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 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 km segments

Breckenridge et al., 2003 Faults Red: Holocene Orange: Late Quaternary Yellow: Quaternary Green: Tertiary Note Borah etc faults Sawtooth fault: Late Q Note Borah etc faults Sawtooth fault: Late Q HL BP

LiDAR datasets in two range front areas. Earthquake epicenters Data Source: USGS Years of record:

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 ka) 6 m offset (avg. 4-8 m) 2-3 events inferred ca 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 surface LiDAR-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 o 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 documented fault scarp not documented 10 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 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 time 10 km

Potential Hazards 1. Stanley (6 km, pop. 300), high summer visitation 2. Large, moraine- dammed lakes have high summer usage 3. Sun Valley (40 km, pop ) 4. Boise metropolitan area (100 km, pop. 400,000) Earthquake epicenters >M4.5 Data Source: USGS Years of record:

Conclusions The Sawtooth Fault has experienced 2-3 events since ka The scarp indicates 2-4 m offset per event Active fault length is km, with a possible segment boundary separating fault segments of 40 km and 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