Preliminary Evaluation of a Newly Discovered Holocene Scarp on the Sawtooth Fault, Central Idaho

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Preliminary Evaluation of a Newly Discovered Holocene Scarp on the Sawtooth Fault, Central Idaho. Glenn D. Thackray, David W. Rodgers, Eric Johnson and Mark Shapley Idaho State University Department of Geosciences. Significant findings. - PowerPoint PPT Presentation

Text of Preliminary Evaluation of a Newly Discovered Holocene Scarp on the Sawtooth Fault, Central Idaho

  • Preliminary Evaluation of a Newly Discovered Holocene Scarp on the Sawtooth Fault, Central IdahoGlenn D. Thackray, David W. Rodgers, Eric Johnson and Mark Shapley

    Idaho State UniversityDepartment of Geosciences

  • Significant findingsThe Sawtooth Fault is a very active range-bounding normal faultThe fault has experienced 2-3 postglacial events with 2-4 m average offsetA segment boundary may divide the fault into 40 km and 15-25 km segments

  • Breckenridge et al., 2003FaultsRed: HoloceneOrange: Late QuaternaryYellow: QuaternaryGreen: TertiaryNote Borah etc faultsSawtooth fault: Late QHLBP

  • Earthquake epicentersData Source: USGSYears of record: 1872-1993

  • LiDAR derived topography, Redfish Lake area, converted to Bare-Earth DEM with 1 m resolution5000 m

  • 1000 m

  • Offset of Late Pleistocene deglacial surface (ca. 11.5-14 ka)

    6 m offset (avg. 4-8 m)2-3 events inferredca. 0.3-0.5 m/ka offset rate6 m offsetLate Pleistocene surfaceOffset of Holocene fluvial surface

    2.5 m offset (avg. 2-3 m)1 event inferred2.5 m offsetHolocene surfaceLiDAR-derived scarp profiles

  • fault scarp documented

    fault scarp not documentedField observationsScarp is consistently 5-8 m high where observed in latest Pleistocene surfaces10 km?

  • Fault scarp reconnaissanceFault scarp is 5-8 m high in de-glacial landscapes in most locations visitedFault scarp generally slopes 30-35o and is composite, with no readily apparent younger scarp in most locationsFault scarp appears to be a single strand in most areasIs the fault scarp segmented?When were the most recent events?

    Outstanding questions

  • Pettit Lake: 7 m scarp with steeper lower portion

  • fault scarp documentedfault scarp not documented-range front is less dramatic in this area-scarp is less apparent or absent in this area-scarp is less continuous south of this area-40 km and 15-25 km segments Possible fault segment boundary10 km?

  • Post-glacial fault historyLake sediments provide potential paleoseismic archivesThree lakes contain observed disturbance event stratigraphyMost recent event occurred in mid-Holocene time10 km

  • Potential Hazards1. Stanley (6 km, pop. 300), high summer visitation2. Large, moraine-dammed lakes have high summer usage3. Sun Valley (40 km, pop. 1500+)4. Boise metropolitan area (100 km, pop. 400,000)

  • ConclusionsThe Sawtooth Fault has experienced 2-3 events since 11-14 kaThe scarp indicates 2-4 m offset per eventActive fault length is 55-65 km, with a possible segment boundary separating fault segments of 40 km and 15-25 kmLake sediments provide an apparent paleoseismic record, with the most recent event in mid-Holocene timeThe fault activity may present significant hazards to population centers, including Boise

  • AcknowledgmentsLiDAR acquisition funded by Idaho NASA EPSCoR programLiDAR data processed by David Streutker and Nancy Glenn, ISUPortions of field mapping and fault scarp profiling completed by Joshua Keeley and Chris Kemp, ISU

    Northern B and R to east: west dipping normal faults, Holocene activitySawtooth Fault: east dipping normal fault: late Quaternary activity