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You have been provided with two case studies about the causes of and responses to major
landslides that affected communities in South America and Europe. Read the case studies and
study the corresponding images. When you are finished, you will be provided with some
discussion questions based on what you learned and thought about as you read the articles.
landslidesandsociety_casestudies.pdf

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Slope failure and society – case studies from Peru and Italy
Sarah Hall, College of the Atlantic
Becca Walker, Mt. San Antonio College
You have been provided with two case studies about the causes of and responses to major
landslides that affected communities in South America and Europe. Read the case studies and
study the corresponding images. When you are finished, you will be provided with some
discussion questions based on what you learned and thought about as you read the articles.
The town of Yungay, Peru and surrounding rural area with Huascarán in the background before (A) and
after (B) the May 31, 1970 debris flow. Courtesy of the National Information Service for Earthquake
Engineering, University of California, Berkeley. http://nisee.berkeley.edu/elibrary/Image/S3809.*
Case study locations – Peru and Sicily,
Italy
https://commons.wikimedia.org/wiki/File:Wi
nkel_triple_projection_SW.jpg
Italy
Peru
Approximate area of mass movement in San Fratello, Italy in 2010. Base image
from Google Earth. Figure after Figure 3 in
http://sgi1.isprambiente.it/geoportalenews/sanfratello/Relazione_San_Fratello.pdf
*Nisee-Peer gives permission to use
image in the module “Surface Process Hazards” non-exclusively, in perpetuity, worldwide distribution, in all
formats directly related to original work. Derivatives require separate permission.
Case Study 1: Nevado Huascarán (Peru), May 31, 1970
Summary and location of the event
Figure 1. Nevado Huascarán, the highest
peak in Peru (elevation 6768 m). The scar of
the ice/rock avalanche is evident on the north
peak (arrow). Image S. Hall.
Nevado Huascarán is the highest peak in Peru
and second highest in all of the Andes Mountains
(Figure 1). Rock and ice falls originating from
Nevado Huascarán have occurred throughout
history and will likely continue to occur with
ongoing glacier melt and active seismicity in the
area (Figure 2). Coincident with a magnitude 7.9
earthquake on May 31, 1970, a “rock avalanche”
(rock and ice fall) from the west face of the north
peak (primarily granodiorite with fractures and
joints throughout) converted into a high-velocity
debris flow (mud and sediments) which buried
towns along the Rio Santa valley on its way to
the Pacific Ocean, ~150km (~93 miles) away.
N
S
C
2a
Y
M
R
2b
Figure 2. (a) Location map of the Cordillera Blanca in north-central Peru. (b) The central portion of the
Cordillera Blanca containing the highest peaks in the range, the north (N) and south (S) peak of
Nevado Huascarán. These peaks, as well as other nearby glaciated peaks, have sourced many rock
avalanches during at least the Pleistocene. While there have been few seismic events along the active
Cordillera Blanca Fault (orange line, 2b), seismicity related to subduction along the Peruvian coast
triggered a rock avalanche from the north peak of Nevado Huascarán resulting in a debris flow (path
shown in yellow.) The location of the buried towns are shown by letters: Y (Yungary), R (Ranrihirca),
and M (Matacoto), and the location of Cemetery Hill is shown with the letter C. Modified after Evans et
al., 2009; imagery from Google Earth.
Within 3-4
minutes
from the
start of the event, the valley towns of Yungay, Ranrihirca, and Matacoto were buried, killing an
estimated 6,000-20,000 residents (Evans et al., 2009). The 1970 event was not the first time in
recent history that a large rock avalanche/debris flow originated from this peak. Just 8 years
earlier, a similar, although smaller, event buried part of the town of Ranrihirca. On this Sunday
afternoon, many of the town residents were busy at the market or watching a football match on
TV. When the earthquake began at ~3:23pm, the shaking and swaying made it difficult to walk,
and the many adobe-style structures collapsed. The shaking stopped after ~45 seconds, and this
is when a thunderous noise and cloud of dust was seen rising from the area near the base of
Huascarán. Some say they saw the debris flow fly over a small ridge on the west side of town
taking the form of a huge breaking wave. People in the town of Yungay ran towards Cemetery
Hill, the high ground of town where a large statue of Jesus stands with outstretched arms (Figure
3). Many claimed to feel a very strong gust of wind just before the arrival of the debris flow.
Some say the wind was so strong it stripped the trees of leaves. One account says ~90 people
made it to the top of the hill in the ~3 minutes they had between when the strongest shaking
stopped and when the debris flow buried the town (Cluff, 1971).
3A
3B
Figure 3. (a) Cemetery Hill (left) and memorial gardens built on top of the buried city of Yungay. (b)
Yungay memorial site with partially buried remnants of the old church. Images S. Hall.
Geologic, hydrologic, and geomorphic setting
A variety of geologic, hydrologic, and landscape features make catastrophic mass wasting events
somewhat common in this glaciated, seismically active, high-relief landscape. The Callejón de
Huaylas Valley is covered by moraine material from past glaciations and debris flow deposits
from previous mass wasting events. These glacial and debris flow deposits are exposed by the
action of the north-flowing Rio Santa as it moves through the subsiding valley. Water from
seasonal precipitation, rapidly melting glaciers, and periodic high-precipitation events associated
with the El Nino phase of ENSO also enhance the threat of mass wasting events. Erosion by the
Rio Santa has exposed debris flow deposits from events that have occurred in the area during the
past ~2 million-10,000 years, allowing for scientific study of past events. Scientists have found
evidence in these deposits for debris flows similar in style and scale to the 1970 debris flow
(Figure 4).
Tectonics also play a role in triggering
mass wasting events in the area. The
trigger for the 1970 event was a large
(magnitude 7.9) subduction zone
earthquake along the Peruvian coast
where the Nazca Plate subducts beneath
the South American Plate (Figure 5). In
addition, the Cordillera Blanca Fault,
located approximately 5 km (3 miles)
from the peak of Nevado Huascarán on
the western side of the Cordillera Blanca
Range, poses a seismic threat to the
region. Although very little historical
seismicity has been recorded along this
Figure 4 1970 debris flow deposit at the based of
Cemetery Hill in Yungay, Peru. Note the large
structure, scientists have identified fault
angular clasts in a fine-grained matrix. Image S. Hall.
scarps that offset ~16ka moraines,
indicating that the fault has been active
in the recent geologic past (Farber et al., 2005).
South
American
Plate
Cordillera
Blanca
10°S
15°S
25°S
30°S
Nazca
Plate
o
m/yr
68 m
n
ipla
Alt
20°S
N
35°S
85°W 80°W 75°W 70°W 65°W 60°W
Figure 5. Physiographic map of western
South America and the Pacific Ocean. The
red line indicates the plate boundary
between the Nazca and South American
Plates. This subduction zone is capable of
hosting magnitude earthquakes as large as
magnitude 9! Shaking from a large
earthquake at the coast, such as in 1970,
can be felt well inland.
Consequences on the landscape and
infrastructure
In the immediate aftermath of the debris flow, ~4
palm trees stood sticking out of the mud along
with pieces of the town church and a regional bus,
all of which still remain in the deposit today
(Figure 3b). The town of Yungay is now a
memorial site where thousands of Peruvian and
international visitors come each year. The
surrounding area is being resettled and the town
of Nueva Yungay has been established nearby, yet
15°S
away from the paths of the past flows (Figure 6).
Next steps: hazards and future planning
With climate change contributing to rapidly
melting glaciers and more variable precipitation
patterns (Urrutia and Viulle, 2009; Schauwecker
et al., 2014), we can expect an elevated risk in the
future for debris flows in this high-relief and
seismically active region. In addition to the risk of
rock avalanches like the 1970 event, debris flows
have also been generated by the overtopping or
bursting of lakes that are dammed by moraines.
Figure 6. (a) Aerial imagery, oblique view to east, just after the debris flow. From Servicío
Aerofotografíco Nacional de Perú; June 13, 1970. (b) Google Earth image from today, oblique view
to east. Y = Yungay; R = Ranrihirca. While much of the surrounding area has been resettled and revegetated, the site of old Yungay remains as a memorial site.
Glacial landscapes are often full of loose sediment – moraine material, boulders, colluvium – and
have lakes or bogs filling areas
upstream of end moraines that cross
the U-shaped valleys (Figure 7). If a
mass, a block of ice, rock, landslide
material, or merely a large amount of
water falls into one of these
moraine-dammed lakes and the lake
level rises rapidly, the dam may fail
or overflow, creating a flood and
subsequent downstream debris flow.
Many of these events historic and
ancient events have been
documented in this region (Carey,
Figure 7. End moraines composed of till (a mix of clay,
gravel, cobbles, and boulders) are left behind as glaciers
melt and retreat up valley. Moraines such as these block
the flow of water through the valley and host lakes that
could burst through the moraines in an especially wet year
or as result of shaking or ice/rock fall into the lake. Image S.
Hall.
2010; for more info, check this out: http://glaciers.uoregon.edu/hazards.html).
Following the 1970 event, geohazards assessments of several avalanches from the glaciated high
peaks of the Cordillera Blanca Range were undertaken to understand the causes of the
avalanches, come up with strategies for preventing avalanche-related loss of life in the future,
and establish evacuation plans. In addition, extensive surveys in the region were conducted to
describe and monitor all of the ~380 regional lakes and ~600 glaciers, especially when the
feature is located upstream of a populated area, next to a melting glacier, or in steep terrain (most
localities). In some cases, officials have lowered the lake levels as a preventative measure to
alleviate some stress on the dam in the event of a water-raising event. One such monitored lake,
Lake Palcacocha, is upstream of the city of Huaraz, the largest city in the Cordillera Blanca with
a population of ~101,000 people. While quite rural, this region has a large population living both
in the urban settings and spread out over the land farming and raising livestock. Further, the
tourism industry in the region is of huge economic importance as this region is a world-class
climbing destination as well as home to one of Peru’s most visited national parks (Huascarán
National Park) and multiple UNESCO world heritage sites.
Case Study 2: San Fratello (Sicily, Italy), February 13-15, 2010
Summary and location of the event
Figure 8. Maps showing the location of San Fratello within Italy (top left), the proximity to the Nebrodi
Mountains (center), and the relationship of San Fratello with the rivers and topography of the region (top
right). Imagery from Google Earth.
San Fratello is a hillside town of roughly 4500 residents in the Messina Province of NE Sicily
(Figure 8). Lying on the northwest side of the Nebrodi Mountains (part of a larger range called
the Apennines), San Fratello sits on a steep ridge with 2 river valleys on either side–Furiano
Creek and Inganno Creek (Figure 8).
Figure 9. Infrastructure damage as a result of the 2010 landslide. ISPRA.
http://sgi1.isprambiente.it/geoportalenews/Sanfratellodoc.html
On February 13, 2010, after a winter of higher than
average precipitation and several days of persistent
rain, ground motion began across a 1.2 mile wide area
just east of town and continued moving for 2 days.
The mayor reported that “they were watching the town
disappear before their eyes.” Ultimately, the landslide
area covered approximately 250 acres. Roughly 90%
of this slide area was outside of the town of San
Fratello and the other 10% within a populated area
including the San Benedetto, Riana, Porcaro, and
Stazzone districts of town (Figure 8). Approximately
2000 people were evacuated from the area at the onset
of ground motion, and although no fatalities were
reported, dozens of historic and modern buildings
were destroyed, and about 300 buildings sustained
damage. Agricultural areas on the outskirts of town
were also affected by the landslide, and much of the
municipality’s water, sewer, and drainage systems
were destroyed. Despite the extensive property and
infrastructure damage (Figure 9), residents still held
Figure 10. Area of the historical
their annual, pre-Easter Jewish celebration a month
landslides in the region (1754,
later. The 2010 event was not the first time that ground
1922, 2010). Modified from Bardi,
et al., 2014.
motion had impacted San Fratello; documented
landslides in 1754 (during which the northeast portion
of town was damaged); 1922 (when an area along the western hillside of the town was totally
destroyed); and 2000 also caused considerable damage to the town and its infrastructure (Figure
10).
Geologic, hydrologic, and geomorphic setting
The Sicilian coastline is generally steep and dissected by river valleys. The San Fratello landslide
was located in an area with a slope angle greater than 30° and has been characterized as a
“complex slide” because of the geologic and hydrologic factors that contributed to the event.
Geologically, the three principal bedrock lithologies in the area include, from oldest to youngest:
•
•
•
Cretaceous Monte Soro Complex: quartz arenite sandstones and clays;
Cretaceous Scaly Clays: marly claystones with decimeter-scale beds of limestone and
limestone marl;
Eocene Frazzano Complex: sandstones and clays with interbedded conglomerate,
sandstone, and gneiss
Most of this bedrock geology is covered with thin (in most places, less than 10 m or 30 ft)
deposits of unconsolidated clays and silty clays. The 2010 landslide occurred within this surficial
layer.
Interestingly, studies of the 2010 event reveal that the slide area is not one coherent block but
involved different types of movement at different depths. For example, the area that affected the
San Benedetto district was characterized as a type of mass wasting event called a rotational slide
with a curved slip surface located roughly 11.5 m (~38 ft) below the surface. In contrast, the area
that damaged the Stazzone district had a flatter slip surface with a depth of roughly 30 feet. A
slip surface was not identified for the slide area near Riana.
Figure 11. Precipitation data (in mm) recorded in the San Fratello area between October 2009-2010.
Modified from Bardi et al., 2014.
Surface water and groundwater also played an important role in the generation of the 2010 event.
The winter preceding the landslide brought approximately 35 inches of precipitation to the area
from October 2009-January 2010 (Figure 11). In the 8 days prior to the initiation of the landslide,
San Fratello received roughly 4 inches of rain. Scientists used borehole studies to measure the
depth to the water table and found that the location of the water table corresponded closely to the
location of the landslide slip surfaces.
Consequences on the landscape and infrastructure
Following the 2010 event, a variety of landslide-inducing surface features were observed in and
near San Fratello, including traction cracks (the ground surface pulls away and creates a raised
pattern) and extensional and compressive fractures (Figure 12). Observed landslide scarps (steep
regions where the slip surface has reached the Earth’s surface and caused a ground rupture–see
figure 15) 5-10 m (~15-30 ft) high developed about 1/4 mile from the center of town. With the
modification of the Earth’s surface came rapid changes in surface water flow patterns and led to
the creation of several small “landslide lakes”(Figure 13). An earth flow was generated in the
lower portion of the landslide area and entered a tributary of Inganno Creek. Infrastructure
damage was extensive. Water and sewer pipelines, schools, churches, and homes were damaged
and in some cases, completely destroyed (Figure 9).
Figure 12. Diagram illustrating some of the main parts of a complex slide. Pay particular
attention to the landslide crown and scarp, features mentioned in the article. From USGS.
Next
steps: hazards and future planning
An important part of planning for future landslides involves understanding the causes and
geographic extent of the landslides that have occurred in the area in the past. The ability to map a
landslide allows scientists to identify areas of relatively low, medium, and high risk and use
these hazard maps to make decisions about future land use and actions that should be taken to
mitigate (decrease) the risk of life and property loss from future events. This is challenging in an
area like San Fratello for several reasons. Multiple landslide events have resulted in the
“overprinting” of older landslides by more recent deposits. In addition, the ground surface is
partially obscured by vegetation and has been modified by environmental processes (example:
erosion) and human activities (example: construction, agriculture.)
Figure 13. Map of the 2010 San Fratello landslide. Effects of the landslide, including counterslopes,
torsional movements, landslide lakes, ground and building damage, erosion, and damaged pipelines
are illustrated. Districts mentioned in the article are Porcaro (1); Stazzone (2); Riana (3); and San
Benedetto (4). Modified from Bardi et al., 2014
A variety of strategies have been employed in San Fratello since the 2010 slide to attempt to
stabilize the ground, and there are additional plans for hazard mitigation in the area. Concrete
drains to collect surface water, adjacent drain pipes to transport the water, and perforated pipes to
remove water from the subsurface have been installed in and around the landslide area. In 2012,
several global positioning system (GPS) receivers were installed on buildings and drainage wells
within the landslide area to monitor ground movement. Research using a geodetic technique
called InSAR was also conducted in the area to measure the amount of ground displacement that
occurred between 2010 and 2013. The study results provided valuable information related to
understanding the extent of the landslide and documenting that slow ground deformation
continued years after the onset of ground movement in 2010 (For more, see Bardi et al., 2014).
In particular, the InSAR data suggest that the greatest amount of ground deformation within the
study area occurred in the San Benedetto (2294 mm, or ~7.5 ft, of displacement!) and Porcaro
(2178 mm, or ~7.1 ft) districts. In contrast, the Stazzone and Riana quarters experienced 604 mm
(~2 feet) and 545 mm (~1.8 ft) of displacement, respectively, during the study period.
GEOS 541 Geomorphology
Mass Wasting Reading Assignment
Due Monday, April 30th
Part I. Peru and Italy Case study discussion questions
1. Nevado Huascarán is composed primarily of granodiorite. Based on the tectonic
setting of the area, propose a hypothesis about how Nevado Huascarán formed.
2. Would you expect seismicity to be a risk factor in the generation of landslides in the
Nevado Huascarán and/or San Fratello study sites? Why/why not? If so, in which
study area would you be MOST concerned about seismic hazards? Why?
3. Propose some factors that could have contributed to the Nevado Huascarán rock
and ice fall turning into a debris flow.
4. In both study areas, scientific and historical evidence exists suggesting that masswasting events have occurred in the past. If this is the case, why do people continue
to inhabit these areas?
5. What was water’s role in the Nevado Huascarán and San Fratello events?
6. Mass-wasting events also occur in areas that receive very little precipitation.
Propose …
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