Seismic Reflection Project Near the Southern Terminations of the Lost River and Lemhi Faults, Eastern Snake River Plain, Idaho

Abstract

SUMMARYThirteen seismic reflection lines were processed and interpreted to determine the southern terminations of the Lost River and Lemhi faults along the northwest boundary of the eastern Snake River Plain (ESRP). The southernmost terminations of the Arco and Howe segments were determined to support characterization of the Lost River and Lemhi fault sources, respectively, for the INL probabilistic seismic hazard analysis.Four commercial seismic reflection lines (Arco lines 81-1 and 81-2; Howe lines 81-3 and 82-2) were obtained from the Montana Power Company. The seismic data were collected in the early 1980's using a Vibroseis source with station and shot point locations that resulted in 12-fold data. Arco lines 81-1 and 81-2 and Howe lines 81-3 and 82-2 are located within the basins adjacent to the Arco and Howe segments, respectively.Seven seismic lines (Arco lines A1, A2, A3, and A4 and Howe lines H1, H2, and H3) were acquired by EG&G Idaho, Inc. Geosciences for this study using multiple impacts with an accelerated weight drop source. Station and shot point locations yielded 12-fold data. The seismic reflection lines are oriented perpendicular to and at locations along the projected extensions of the Arco and Howe fault segments within the ESRP.Two seismic lines (Arco line S2 and Howe line S4) were obtained from Sierra Geophysics. In 1984, they acquired seismic reflection data using an accelerated weight drop source with station and shot point locations that yielded 6-fold data. The two seismic reflection lines are oriented perpendicular to and at locations along the projected extensions of the Arco and Howe fault segments within the ESRP. In 1992 for this study, Geotrace Technologies Inc. processed all of the seismic reflection data using industry standard processing techniques.

Strain rates and contemporary deformation in the Snake River Plain and surrounding Basin and Range from GPS and seismicity

“…The mountain ranges adjacent to these normal faults end abruptly at the physiographic boundary of the eastern Snake River Plain. At the ends of the ranges, bedrock structural features, gravity data, and seismic refl ection data suggest that significant seismogenic fault movements do not extend into the eastern Snake River Plain (Mabey, 1978;Bruhn et al, 1992;Jackson et al, 2006). Some investigators, however, suggest that the normal faults extend into the eastern Snake River Plain and may be covered over by Quaternary basalt fl ows (e.g., Rodgers et al, 2002).…”

Extension-driven right-lateral shear in the Centennial shear zone adjacent to the eastern Snake River Plain, Idaho

“…Recent faulting is evidenced by the 1983 moment magnitude (M) 6.9 Borah Peak, Idaho, earthquake, which ruptured the central segment of the Lost River fault with oblique normal slip that included a 17% sinistral component (Crone et al, 1987). At the southern ends of the three normal faults and within the proposed Centennial shear zone, bedrock structural features inferred from gravity and seismic-refl ection data suggest that signifi cant fault offsets do not extend into the eastern Snake River Plain (Mabey, 1978;Jackson et al, 2006). However, short and generally left-stepping, en echelon, NWtrending normal faults and monoclinal fl exures expressed in 400-730 ka basalt fl ows within the eastern Snake River Plain may be interpreted as slip on the Lost River fault (Kuntz et al, 1994(Kuntz et al, , 2002.…”

SSHAC Level 1 Probabilistic Seismic Hazard Analysis for the Idaho National Laboratory

“…Alternatively, others interpret the short normal faults and monoclines to be tectonic in origin and related to slip on the Arco segment (Wu and Bruhn, 1994;Kuntz et al 2002). Jackson et al (2006) interpreted both west-and east-dipping faults on seismic reflection lines AR2 and AR3 in Figure 4-16 acquired in the Arco VRZ and suggested that the normal faults and monoclines could be products of dike intrusion. They also suggested that the westdipping faults on lines AR1 and AR2 may be associated with slip on the Arco segment to a distance of 4.6 km south of its mapped location at the end of the Lost River Range (Figure 4-16).…”

“…The Lemhi fault may terminate just south of the Lemhi range (Figure 4-16). Seismic reflection line SC1 shows a southwest dipping fault, which is thought to be the southernmost extent of the Lemhi fault and two the south, lines SC2 and SC3 show flat lying continuous reflections (Jackson et al, 2006). Although limited investigations have been performed, interpretations of regional gravity data suggest the Birch Creek valley may partially extend into the ESRP and that a volcanic vent may be aligned with a splay fault of the Beaverhead fault (Figure 4-10) (Woodward-Clyde Federal Services et al, 1996).…”