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 PresentationTRANSCRIPT
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 UniversityDepartment of Geosciences
Significant findings• The Sawtooth Fault is a
very active range-bounding normal fault
• The fault has experienced 2-3 postglacial events with 2-4 m average offset
• A segment boundary may divide the fault into 40 km and 15-25 km segments
Breckenridge et al., 2003
FaultsRed: HoloceneOrange: Late QuaternaryYellow: QuaternaryGreen: Tertiary HL
BP
LiDAR datasets in two range front areas.
Earthquake epicentersData Source: USGSYears of record: 1872-1993
LiDAR derived topography, Redfish Lake area, converted to Bare-Earth DEM with 1 m resolution
5000 m
1000 m
Offset of Late Pleistocene deglacial surface (ca. 11.5-14 ka)
•6 m offset (avg. 4-8 m)•2-3 events inferred•ca. 0.3-0.5 m/ka offset rate
6 m offset
Late Pleistocene surface
Offset of Holocene fluvial surface
•2.5 m offset (avg. 2-3 m)•1 event inferred
2.5 m offset
Holocene surfaceLiDAR-derived scarp profiles
fault scarp documented
fault scarp not documented
10 km
Field observations
Scarp is consistently 5-8 m high where observed in latest Pleistocene surfaces
10 km
?
Fault scarp reconnaissance• Fault scarp is 5-8 m high in de-glacial
landscapes in most locations visited• Fault scarp generally slopes 30-35o and is
composite, with no readily apparent younger scarp in most locations
• Fault scarp appears to be a single strand in most areas
• Is 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 documented10 km
-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 boundary
10 km
?
Post-glacial fault history
• Lake sediments provide potential paleoseismic archives
• Three lakes contain observed disturbance event stratigraphy
• Most recent event occurred in mid-Holocene time10 km10 km
Potential Hazards1. Stanley (6 km,
pop. 300), high summer visitation
2. Large, moraine-dammed lakes have high summer usage
3. Sun Valley (40 km, pop. 1500+)
4. Boise metropolitan area (100 km, pop. 400,000)
Earthquake epicenters >M4.5Data Source: USGSYears of record: 1872-1993
Conclusions• The Sawtooth Fault has experienced 2-3
events since 11-14 ka• The scarp indicates 2-4 m offset per event• Active fault length is 55-65 km, with a
possible segment boundary separating fault segments of 40 km and 15-25 km
• Lake sediments provide an apparent paleoseismic record, with the most recent event in mid-Holocene time
• The fault activity may present significant hazards to population centers, including Boise
Acknowledgments• LiDAR acquisition funded by Idaho
NASA EPSCoR program• LiDAR data processed by David
Streutker and Nancy Glenn, ISU• Portions of field mapping and fault
scarp profiling completed by Joshua Keeley and Chris Kemp, ISU