Revisiting the Borah Peak Leveling Line: 30+ years of interseismic deformation across the Lost River fault

The 1983 Borah Peak, Idaho, earthquake (M 7.0) produced both local ground surface rupture and notable far-field geodetic elevation changes that inspired a suite of investigations into coseismic flexural response. Shortly after the earthquake, Stein and Barrientos revisited a 50 km leveling line that runs roughly perpendicular to and spanning the Lost River normal fault. They found 1 meter of surface subsidence adjacent to the fault on the hanging wall that decays to no detectable change over 25 km distance from the fault. On the footwall, 20 cm of surface uplift was observed adjacent to the fault, decaying to zero change over 17 km. Though the changes in elevation are calculated as a difference between the first leveling in 1933 and the post-event leveling in 1984, they treat this change as the coseismic period, assuming little change between 1933 and 1983. A subsequent survey in 1985 revealed no significant change, suggesting that postseismic relaxation was complete. We evaluate the assumption that no detectable interseismic slip occurred between 1933 and the Borah Peak event by resurveying the line and differencing elevations between 2017 and 1985. If interseismic slip is insignificant, then there should be no detectable change over these 32 years. Using RTK GNSS with a 3D error ellipse of 0.9 cm, we resurveyed all leveling monuments in June, 2017. Significant deformation was observed. Between 1985 and 2017, 28 cm of displacement occurred across the fault. The hanging wall, adjacent to the fault, subsided 8 cm while the footwall rose 20 cm. Subsidence on the hanging wall increases slightly with distance away from the fault, reaching a maximum of 10 cm at a distance of 4 km from the fault and decaying to zero by 17 km. On the footwall surface uplift increases from 20 cm at the fault to 42 cm by 6.5 km before decaying. Clearly interseismic deformation has occurred over the last 32 years, including both discrete slip at the fault and distributed subsidence or surface uplift with distance away from the fault. A difference between the 2017 and 1933 data reveal that the opposing patterns of deformation pre and post event at on the footwall largely balance each other out, creating block-like surface uplift. These vertical changes are complemented by observations from continuous geodetic GNSS that corroborate the interseismic extension.