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