. Four recent minor earthquakes, one near Bengough in 1972, two near Radville, and one at Esterhazy in 1976, were all strongly felt over small areas.There is a general spatial correlation between the observed seismicity and the more structurally disturbed areas of the Precambrian basement and overlying sedimentary sequence in the Williston Basin region. In south-central Saskatchewan, four of the instrumentally located earthquakes show a spatial correlation with a large multi-state salt-solution structure marked by a major anomaly in electrical conductivity and a steep Bouguer gravity gradient. The abundant evidence for zones of basement weakness and their control on structures in the Phanerozoic sequence suggests that reactivation of these zones in response to the contemporary stress field may also be responsible for the seismicity.Support is given for the vertical strike-slip mechanism proposed for the 1972 Bengough earthquake. Other mechanisms considered possible for generating earthquakes in Saskatchewan are related to cavity formation in the Prairie Evaporite by either natural or industrial processes, specifically tensional faulting in the cap rock due to overburden pressure. Cavity collapse is not thought to be a likely mechanism for generating detectable seismic waves. Although there is a spatial correlation between some of the observed seismicity and areas of subsurface fluid injection, the absence of any direct in situ stress measurements, of knowledge of the cohesive strength of the rock in areas of fluid injection, and of accurate focal depths precludes any direct correlation.