SEGMENTATION OF HANGINGWALLS AT THE BASE OF FAULT BLOCK MOUNTAINS IN IDAHO: THEIR INFLUENCE ON THE EXTENT AND AFTERSHOCK DISTRIBUTION OF THE BORAH PEAK EARTHQUAKE OF 1983

The valley floors on the hangingwall adjacent to a segmented mountain front faults are also segmented. The valley segments are defined by gravity saddles extending from the fault segment boundaries on the footwall across the hangingwall valley to the back slope of the adjacent fault block mountain. In the Lost River Valley, the hangingwall blocks are ~25 km wide and ~16 km thick.

During the Borah Peak Earthquake, a hangingwall block collapsed under gravity toward the Lost River Fault leaving the two adjacent blocks at their pre-collapse elevation; The blocks that remained standing in relief acted as offset barriers to seismic energy. Seismic energy spreading outward from the focus toward the adjacent offset barriers may be absorbed or diverted by them. Barriers standing in higher relief may completely stop the seismic waves whereas those in lower relief may allow leakage into the barrier block. The distribution of aftershocks in the collapsed block is controlled by (1) the offset barriers and (2) the shape of the collapse bend. A survey after the earthquake showed the land surface bending downward toward the fault in a way that is suggestive of the actual shape of the collapsed block. Borah Peak aftershocks were concentrated at the beginning of the bend about 10 km to 20 km from the fault.