Lasers Finding Faults in Central Idaho

Idaho, with a box around the Sawtooth Mountains where the fault was recently found. Future movement on the fault will be felt in Boise, the largest city in Idaho. Figure made using TopoCreator.com.

ISU geology professor and department chair Glenn Thackray led a team that recently discovered a previously unknown earthquake-producing fault in the Sawtooth Mountain Range in central Idaho, USA. The fault is about 40 miles long and thought to be comprised of two segments.  Capable of producing a magnitude 7.0 earthquake (similar to the 1989 San Francisco Earthquake), it was last active 4000 and 7000 years ago. If the fault ruptured again, it could cause major damage to the Sawtooth National Recreation Area and Sun Valley Ski Resort. A major earthquake could produce moderate shaking in the Wood River Valley (pop. 22,000), located 25 miles south, and the Boise metropolitan area (pop. 617,000), located 65 miles southwest.

The Sawtooth Mountain Fault. Aerial photo one left shows vegetation that prevented previous identification of the fault. The fault is highlighted in hot pink in the shaded relief map on the right. Note: images not at exact same scale. Modified from Thackray et al. (2013).

The geoscientists from Idaho State University mapped the fault using Light Detection and Ranging (LiDAR) technology.  LiDAR, a light-based remote sensing method, is similar to radar or sonar; it measures the time it takes for a light shined at a surface to return to its source, calculating the distance from the surface to the receiver using the speed of light.  Thousands of data points are combined to create a detailed map of the earth’s surface.  LiDAR is ideal for studying faults that are difficult to detect using traditional mapping techniques because it allows for remote access to inaccessible terrain. In this study, data was collected by an airplane equipped with a LiDAR scanner that allowed safe and rapid data collection in a relatively remote part of Idaho.

One of the challenges that comes with using LiDAR is that it records trees, bushes, and other vegetation as well as the earth’s surface below.   While vegetation mapping is perfect for some applications, including mapping fuel loads for forest fires, it masks the rock surfaces that are indicative of faults.   This study shows that the current software for removing vegetation from the LiDAR images is precise enough to reveal faults in the surface of the earth if the resolution of data is high enough – in this case, point spacing of less than a yard was sufficient to find the previously unknown fault through the dense forest growing over it.

Mapping via LiDAR through vegetation has major implications for studying faults and other landforms.  There are many places where rocks are not exposed or are only exposed in small places; geologists can now map these zones with more detail thanks to LiDAR, including finding more unknown faults like the one in the Sawtooth Mountains.  

Original article: Thackray, Glenn D., David W. Rodgers, and David Streutker. "Holocene scarp on the Sawtooth fault, central Idaho, USA, documented through lidar topographic analysis." Geology 41.6 (2013): 639-642.