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

Abstract

Thirteen 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. Keywords:Keywords are required forExternal Release Review*Keywords Keywords *Contacts (Type and Name are required for each row) Type ofContactContact Name POC Editor RecordFour 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 aremore » 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. The southern termination of the Howe segment of the Lemhi fault was placed between Howe lines H1 and H2, 2.2 km south of the fault’s southernmost surface expression. In the adjacent basin, south-dipping normal faults at the northern end of Howe line 81-3 and two southwest-dipping normal faults at the northeastern end of Howe line 82-2 that can be correlated with Howe segment. South of the surface expression, two southwest-dipping normal faults on Howe line H1 can be correlated with the Howe segment. Further into the ESRP, Howe lines H2, H3, and S4 show continuous flat lying reflectors and indicate no fault offset. The southern termination of the Arco segment of the Lost River fault was placed between Arco lines S2 and A3, a distance of 4.6 km south of the fault’s southernmost surface expression. Within the basin, west-dipping normal faults interpreted on Arco lines 81-1 and 81-2 can be correlated with the Arco segment. Further south within the Arco volcanic rift zone (VRZ), three seismic lines (Arco lines A2, S2, and A3) permit two interpretations. The west- and south-dipping normal faults on Arco lines A2 and S2 could be associated with slip along the Arco segment. These normal faults have an opposite dip to an east-dipping fault on Arco line A3. The observed small-offsets (< 85 m) along the oppositely dipping normal faults can be interpreted as a graben structure that resulted from dike intrusion within the Arco VRZ. Arco line A4 further south within the Arco VRZ shows flat lyin« less