The next time you find yourself at the bottom of a cliff, make sure to look up.
On 24 June 2020, a M5.8 earthquake near Lone Pine, California, shook some huge boulders loose on the side of a cliff overlooking the visitor facilities at Whitney Portal, the stepping-off site for hikers climbing Mount Whitney (the highest point in the contiguous United States) and 28 km (17 mi) distant from the epicenter. Several large boulders and smaller debris fell from the cliff, and some large pieces came to rest in one of the campgrounds and a parking lot. The campground was closed due to COVID-19, but the other facilities in the area and the parking lot were open for day visitors and hikers. No buildings were hit, and only one of the approximately 40 cars that were in the parking lot was damaged, although nearly everything was covered with rockfall dust. Fortunately, no one was hurt.
Rock faces in mountain ranges are prone to rockfalls by nature of their steep slopes and exposure to the forces of weathering. We have a basic understanding of how rockfalls work, but the devil is in the details. The pile of rocks and boulders at the base of a steep slope or cliff, the talus slope, indicates where most of the rocks land when they fall, but here’s the thing… sometimes they land somewhere else. The very ones that are most likely to do damage are also the ones that are likely to travel farther than the talus slope – the so-called “outliers.” Massive boulders with a lot of momentum can hit the talus and keep rolling (or move a boulder already on the talus pile), careening off trees and each other and spinning off bits of “flyrock” – fragments that shoot off in unpredictable directions – in the process. The Whitney Portal rockfall offered an opportunity for scientists to gather information about these details to help understand the hazards in this and similar areas, as well as the risks they pose.
The M5.8 earthquake near Lone Pine, 28 km (17 mi) to the southeast of Whitney Portal, caused shaking at a relatively low intensity of MMI V (moderate shaking), but it was enough to loosen the rocks high up on the cliff about 270 m (885 ft) above the parking lot. The rockfall buried an 85 m (280 ft) section of the hiking trail at the base of the talus slope, destroyed 190 trees and several campground sites, damaged a parking lot, and narrowly missed a nearby hiker. Three U.S. Geological Survey (USGS) scientists and one U.S. Forest Service (USFS) scientist arrived on site within days to map the rockfall debris area and its impacts. They returned two more times, a few weeks later and again about five months later, to gather additional details of the event in an attempt to reconstruct exactly what happened during the rockfall event… conducting what was much like a crime scene investigation (CSI).
With various mapping techniques, painstaking documentation of impact locations, and a modeling software package aptly called “RockyFor3D,” USGS scientists were able to piece together the details of those harrowing seconds between the start of the shaking and when the final boulder came to rest in the parking lot. According to the forensic evidence, the shaking from the earthquake loosened a total of about 2500 cubic meters (3270 cubic yd; for reference, a standard dump truck carries ~12 cubic yd) of rockfall debris from three areas on the side of the cliff. One mass of debris hit the talus and sent three boulders beyond the edge of the existing talus slope, one of which caused most of the damage in the campground.
The winner of the “farthest travelled award” was a ~0.5 cubic meter (18 cubic ft) piece of flyrock that was generated at the base of the talus when two boulders collided. It landed 92 m (302 ft) from the location of the collision, leaving a 30 cm-long (12 in) gash in the parking lot where it bounced before it came to rest. About 190 trees were destroyed, some with trunks of more than 90 cm (35 in) in diameter; some were at the base of the cliff, but several were in the campground area beyond the edge of the talus slope. Interestingly, one can estimate where previous rockfalls have occurred not only by the location of the talus slopes, but also by the less dense forests at the foot of similar cliffs.
The take-aways from this research were that to determine the most accurate hazard from potential rockfalls, one must consider the size of the boulders involved and their interactions with each other, with trees, and with the talus boulders, as well as the flyrocks that can be generated by collisions. Even if you don’t look up when you’re beneath a steep cliff, observing the nature of the forest and the location of the talus slope and outlying boulders can give you an idea of the natural hazards where you are.
- written by Lisa Wald, February 4, 2022
Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
More Information:
- M5.8 Lone Pine, CA earthquake webpage
- Collins, Brian.D., Corbett, Skye C., Horton, Elizabeth J., and Gallegos, Alan J. (2021). Rockfall Kinematics from Massive Rock Cliffs: Outlier Boulders and Flyrock from Whitney Portal, California, Rockfalls, Environmental & Engineering Geoscience, v27, no4, pp1–22.
The Scientist Behind the Science
Brian Collins is a Research Civil Engineer with the USGS in California. He studies landslides of all types (rock falls, rock slides, shallow and deep landslides) with a particular focus on hazards that they can cause as well as the mechanics of their initiation – that is, at what point stable soil or rock suddenly begins to move. When not working, he can be found traversing (hopefully) stable landscapes while rock climbing, running, or biking.
- Overview
The next time you find yourself at the bottom of a cliff, make sure to look up.
Sources/Usage: Public Domain. Visit Media to see details.An image of 24 June 2020 (B4) and prehistoric (P2) outlier rockfall boulders within the Mount Whitney Trailhead Campground. Boulder P2 is approximately 3 m tall. The blue dashed line indicates one of several tent sites affected by the 2020 rockfalls. On 24 June 2020, a M5.8 earthquake near Lone Pine, California, shook some huge boulders loose on the side of a cliff overlooking the visitor facilities at Whitney Portal, the stepping-off site for hikers climbing Mount Whitney (the highest point in the contiguous United States) and 28 km (17 mi) distant from the epicenter. Several large boulders and smaller debris fell from the cliff, and some large pieces came to rest in one of the campgrounds and a parking lot. The campground was closed due to COVID-19, but the other facilities in the area and the parking lot were open for day visitors and hikers. No buildings were hit, and only one of the approximately 40 cars that were in the parking lot was damaged, although nearly everything was covered with rockfall dust. Fortunately, no one was hurt.
Sources/Usage: Public Domain. Visit Media to see details.Aerial view of Whitney Portal, CA, showing the 24 June 2020 rockfall source areas (SA) and the runout/deposit zone that covers approximately 2.4 hectares of the talus slope. Rockfall effects include damage to a hiking trail, a campground, and a parking lot. Note that the main map image is oriented with north pointing down. The pre-event (16 July 2016) base image is from Google Earth. Tick marks indicate latitude and longitude in the WGS84 coordinate system. Elevations are referenced to NAVD88 in meters. Rock faces in mountain ranges are prone to rockfalls by nature of their steep slopes and exposure to the forces of weathering. We have a basic understanding of how rockfalls work, but the devil is in the details. The pile of rocks and boulders at the base of a steep slope or cliff, the talus slope, indicates where most of the rocks land when they fall, but here’s the thing… sometimes they land somewhere else. The very ones that are most likely to do damage are also the ones that are likely to travel farther than the talus slope – the so-called “outliers.” Massive boulders with a lot of momentum can hit the talus and keep rolling (or move a boulder already on the talus pile), careening off trees and each other and spinning off bits of “flyrock” – fragments that shoot off in unpredictable directions – in the process. The Whitney Portal rockfall offered an opportunity for scientists to gather information about these details to help understand the hazards in this and similar areas, as well as the risks they pose.
Sources/Usage: Public Domain. Visit Media to see details.The Whitney Portal rockfall was triggered by shaking from the M5.8 Lone Pine earthquake, 28 km (17 mi) to the southeast on 24 June 2020. The M5.8 earthquake near Lone Pine, 28 km (17 mi) to the southeast of Whitney Portal, caused shaking at a relatively low intensity of MMI V (moderate shaking), but it was enough to loosen the rocks high up on the cliff about 270 m (885 ft) above the parking lot. The rockfall buried an 85 m (280 ft) section of the hiking trail at the base of the talus slope, destroyed 190 trees and several campground sites, damaged a parking lot, and narrowly missed a nearby hiker. Three U.S. Geological Survey (USGS) scientists and one U.S. Forest Service (USFS) scientist arrived on site within days to map the rockfall debris area and its impacts. They returned two more times, a few weeks later and again about five months later, to gather additional details of the event in an attempt to reconstruct exactly what happened during the rockfall event… conducting what was much like a crime scene investigation (CSI).
Sources/Usage: Public Domain. Visit Media to see details.An overview map of the 24 June 2020 rockfall deposit zone at Whitney Portal showing major debris tracks referenced to source location (SA; see previous image), locations of the talus edge and outlier boulders (B), and flyrock (FR) trajectory impact locations (1, 2, 3, 4) referenced in the event description. “SB” indicates the location of an existing 450 m3 boulder around which debris split into two lobes. The map is oriented with north pointing down. The post-event (29 June 2020) base image is from Digital Globe as displayed in Google Earth. With various mapping techniques, painstaking documentation of impact locations, and a modeling software package aptly called “RockyFor3D,” USGS scientists were able to piece together the details of those harrowing seconds between the start of the shaking and when the final boulder came to rest in the parking lot. According to the forensic evidence, the shaking from the earthquake loosened a total of about 2500 cubic meters (3270 cubic yd; for reference, a standard dump truck carries ~12 cubic yd) of rockfall debris from three areas on the side of the cliff. One mass of debris hit the talus and sent three boulders beyond the edge of the existing talus slope, one of which caused most of the damage in the campground.
Sources/Usage: Public Domain. Visit Media to see details.Images showing the flyrock trajectory from the 24 June 2020 rockfall (see also Figure 3). Rock impacts at the base of the talus slope split and moved boulders, launching a 0.5 cubic meter boulder 92 m into the lower parking lot at Whitney Portal. (A) The impact of combined boulders B6 and B7 with the 50 cubic meter boulder resulted in translation of the 300 cubic meter boulder above it, splitting of B6 from B7, splitting of the 50 cubic meter boulder, and the ballistic trajectory of flyrock fragment FR1. The horizontal dimension of the 300 cubic meter boulder is approximately 10 m. (B) The flyrock bounced in the parking lot twice before coming to rest. The second bounce caused additional smaller fragments (FR2) to damage a vehicle on the other side of the parking lot. Impact locations 1, 3, and 4 are referenced to Figure 3. Photograph in (B) is from the Inyo County Sheriff’s Office; inset photograph by Brian Olson, California Geological Survey. Photo date: (A) 7 July 2020, (B) 24 June 2020 (Inset 24 July 2020). The winner of the “farthest travelled award” was a ~0.5 cubic meter (18 cubic ft) piece of flyrock that was generated at the base of the talus when two boulders collided. It landed 92 m (302 ft) from the location of the collision, leaving a 30 cm-long (12 in) gash in the parking lot where it bounced before it came to rest. About 190 trees were destroyed, some with trunks of more than 90 cm (35 in) in diameter; some were at the base of the cliff, but several were in the campground area beyond the edge of the talus slope. Interestingly, one can estimate where previous rockfalls have occurred not only by the location of the talus slopes, but also by the less dense forests at the foot of similar cliffs.
The take-aways from this research were that to determine the most accurate hazard from potential rockfalls, one must consider the size of the boulders involved and their interactions with each other, with trees, and with the talus boulders, as well as the flyrocks that can be generated by collisions. Even if you don’t look up when you’re beneath a steep cliff, observing the nature of the forest and the location of the talus slope and outlying boulders can give you an idea of the natural hazards where you are.
- written by Lisa Wald, February 4, 2022
Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
More Information:
- M5.8 Lone Pine, CA earthquake webpage
- Collins, Brian.D., Corbett, Skye C., Horton, Elizabeth J., and Gallegos, Alan J. (2021). Rockfall Kinematics from Massive Rock Cliffs: Outlier Boulders and Flyrock from Whitney Portal, California, Rockfalls, Environmental & Engineering Geoscience, v27, no4, pp1–22.
The Scientist Behind the Science
Sources/Usage: Public Domain. Visit Media to see details.Brian Collins near Barry Glacier, Alaska. Brian Collins is a Research Civil Engineer with the USGS in California. He studies landslides of all types (rock falls, rock slides, shallow and deep landslides) with a particular focus on hazards that they can cause as well as the mechanics of their initiation – that is, at what point stable soil or rock suddenly begins to move. When not working, he can be found traversing (hopefully) stable landscapes while rock climbing, running, or biking.