ABSTRACT

The Intermountain Seismic Belt (ISB) defines the eastern limits of extending crust in the western contiguous United States. The ISB transects several older tectonic provinces and regional lineaments in the northern Rocky Mountains, while paralleling others. Since 1982, over 2300 earthquakes have occurred in northwest Montana, defining a belt of seismicity up to 150 km wide that ends abruptly in the northern Flathead Valley (south end of the Rocky Mountain trench). Two types of earthquake sequences characterize northwest Montana, discrete event earthquakes followed by a decaying sequence of aftershocks, and swarms of earthquakes in a specific area that occur over weeks to months. The vast majority of earthquake fault-plane solutions in northwest Montana display strike-slip or oblique-slip focal mechanisms on north-west-striking faults (right-lateral) or northeast-striking faults (left-lateral), with north-south compression (P-axis) and east-west tension (T-axis). Notable earthquakes during the 20th century in northwest Montana include the 1935 Helena series, the 1945 and 1952 Flathead Lake quakes, the 1969 and 1971 swarms near the southwest shore of Flathead Lake, and the 1975 quake southeast of Kalispell. The largest northwest Montana earthquake since 1982 (the time since the Montana Bureau of Mines and Geology began to catalog earthquakes) occurred in 1985 in the southern Swan Range (M 4.9), and the most significant earthquake swarm during this time period occurred in 1995 at Kila, southwest of Kalispell. Recent seismicity is conspicuously absent in the vicinity of the down-dip projection of the southern Mission Fault, despite 7 m high Holocene fault scarps. Several earthquakes located within or near the Lewis and Clark zone (LCZ) have fault-plane solutions that are consistent with continuing dextral slip on steeply dipping, west northwest-trending faults that characterize the LCZ.

Our model of regional extension places the northern limit of the structural Basin and Range province at the north end of the Flathead Valley, in the southern Rocky Mountain trench. To the south, the northern Rockies are extending westward in five quasi-coherent crustal domains bounded by right-lateral, strike-slip and oblique-slip accommodation and transfer zones, with each south-side domain translating further west than those to the north. These domains are the Flathead zone, Lewis and Clark zone, Big Belt zone, Madison-Lost River zone, and the Idaho batholith zone. The main accommodation zones across the northern Rockies and Pacific Northwest are the Lewis and Clark zone, the Olympic-Wallowa lineament, the Vale zone, and the Brothers fault zone. This model predicts a horizontal velocity field (westward extension accompanied by clockwise rotation) for the region between the Snake River Plain and north-west Montana that will be tested in the near future with data from new continuous Global Positioning System (GPS) stations.