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A B C+20 Surface Change (cm) Date and time Volume Data sources
8m (when known) (m3) for detection
+3
30 November 2017 39.5 ± 3.8 SfM - SfM
8m 16:37 PST
+0.2 B 20–22 October 2017 187.2 ± 19.5 SfM - SfM Figure 2. Spatial and temporal pro-
(4 events) gression of rockfalls from the
0.0 southeast face of El Capitan
-2.0 28 September 2017 9,811.0 ± 408.2 SfM - SfM between Oct. 2010 and Nov. 2017.
-4.0 14:21 PST (A) Structure-from-Motion (SfM)
-6.0 model with color overlay showing
27 September 2017 453.3 ± 42.3 TLS - SfM the spatial progression of rock-
13:51–16:54 PST (7 events) falls, derived by comparing SfM
models against earlier terrestrial
9 October 2015–12 October 2016 1.1 ± 0.2 TLS - TLS laser scanning data (TLS). Nega-
(1 event) tive surface change represents
rockfall thicknesses; positive sur-
11 June 2014 977.0 ± 28.1 TLS - TLS face change, shown in (B), repre-
03:30 PST sents outward displacement of a
rock sheet by up to 20 cm. (C) Tem-
22 June 2012–11 October 2013 55.4 ± 4.6 TLS - TLS poral progression of rockfalls
(2 events) occurring between Oct. 2010 and
Nov. 2017.
Surface Change (m) 12 October 2010–22 June 2012 21.5 ± 2.5 TLS - TLS
50 m (7 events)
11 October 2010 653.0 ± 172.9 SfM - TLS
10:25 PST
Rock detached by impact from 430.2 ± 68.0 SfM - SfM
rockfalls above
50 m
-8.0
after the larger 28 Sept. rockfall, comparing rockfalls mostly consisted of rock sheets terrestrial lidar data—The 2005 event of the west
the new SfM model against data collected tens of meters tall and wide but usually <1 m face of the Drus (Mont Blanc massif): Natural
the previous day (Fig. 2). The 28 Sept. rock- thick (Fig. 2A); more widely spaced regional Hazards and Earth System Sciences, v. 17,
fall was 120 m tall, 45 m wide, and up to joints influenced detachment of the larger- p. 1207–1220, https://doi.org/10.5194/nhess-17-
8 m thick, with a total volume of 9,811.0 ± volume rockfalls. Finally, whereas differenc- 1207-2017.
408.2 m3. The impact of the collapsed slab ing of SfM and TLS models typically yields Matasci, B., Stock, G.M., Jaboyedoff, M., Carrea, D.,
on the cliff below dislodged another 430.2 ± negative surface change indicative of mate- Collins, B.D., Guerin, A., Matasci, G., and
68.0 m3. Thus, the 28 Sept. rockfall was 23 rial loss, models generated after the Oct. Ravanel, L., 2018, Assessing rockfall susceptibility
times larger than the rockfalls that occurred 2017 rockfalls revealed an area of positive in steep and overhanging slopes using three-
the previous day. Within 24 hours, the NPS surface change. Here, a rock sheet 23 m tall, dimensional analysis of failure mechanisms:
was able to disseminate this information to 14 m wide, and tens of cm thick rotated out- Landslides, v. 15, p. 859–878, https://doi.org/10
the public via press releases and social ward up to 20 cm along a vertical hinge line .1007/s10346-017-0911-y.
media. on its western side (Fig. 2B). The sheet is Stock, G.M., and Collins, B.D., 2014, Reducing
bounded on three sides by rockfall scars, rockfall risk in Yosemite National Park: Eos, v. 95,
Importantly, the data also informed NPS and likely displaced during or immediately 22 July 2014, https://eos.org/project-updates/
decisions regarding public safety. Structural after the 22 Oct. 2017 rockfall. This geom- reducing-rockfall-risk-yosemite-national-park (last
assessments of discontinuities and plausible etry, combined with a simplified fracture accessed 17 May 2017).
future rockfall volumes, enabled by the 3D mechanics analysis, indicates that the sheet Stock, G.M., Martel, S.J., Collins, B.D., and Harp,
data, indicated low potential for an immi- should detach with another 20% of fractur- E.L., 2012, Progressive failure of sheeted rock
nent rockfall that could reach the road, ing along the partially attached side. slopes: The 2009–2010 Rhombus Wall rock falls
allowing the road to be reopened. Although the 3-D data do not allow us to in Yosemite Valley, California, USA: Earth Surface
Comparing the volumetric data with histori- predict exactly when this will occur, they Processes and Landforms, v. 37, p. 546–561,
cal events (Stock et al., 2013) puts these do define the precise location and volume https://doi.org/10.1002/esp.3192.
rockfalls in perspective: the 28 Sept. rockfall of this future rockfall. Stock, G.M., Collins, B.D., Santaniello, D.J., Zimmer,
was the 29th largest rockfall occurring in V.L., Wieczorek, G.F., and Snyder, J.B., 2013,
Yosemite since 1857, and has a return period Our analysis of the El Capitan rockfalls Historical rock fall in Yosemite National Park,
of ~6 years. demonstrates the utility of SfM for quickly California (1857–2011): U.S. Geological Survey
generating 3-D cliff models that quantify Data Series 746, 17 p. and data files, https://pubs
After the immediate crisis had passed, rockfalls, and reinforces the value of having .usgs.gov/ds/746/ (last accessed 17 May 2018).
subsequent analyses offered further insights baseline data in place prior to a critical Stock, G.M., Guerin, A., Matasci, B., Jaboyedoff, M.,
into the longer-term evolution of the cliff. event. The ability to rapidly collect, analyze, Derron, M.-H., and Collins, B.D., 2017,
This area of El Capitan became active in and disseminate rockfall data in near-real Quantifying 40 years of rockfall activity in
Oct. 2010 (the first activity since at least time represents a significant stride forward Yosemite Valley with Structure-from-Motion and
1976), with rockfalls occurring sporadically in informing land managers and the public terrestrial lidar analyses: Geological Society of
over the next several years, culminating in about this potent natural process. America Abstracts with Programs, v. 49, no. 6,
the large rockfalls on 27–28 Sept. 2017 (Fig. https://gsa.confex.com/gsa/2017AM/webprogram/
2C). Subsequently, several smaller rocks fell REFERENCES CITED Paper299103.html (last accessed 17 May 2018).
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the rockfalls generally propagated upward Derron, M.-H., and Ravanel, L., 2017, Brief ‘Structure-from-Motion’ photogrammetry: A low-
from the location of the first event. The communication: 3-D reconstruction of a collapsed cost, effective tool for geoscience applications:
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Manuscript received 17 April 2018
Revised manuscript received 10 May 2018
Manuscript accepted 11 May 2018
www.geosociety.org/gsatoday 29
