Shallow structure from a seismic reflection profile across the Borah Peak, Idaho, fault scarp

Abstract

A short 12-fold CDP seismic-reflection survey was performed along the road to Doublespring Pass across the fault scarp formed by the October 28, 1983, magnitude-7.3, Idaho earthquake. This high-resolution reflection survey was conducted to determine the feasibility of using reflection seismology to delineate shallow structures in a fault zone. Field-recording parameters were designed to optimize seismic reflections in the 30-150 msec range corresponding to 10-100 m in depth. A modified 30-06 hunting rifle was used as the energy source. Single 100-Hz geophones at 1.5-m group intervals in conjunction with 220-Hz low-cut recording filters (24 dB/octave) provided dominant frequencies above 150 Hz on field records. As would be expected from geologic considerations, the processed data suggest the existence of faulting in the subsurface. Strong events between 30 and 80 msec on the upthrown side of the scarp are of distinctly different character and frequency than those on the downdropped side at similar times. This indicates different geologic units are present at approximately the same reflection time on opposite sides of the fault zone. The northeastern edge of the scarp may not represent the true subsurface boundary of the upthrown block. Projection to the surface of the northeasternmost edge of the seismically determined subsurface graben is 10-15 m farther northeast than expected from surface faulting. High-frequency energy present within the subsurface expression of the graben is primarily noise and is related to the deformed and incoherent nature of materials within the graben.