PALEOSEISMIC INVESTIGATION OF AN ACTIVE FAULT NEAR BUTTE, MT

The Montana Bureau of Mines and Geology (MBMG) trenched an active fault near Butte, MT to find slip rate and earthquake recurrence intervals. The trench—the first by the MBMG in 33 years—is within the northern Intermountain Seismic Belt and at the northeastern edge of the Basin and Range extensional province. Faults in the region have produced earthquakes as large as the 1959 M_w 7.2 Hebgen Lake earthquake. Slip rates and earthquake recurrence intervals of most faults in the region remain unknown.

Extension near Butte is accommodated by several normal to oblique slip faults that create a prominent range front and 360–460 m deep sedimentary basin. Previous assessments suggest that faults in the area may pose a risk to Butte, but land surface modification by mining and development since the 1860s precludes paleoseismic trenching on the main structure. Fortunately, we can trace a splay north into a small basin called Elk Park, where Quaternary deformation is evident.

Our trench crossed a >2 m west (uphill) facing scarp at the northern end of Elk Park. The scarp cuts hummocky topography consistent with liquefaction or low angle mass movement, forming a series of ponds. Sand and clay layers (unit 1) at the base of the trench are highly deformed and show evidence of low-angle movement towards the east. Unit 1 is overlain and incised by a clast-supported gravel (unit 2), with a highly irregular top surface. Unit 2 is overlain by fluvial to lacustrine sand (unit 3) that increases in clay content towards the lower end of the trench and is capped by a prominent paleosol. An ash in unit 3 correlates with the 11,200 ¹⁴C yr BP Glacier Peak eruption. Units 1 through 3 are offset by two high angle fractures with ~25 cm of normal movement. The top sediment in the trench is grey ashy sand (unit 4) that transitions west into four clay units (units 4a through 4d). Units 3, 4a and 4b are offset by small fractures with up to 10 cm normal movement.

We propose that the Elk Park scarp formed by fault propagation folding. If so, it reflects at least 3.5 m of folding, 1 m of which occurred after 11,200 ¹⁴C yr BP. It is possible that there are multiple hinges converging towards the center of the scarp where the fault will eventually break through as it propagates upward. Future movement could potentially produce earthquakes, posing a hazard to the city of Butte.