JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B8, 2380, doi:10.1029/2002JB002174, 2003
Granular memory and its effect on the triggering and distribution of
rock avalanche events
S. J. Friedmann
Department of Geology, University of Maryland, College Park, Maryland, USA
G. Kwon and W. Losert
Department of Physics, International Solar-Terrestrial Physics and Institute for Research in Electronics and Applied Physics,
University of Maryland, College Park, Maryland, USA
Received 27 August 2002; revised 13 February 2003; accepted 21 March 2003; published 15 August 2003.
[1] Rock avalanches are uncommon, large, catastrophic events in which granular
masses of rock flow at high speeds, commonly with unusually long runouts. Necessary
conditions for rock avalanche occurrence include high relief, steep slopes, and a
prefractured (granular) groundmass. An additional parameter, antecedent shear of the
granular mass in the direction of failure, may be prerequisite. In many instances
the basement mass of the eventual landslide underwent internal strain subparallel to the
direction of mass wasting, commonly focused along a basal surface (e.g., fault). New
experimental data from laboratory experiments indicate that shear of a granular mass
produces subtle anisotropy in grain arrangements that strongly affects the subsequent
distribution of deformation and rearrangement, the failure threshold, and the rate at
which steady state conditions are reached. This phenomenon may help to explain the
spatial clustering of rock avalanche deposits. If memory of prior shear is also relevant
on larger scales, taking deformation history into account could greatly enhance
INDEX TERMS: 1815 Hydrology: Erosion and
prediction of rock avalanche occurrence.
sedimentation; 1869 Hydrology: Stochastic processes; 7223 Seismology: Seismic hazard assessment and
prediction; 8010 Structural Geology: Fractures and faults; 9820 General or Miscellaneous: Techniques
applicable in three or more fields; KEYWORDS: Rock avalanche, granular flow, landslide hazard, seismic
triggering
Citation: Friedmann, S. J., G. Kwon, and W. Losert, Granular memory and its effect on the triggering and distribution of rock
avalanche events, J. Geophys. Res., 108(B8), 2380, doi:10.1029/2002JB002174, 2003.
1. Introduction
[2] Rock avalanches are spectacular and catastrophic
examples of natural granular flows of great magnitude.
Workers have recognized deposits from these events for over
100 years under a variety of circumstances and settings [e.g.,
Heim, 1932; Shreve, 1968; Friedmann, 1997]. Tectonically
active regions constitute by far the most common setting for
rock avalanching [Keefer, 1984a, 1993], and while some
events started due to undercutting [e.g., Daly et al., 1912;
Heim, 1932], the vast majority are believed to have begun due
to ground accelerations associated with seismic events [e.g.,
McSaveney, 1978; Hadley, 1978]. These events are common
in arid and semi-arid settings [e.g., Keefer and Wilson, 1989;
Yarnold, 1993], although they occur in humid settings,
undersea [e.g., Moore et al., 1989], and on other planets
[e.g., Howard, 1973; Luchitta, 1979; McEwan, 1989].
[3] They also represent a significant hazard, associated
historically with >20,000 deaths and major property destruction in populated areas [e.g., Pflaker and Ericksen,
1978]. Historic landslides in these areas have generated
Copyright 2003 by the American Geophysical Union.
0148-0227/03/2002JB002174$09.00
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eyewitness accounts that provide key insight into the
physics of rock avalanche transport. However, there are
no eyewitness accounts of rock avalanche triggering. Because these events are uncommon and difficult to predict,
little is known about how to best predict their occurrence in
high-risk regions, including seismically active areas.
[4] This paper attempts to improve the understanding of
probability of event occurrence and distribution by examining the role of basement deformation history on triggering
through a combination of field, literature, and experimental
approaches. Field and literature studies focus on the direction
of basal deformation relative to the direction of flow, as a
factor that may influence avalanche probability and runout
length. The experimental work specifically builds upon
recent advances in the physics of granular materials.
2. Rock Avalanche Occurrence
[5] Much previous work has focused on the physics of
transport for rock avalanches. Their unusual physics centers
on a key feature: exceptionally long runout. On earth, rock
and debris avalanches runout between 3 and 10 times their
fall heights; on Mars, over 25 times [Shaller, 1991]. This
requires extremely low internal angles of friction, often less
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Figure 1. Source characteristics for avalanching behavior. (a) Plot of relief (m) versus slope (degrees)
for 47 rock avalanche deposits. All events require an initial slope >25° (after Keefer [1984b]). (b) Plot of
failure angles for steady state avalanches. Height (h) is measured in grain diameters (d). For flows thicker
than 10 grain diameters, failure also requires a 25° slope (after Daerr and Douady [1999a]).
than 10° (most natural landslides, for comparison, have
internal fiction angles near 30°). This long runout, farböschung or drop height versus runout length (H/L), is what
makes rock avalanches both extremely devastating and
difficult to predict. Despite hundreds of examples and observations, there is no agreement among workers concerning the
physics that produce this effect. Many disparate transport
models exist, including air lubrication [Shreve, 1968], inelastic collision [Bagnold, 1954; Campbell et al., 1995] frictional
melting of the rocks themselves [Erissman, 1979], and
fluidization due to internal vibrational modes of the rocks
(known under the potentially misleading name of acoustic
fluidization) [Melosh, 1979, 1987]. Most of these models are
consistent with local observations but inconsistent with
universal observations or physical constraints [Shaller,
1991; Yarnold, 1993; Friedmann, 1997]. To date, few of
these models have received direct tests through physical or
numerical experiments (exceptions include Campbell et al.
[1995] and Hsu [1975]). No tests have attempted to determine the importance of various processes within these
models, nor attempted to parameterize key inputs. Moreover,
no model currently predicts under what circumstances long
runout will occur, nor what that runout will be.
[6] Relatively little work has focused on the necessary
conditions for rock avalanche initiation. Keefer [1984b]
documented three critical preconditions for flow initiation
by examining the basement terrains of many rock avalanche
systems. He demonstrated that high relief (>150 m drop
height), steep slopes (>25°), and a prefractured (granular)
groundmass were necessary conditions for flow initiation
(Figure 1a). The last point, a prefractured groundmass, is
central to this discussion because it implies that insights
from granular physics may help to better understand the
initiation process. In locations without a penetratively
fractured basement, failure initiates in other modes (e.g.,
rock slide, slump) that do not exhibit long runout. Keefer
[1993] continued to assess the potential for rock avalanche
triggering by looking for factors that would increase the
potential for flow-failure mode (e.g., bedding surfaces
dipping out of the rock face), producing a logic-tree for
gauging the relative potential for flow initiation. Schmidt
and Montgomery [1995] argue that some critical relief must
be exceeded before deep-rooted failure can occur. In short,
as relief increases, the force of the rock’s mass ultimately
exceeds the local yield strength, determined by a variety of
geological factors. This is consistent with Keefer’s [1984b]
recognition of the minimum critical relief threshold for rock
avalanche failure.
[7] In recent and ancient settings, rock avalanche deposits exhibit a dense clustering of events wherein successive
events occur directly on top of or adjacent to previous
events. Although many studies document this spatial
clustering [e.g., Mudge, 1965; Shreve, 1968; Eppler et
al., 1987], they have not examined the underlying causes
that lead to multiple events within a restricted spatial
framework. Other aspects of the spatial distribution (e.g.,
why an event initiates in one portion of the range versus
another) also have not received significant research. The
spatial distribution may hold further clues about the
avalanche triggering mechanism and valuable information
for hazard assessment.
[8] To shed light on these key issues, we have chosen
three examples from the literature in which the spatial
occurrence of the rock avalanche event is noteworthy and
points to additional root circumstances to failure. These
studies include two exceptionally well know cases, one
recent and one historic, and one ancient case with a large
number of rock avalanche occurrences.
3. Blackhawk Landslide
3.1. Background
[9] Along with the Frank, Elm, and Sherman Glacier
slides, the Blackhawk serves as an archetypical example of
FRIEDMANN ET AL.: GRANULAR PHYSICS AND ROCK AVALANCHE TRIGGERING
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Figure 2. Blackhawk Landslide, California. (a) Birds-eye view looking southward of the landslide and
the source region. Landslide is 1 mile wide. (b) Close-up image of transverse flow waves from landslide
surface. Ridges are commonly 4– 6 m in relief and spaced 50 m apart. (c) Granular framework of
landslide with inherited bedding features; white bar is 2 m. (d) Jigsaw breccia within landslide (close-up
of Figure 2c). (e) Trace of map view extent of Blackhawk and Silver Reef rock avalanche deposits, north
= top. Note close spatial arrangement and potential for multiple Silver Reef lobes or events.
rock avalanche deposits. One of the best studied examples
in the world [Shreve, 1968; Johnson, 1978; Shreve, 1989],
the landslide sits in the northern San Bernardino Mountains
near Apple Valley, CA, and comprises 3 108 m3 of
granulated rock (Figure 2). It features many characteristics
classically attributed to rock avalanche deposits, including a
penetratively deformed granular framework, ‘‘jigsaw’’ breccias, and inherited stratigraphy and structures. Transverse
waves of constant height and spacing transect the surface of
the slide, developed either during or near the cessation of
flow (Figure 2b). The flow is broadly lobate, and displays
prominent raised levees along the edges of flow. The slide
ran out very far, with an estimated internal angle of friction
near 10°.
[10] Despite it’s prominence in the landscape and literature, it is only the best exposed and expressed of several
landslides in that area. At least one and potentially two other
events called the Silver Reef slide(s) occur adjacent to the
Blackhawk [Shreve, 1968] (Figure 2e). Many of the same
features found within the Blackhawk are found in the Silver
Reef slides, including transverse flow waves, a penetratively
deformed granular framework, long runout, ‘‘jigsaw’’ breccias, and inherited stratigraphy and structures. On the basis of
their quality of preservation and shallow burial depth, these
landslides are probably late Pleistocene and close in age to
the Blackhawk, and most workers consider the Silver Reef
event(s) to be identical in origin and nature [Shreve, 1968;
Shaller, 1991].
[11] Importantly, these landslides are the only Quaternary
rock avalanche deposits to be found nearby. The nearest
documented rock avalanche deposit, the Martinez Mountain
Landslide, occurs over 150 km to the south [Jennings,
1973]. However, the other preconditions for rock avalanche
triggering are maintained along the range front, including
steep, high-relief terrain, fractured basement, and bedding
surfaces dipping out of the rock face. This region is well
known for large seismic shaking associated with earthquake
faults and is part of an active fold/thrust belt several
hundred kilometers in length [e.g., Agnew et al., 1992;
Hutton et al., 1991]. Despite the abundance of seismictriggering events in the Pleistocene, there are no other rock
avalanche events exposed along the San Bernardino Mountains, along strike, north or south side. This suggests that
local characteristics in Blackhawk canyon are also critical to
the initiation of rock avalanche behavior.
3.2. Basement Configuration
[12] The source regions for the Blackhawk and Silver Reef
landslides are well exposed and easily accessed reentrants
within the San Bernardino mountain front (Figure 3). Several
workers [Shreve, 1968; P. Sadler, unpublished maps] have
mapped the basement source region for these landslides
and documented key stratigraphic and structural relationship. One relationship is noteworthy. Along much of the
range front, older reverse faults crop out near the piedmont. These are mostly north vergent, dipping to the south
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Figure 3. Evacuated source area reentrants of Blackhawk and Silver Reef landslides, birds-eye view.
Upper arrow points to Blackhawk Canyon. Note contact between light and dark units dipping at 15°
into valley at right. This contact is the folded Voorhies thrust. The lower arrow points into Grapevine
Canyon, the source area for the Silver Reef event. Arrow 0.5 miles in length.
(Figure 4a). In Blackhawk Canyon reentrants, however,
the Voorhies Thrust and its imbricates dip both to the
south and north due to folding by lower structures. This
geometry is due to a local structural culmination.
[13] Here, both the antecedent strata and the subjacent
bounding fault dip northward toward the adjacent basin
(Figure 4b). This means the material above the fault underwent strain synthetic and sub-parallel to the vector of landslide initiation. In other words, before exhumation, the
basement rocks underwent bulk shear in the same direction
as the subsequent flow to a first order, moving out of the
range front and downward into the basin. This is true only
within the Blackhawk source reentrant. Elsewhere along the
range front, the basement rocks and their key structural
surfaces dip in the opposite direction from previous shear.
4. Frank Landslide
4.1. Background
[14] The Frank Landslide in Alberta is Canada’s greatest
landslide disaster and one of its 10 greatest natural disasters
(Figure 5). More than 12% of the population of Frank, at least
70 people, died in the event, and 75% of the homes were
destroyed [Daly et al., 1912; Anderson, 1968]. The failure
was instigated by coal mining underneath the eventual failure
plane, similar to the undermining of the Elm landslide
basement [Heim, 1932]. The rock avalanche flowed at an
estimated 40 m/s, moving across the valley and up the
opposite side 130 m and leaving a thickness of debris up to
45 m. Volume estimates are on the order 3 107 m3.
[15] Although the Frank event is the only major historic
rock avalanche in the vicinity of Turtle Mountain, there is
evidence of previous major landslides there. Local Native
Americans reportedly avoided Turtle Mountain, referring to
Figure 4. Simplified diagrams of basement structure in
Blackhawk source region [after Shreve, 1968]. Each cross
section is roughly 1.5 km in length and runs N-S across
northern San Bernardino Mountains. (a) Cross section 1.3 km
east of Blackhawk Canyon axis, along Miles Canyon. Note
the dip of Voorhies and Santa Fe thrusts out of the mountain
due to subjacent folding. (b) Cross section along east edge of
Blackhawk Canyon. Note the southward tip of the Voorhies
thrust and its imbricates shallow toward the landslide scarp,
suggesting flattening and northward dip within the canyon,
synthetic and subparallel to the landslide paleoflow vector.
See color version of this figure at back of this issue.
FRIEDMANN ET AL.: GRANULAR PHYSICS AND ROCK AVALANCHE TRIGGERING
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it as ‘‘the mountain that walks.’’ There are many other types
of landslide deposits around the mountain as well, including
slumps and debris flows.
4.2. Basement Configuration
[16] Since the Frank event is the only clear rock avalanche
event that crops out near Turtle Mountain, there is no case for
clustering of rock avalanches along strike or nearby. However, the specific locus of failure is of interest (Figure 6).
Early workers argued that the slide initiated along joint
surfaces within the basement [McConnell and Brock, 1904;
Daly et al., 1912] or along bedding surfaces [e.g., Allen,
1933; Norris, 1955]. The joint networks are critical to the
story in order to create a penetratively fractured basement,
forming a granular mass. However, much of the length of
Turtle Mountain shares this characteristic with the slide area,
making this condition nonunique. On this basis, workers
argue that the likelihood of future events is relatively high.
[17] Subsequent mapping of the source area for the slide
reveals important local distinctions [Cruden and Krahn,
1978]. Within the slide area, the failed strata overlie a minor
imbricate reverse fault that verges eastward out of the range
front. Elsewhere along the range front, this fault dips
westward and the overlying rock mass moved to the east.
Within the slide source region, however, the reverse fault
dips eastward, so that the basal slip plane, the overlying
granular mass, and the rock avalanche flow azimuth are all
coincident and verge eastward (Figure 6c). In addition,
bedding-parallel flexural slip occurred along stratal boundaries within the slide region, also with the same sense of
vergence (see their photo 12). Thus, within the source
region, there were multiple modes of shear and deformation
down and outward into the basin.
[18] Cruden and Krahn [1978] also collected shear
strength measurements along various surfaces within the
Frank event source region. They determined that the fault
and flexural-slip surfaces were significantly weaker than
other anisotropic surfaces (joints and bedding planes), even
weaker than a diamond-saw cut. This is consistent with
work-softening along these surfaces in the direction of
vergence. This is reinforced by the observation that deepseated creep apparently preceded the slide event.
5. Shadow Valley Basin
Figure 5. The Frank Landslide, Alberta, Canada. (a) The
slide scarp and headwall on the east side of Turtle mountain.
Photo courtesy of E-an Zen. (b) Birdseye view of the
landslide toward the east. Note railroad track crossing
landslide. Reproduced with the permission of the Minister
of Public Works and Government Services, Canada, 2003,
and courtesy of Natural Resources Canada, Geological
Survey of Canada. (c) Granular framework for the slide.
Reproduced with the permission of the Minister of Public
Works and Government Services, Canada, 2003, and
courtesy of Natural Resources Canada, Geological Survey
of Canada.
5.1. Background
[19] The Shadow Valley basin is a Miocene supradetachment basin of the eastern Mojave Desert [Friedmann et al.,
1996; Friedmann, 1999]. It occurs above the Kingston
Peak-Halloran Hills detachment fault, a west-vergent, lowangle normal fault [Davis et al., 1993]. This fault cuts older,
east-vergent reverse faults in the Clark, Mesquite, and
Mescal Mountains, reversing their sense of shear (Figure 7).
[20] The Shadow Valley basin contains many large volume mass-wasting deposits (30 – 50%), including megabreccias, glide-blocks, lahars, and debris flows [Friedmann et
al., 1996; Friedmann, 1997; Bishop, 1997]. More than
19 megabreccias crop out within the basin, representing
nearly 10% of basin fill with a minimum volume of 3.5 109 m3. All are interpreted to be rock avalanche deposits.
This density and clustering of events is typical of other
supradetachment basins, many of which contain abundant,
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Figure 7. The Shadow Valley basin, eastern Mojave
Desert. Map showing location of basin and Kingston
Peak-Halloran Hills Detachment Fault (thick dashed line).
A, Avawatz Mountains; CM, Clark Mountain; HH, Halloran
Hills; KPP, Kingston Peak Pluton; MM, Mesquite Mountains; MR, Mescal Range; N, Nopah Range; SM, Shadow
Mountains; SDVFZ, Southern Death Valley Fault Zone.
large megabreccia deposits [e.g., Hodges et al., 1989;
Topping, 1993; Yarnold, 1994; Friedmann and Burbank,
1995; Ingersol et al., 1996; Miller and John, 1999].
5.2. Basement Configuration
[21] The source region for the Shadow Valley rock
avalanches is well constrained. On the basis of directional
flow features within the deposits and their lithology, the
megabreccias originated in a structural reentrant along the
eastern basin edge [Friedmann et al., 1996; Friedmann,
1997]. The primary fault surface that formed the reentrant is
curviplanar and corrugated, and rock avalanches preferentially formed within these corrugations (Figure 8).
[22] The sense of slip along the basin-bounding fault was
westward, dipping gently out of the range front. Rock
avalanches entering the basin from the range-bounding fault
would have flowed in the same direction, probably initiating
along synthetic faults or fractures near the main fault. This
is consistent with sense-of-shear indicators in both the rock
avalanche deposits and basement structures within the
reentrants.
Figure 6. Several cross sections through Turtle Mountain,
basement for the Frank event. In all cross sections west = L
and east = R. (a) Through north peak of Turtle Mountain,
after Daly et al. [1912]. (b) Through south peak of Turtle
Mountain, after Norris [1955]. Note prominent fold beneath
the Turtle Mountain fault. (c) Through southern slide area,
after Cruden and Krahn [1978]. Note the small, imbricate
thrust above the Turtle Mountain fault dipping out of the
bedrock surface.
6. Application of Granular Physics:
Granular Memory
[23] Several mechanisms behind rock avalanche phenomena
may be understood from granular physics experiments.
Studies of the physics of granular matter focus on particles
that are orders of magnitude smaller (on the submillimeter to
centimeter scale), and often round. The aim is to reveal the
physical principles that are generic to a system of many,
inelastic particles that interact through direct contact. Some
effects may be purely geometric, others due to friction
between particles. For a review of recent studies of granular
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Figure 8. (a) Map of eastern detachment, including truncated basement units. Both detachment and
basement strata dip toward the west into the basin parallel to the fault’s vergence and displacement.
(b) Outcrop of a portion of the KHDF; image approximately 40 m high. Note thick band of sheared
breccia along fault. (c) Close-up of the fault; field of view approximately 2 m across. Note the
asymmetric sense of shear (top to the left) and the granular nature of the gouge. (d) Strata from the lower
Shadow Valley basin. The four dark ridges are stacked rock avalanche deposits. Image is approximately
500 m wide and shows 150 m strata.
flow, see, for example, Jaeger et al. [1996], Clement [1999],
and Rajchenbach [2000].
[24] How can rock avalanche properties be scaled down
for comparison to simple granular shear flow models? The
scaling law depends on the specific contact forces used in
the model. For the simplest model of inelastic Hertzian
contact forces between spheres, if the lengths are scaled by
a factor c, time and dissipation will have to scale as Sqrt(c),
and the elastic constant requires a more complicated rescaling [Poeschel et al., 2001]. However, several granular shear
flow properties appear robust, independent of shear rates,
pressures, or particle properties. These flow properties, i.e.
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that shear is localized in a shear band, and that the shear
stress is roughly independent of shear rate, may manifest
themselves also in rock avalanches. Other effects such as
frictional heating or comminution are not captured in scaled
experiments.
[25] Here we describe recent work on the physics of
granular shear flow with a focus on the start of flow. We
apply these results to provide a possible explanation for
clustering of avalanche locations and for the dependence of
slide regions on antecedent shear history.
6.1. Experimental Approach
[26] To investigate granular shear flows, the material
(1 or 2 mm glass beads in most experiments) is placed into a
Couette cell, which consists of two independently moveable
concentric cylinders, as shown in Figure 9a. The gap between
cylinders is 44 mm, and a glued-on layer of glass beads
roughens the cylinder walls. The motion of particles on the
top surface is imaged with a high-speed CCD camera that can
capture up to 3000 frames/s (see inset, Figure 9a). Particle
tracking software allows us to extract the motion of grains at
a resolution of 1/20 of one pixel for up to 2000 grains per
frame. This permits measurements of the time-dependent
velocity of particles averaged over different regions away
from the shear surface, as indicated in Figure 9b.
[27] The height of the granular surface, a measure of the
average density, is determined by imaging a region similar
to the one shown in the inset from a shallow angle.
[28] In addition to steady state shear flow of the inner
cylinder, the start of shear flow is studied in great detail.
Preliminary results are presented by Losert and Kwon
[2001]; complete results and a detailed discussion of the
physics will be presented elsewhere (M. Toiya et al., Shear
reversal in granular flows, submitted to Physical Review E,
2003) (hereinafter referred to as Toiya et al., submitted
manuscript, 2003).
6.2. Results
[29] In steady state shear flow, velocity gradients are
confined to a thin shear band, roughly 5 particle diameters
wide [Losert et al., 2000; Mueth et al., 2000]. Figure 9b
shows particle velocity as a function of distance from the
shear surface normalized by the shear rate for a large range of
speeds and pressures, indicating that shear banding is not
sensitive to pressure or shear rate. The location of the shear
band is sensitive to the flow geometry and remains near the
smaller surface area of the inner cylinder at all times, even
when both cylinders, or only the outer cylinder are rotated
[Losert and Kwon, 2001].
[30] How does this shear band start? When the flow is
stopped and then restarted in the same direction (Figure 10a)
the shear band forms immediately, and a steady state velocity
profile is reached before the inner cylinder is moved by one
particle diameter. This implies that the configuration of grains
retains an imprint of the earlier shear history, leading to an
immediate transition to steady state.
[31] However, when the flow is restarted in the direction
opposite to the prior shear, particles far from the sheared
surface move significantly during the initial transient
(Figure 10b) [Losert and Kwon, 2001]. The height of
the granular column, which indicates the density of the
material, is shown in Figure 11a as a function of time after
Figure 9. (a) Schematic and image of a granular shear
cell. Granular material (1 mm glass beads) is confined in a
44 mm gap between movable inner and outer cylinders. The
bottom of the shear cell can move with either cylinder. Flow
of material is imaged from above with a high-speed CCD
camera (see inset). The average velocity of particles within
two regions close to the inner cylinder will be indicated as
solid and dotted lines respectively in subsequent figures. (b)
Distance from the inner shear surface as a function of
velocity (normalized by the shear rate). Shear is confined to
a thin shear band, 5 particle diameters wide. The shearband width is roughly exponential, independent of shear
speed (from 0.004 to 0.3 rotations/s) or pressure (up to more
than 100 N/m2).
FRIEDMANN ET AL.: GRANULAR PHYSICS AND ROCK AVALANCHE TRIGGERING
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has not yet been determined and is the subject of current
experimentation.
[32] A potential mechanism for the behavior under shear
reversal is shown in Figure 11b. During shear, particle
contacts are preferentially in the direction of the main shear
stress, at about 45°. When shear is restarted in the opposite
direction those contacts break, and the system can rearrange
without much shear resistance and compact slightly.
[33] This granular memory of the direction of prior shear
thus appears as the result of shear anisotropies. Two dimensional experiments on birefringent disks have shown that the
network of load bearing particle contacts becomes strongly
anisotropic (B. Behringer, personal communication, 2002) In
addition, experiments have shown an additional kind of
granular memory. Vertically shaken granular matter of exactly the same initial density will evolve differently, depending on the magnitude of prior shaking [Josserand et al.,
2000]. In free flows on a sand pile, the flow properties were
shown to depend on the pile preparation, in addition to the
expected dependence on pile density [Daerr and Douady,
1999b]. For rocks, this implies that in addition to the density
of the arrangement, rock bodies may be stacked in a way that
reflects the direction and magnitude of prior shear.
7. Discussion
[34] The clustering effects of multiple rock avalanche
events and the specific occurrence of individual events can
be explained through the application of granular physics to
flow initiation and transport. Specifically, the most recent
Figure 10. (a) Average velocity of particles in different
regions (two of the regions are highlighted in Figure 9a). As
shown below, shear is stopped and restarted in the same
direction at t = 0 s. The velocity in each region reaches
steady state immediately. (b) Same as Figure 10a, except
that shear is started in the opposite direction. Particles far
from the shear surface move significantly when the shear is
started.
a restart. When shear is restarted in the same direction, the
density does not change. When shear is restarted in the
opposite direction, however, the material compacts by about
0.5 particle diameters over a characteristic length of several
particle diameters. In other words, the granular framework
collapses significantly (Toiya et al., submitted manuscript,
2003). The system then slowly dilates back to the original
density over longer timescales. Whether this gradual dilation is accompanied by a transient increase in shear strength
Figure 11. (a) A drop in layer height of 0.5 particle
diameters accompanies shear reversal. Restart in the same
direction does not alter the layer height. The shear speed is
0.25 particle diameters/s. (b) Possible particle rearrangement during shear reversal. During shear, particles preferentially are in contact along the shear direction; during
reversal, these contacts break, and particles can move until
new contacts form along the new shear direction.
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Figure 12. Hypothetical flow profile derived from observations of rock avalanche deposits (after Friedmann
[1997]).
landslide’s eventual course. In support of this hypothesis,
many recent rock avalanche deposits crop out at the base of
normal faults with a sense of shear sub-parallel to transport
[e.g., Longwell, 1951; Burchfiel, 1966; Blair, 1999]. This
characterization can be used to better predict the future
occurrence and distribution of rock avalanche hazards.
Application of this precondition would significantly reduce
the potential hazard areas, allowing policy makers and local
workers to focus prevention, mitigation, and evacuation
strategies in a small number of locations.
8. Conclusions
previous shear sense is a critical control on the subsequent
shear for granular rock assemblages in terms of locus of
failure, material yield strength, distribution of shear within
the granular mass, and time needed to approach steady state.
Importantly, the experimental results demonstrate that shear
initiation in the direction opposite the previous shear direction requires the reorganization of a large granular volume to
accommodate the new direction of shear stress. It is reasonable to assume that all of these factors affect the initial failure
conditions of rock avalanches. It is also reasonable to assume
that these factors affect the early history of granular interaction, specifically the density of the granular network and the
number of collisions between grains during flow.
[35] As argued by Bagnold [1954], Melosh [1987], and
Campbell et al. [1995], energy is lost through inelastic
granular collisions. This takes place during transport, and
the mode of macro-scale granular interactions distinguishes
rock avalanches form other geostrophic flows. Melosh [1979]
argues that rapid energy loss results in rapid jamming of
grains, terminating runout. Following the experimental
results, synthetic shear reduces the number of granular
collisions, thus reducing the initial lost energy during flow.
This may enhance the likelihood of flow when other triggering forces bring the granular mass to failure.
[36] This is most important as the flow approaches steady
state conditions. Previous experimental results indicate that
at steady state, only a thin shear band of grains is required
for flow to occur, commonly no more than five grain
diameters thickness [Losert et al., 2000] (Figure 9b). Such
a flow configuration would confine frictional or collisional
dissipation to a thin layer, thus potentially reducing overall
energy loss in the flow. This flow profile was predicted for
rock avalanches by Friedmann [1997] and is consistent with
common features of rock avalanche deposits such as jigsaw
breccias, shear bands, inherited stratigraphy, and a basal
mixing zone (Figure 12). It is also consistent with the
Savage-Hutter granular flow model [Savage and Hutter,
1989]. Granular flow initiated with synthetic shear deformation reaches steady state almost immediately. By comparison, antithetic shear deformation produces a much
broader zone of shear and compaction, with potentially
greater frictional and collisional dissipation. It may also
take much more time to evolve into a thin shear band, and
may not ever reach steady state in a gravity-driven flow.
[37] If these observations are accurate and appropriate,
then the most-recent shear history becomes important to
rock avalanche triggering. Specifically, an event will initiate
with a much greater probability if and only if the source
region followed a deformation trajectory similar to the
[38] The sense of shear in rock avalanche source regions
commonly matches the antecedent shear sense. This suggests a link and necessary precondition for rock avalanche
triggering.
[39] Granular materials ‘‘remember’’ sense of shear due
to anisotropies in granular contact network. This ‘‘memory’’
can help to explain the clustering and specific occurrence of
rock avalanches. This new precondition to rock avalanche
initiation may be used to create enhanced hazard assessments for rock avalanche potential.
[40] Acknowledgments. The authors would like to thank Jeff Harp
for field assistance, Peter Sadler for invaluable help and expertise in the
field, and E-an Zen. This manuscript was greatly improved through the
reviews of D. K. Keefer, H. J. Melosh, and one anonymous reviewer.
Special thanks go to the Univ. of Maryland Dept. of Physics and the
College of Computer, Math, and Physical Sciences. This work was partially
funded by NASA grant NAG32736 (proposal 01-OBPR-02-075).
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FRIEDMANN ET AL.: GRANULAR PHYSICS AND ROCK AVALANCHE TRIGGERING
Figure 4. Simplified diagrams of basement structure in Blackhawk source region [after Shreve, 1968].
Each cross section is roughly 1.5 km in length and runs N-S across northern San Bernardino Mountains.
(a) Cross section 1.3 km east of Blackhawk Canyon axis, along Miles Canyon. Note the dip of Voorhies
and Santa Fe thrusts out of the mountain due to subjacent folding. (b) Cross section along east edge of
Blackhawk Canyon. Note the southward tip of the Voorhies thrust and its imbricates shallow toward the
landslide scarp, suggesting flattening and northward dip within the canyon, synthetic and subparallel to
the landslide paleoflow vector.
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