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Gifts and Perils of Landslides
Catastrophic rockslides and related landscape developments are
an integral part of human settlement along upper Indus streams
Kenneth Hewitt
L
arge rockslides and the rock avalanches they can generate will destroy any living thing or built structure
in their path. In mountain valleys they
can form dams impounding lakes that
may burst suddenly with devastating
consequences. The January 4, 2010,
Atabad landslide, which dammed
the Hunza River about 70 kilometers
north of Gilgit, Pakistan, offers a stark
reminder (see sidebar, page 416). Not
surprisingly, the hazards of landslides
have tended to drive scientific study.
Less often noted is how they also create resources and opportunities for
mountain people.
Recently, more than 340 large landslides have been identified along the
Indus streams in the Karakoram, Hindu Kush and northwest Himalayan
ranges. Most predate historical records
but continue to influence landscape
developments. Land use is closely
adapted to landforms controlled by the
landslides. Many villages and some
small towns sit amid landslide rubble,
as do ancient cultural sites, modern
roads, airfields and tourist facilities.
The environmental knowledge and
Kenneth Hewitt is professor emeritus of geography
and environmental studies and Research Associate at the Cold Regions Research Center at Wilfrid
Laurier University. He received his Ph.D. in geomorphology from London University. His research
focuses on risk and disaster theory, with a particular
emphasis on the social geography and human ecology of vulnerability and response to catastrophic
risks. He began investigations of glaciers in the
Karakoram Himalaya in the 1960s and developed
an interest in large landslides there after witnessing
a major event in 1986. As this article was being prepared to go to press, he was continuing his research
in the Upper Indus Basin and was awaiting word of
helicopter availability to visit the site of the January
4, 2010, Atabad rockslide. Address: Department
of Geography and Environmental Studies, Wilfrid
Laurier University, Waterloo, Ontario N2L 3C5.
Email: [email protected]
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American Scientist, Volume 98
stories of local people reveal an understanding of these events, and the
geohazards they present.
The landslide-related benefits are
notable because so much of the region
is inhospitable. Two-thirds of the upper Indus basin lies above 3,500 meters elevation in climates too severe for
permanent settlement. Glaciers cover
20,000 square kilometers and permafrost an even larger area. Rain-shadowed valley floors tend to be arid or
semi-arid whereas, high above, snowfall is heavy. Precipitous rock walls are
the dominant landform. The landslides
themselves reflect this rugged terrain,
the mountain-building forces at work
and valleys deeply excavated in recent
geological time. Indeed, where rivers
are cutting down vigorously into bedrock to match high rates of tectonic
uplift, life has very few and precarious
footholds.
Surprisingly, however, most sections of the upper Indus streams flow
not in bedrock, but over thick and extensive valley fill sediments. Scientific
observers have long been impressed
and puzzled by so much deposition
within rugged mountains. They realized that the deposits must record
events that overwhelm or interrupt
stream incision, causing sedimentation along the valleys. Until recently,
however, very few of the landslides
were identified as such, and their role
went unrecognized. Here I will focus
on how the rivers are responding to
the many landslides that have blocked
valleys. I will also describe for the first
time the positive effect on the availability of habitable land. Although the
article outlines the science behind the
discovery and nature of the landslides
to introduce their place in settlement
geography, we will remain mindful
of the major disaster risks that accompany them.
Rock Slope Failure and Avalanches
A particular class of landslides dominates the Karakoram—events that combine catastrophic rock slope failure and
rock avalanches. They are catastrophic
in having sudden occurrence, great
size and high speed. They are restricted mainly to the world’s more rugged
mountains, because they require large
collapses on steep rock walls and a descent of hundreds of meters. Average
volume for the Karakoram events is
around 200 million cubic meters; at
least 32 rank as megaslides (more than
a cubic kilometer), and the largest exceed 40 cubic kilometers. The crushing
forces involved are so great that, in less
than a minute, huge volumes of solid
bedrock are reduced to rubble, sand
and dust. This transforms the collapsing mass into a rock avalanche—a high
speed run out of broken and crushed
rock that appears to flow, plunge and
surge forward, creating immense dust
clouds like the more familiar snow
avalanches. Their velocity generally
exceeds 100 kilometers per hour and
may reach 250 kilometers per hour.
When movement falls below such high
speeds flow halts abruptly, but within
two or three minutes, large areas are
buried by rubble and dust.
The fresh or undisturbed rock avalanche surface usually consists of
large boulders. However, in the main
body of material, revealed where erosion cuts through, a dense matrix of
crushed and pulverized rock envelops the larger clasts. On relatively
open and level valley floors the debris
spreads to a sheet a few meters thick,
lobate in plan and with only minor
surface irregularities. However, in the
rugged terrain of the Karakoram, local
topography complicates matters. Rock
avalanches traveling directly across a
valley may stall against the opposing
wall and remain very thick. The fast-
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Figure 1. Rockslides and rock avalanches fundamentally shape the landscapes of upper Indus streams in the Hindu Kush, Karokoram and
northwestern Himalaya mountains. Recent surveys have identified more than 340 of these rockslides, with volumes averaging about 200 million cubic meters and the largest exceeding 40 cubic kilometers. Considering that this material moves downslope at more than 100 kilometers
per hour, with peak speeds that may reach 250 kilometers per hour, it’s not surprising that humans and their built environment are no match
for such power. Nonetheless, these rockslides also create landscapes suitable for human habitation, such as this one where a lake has formed
near Hushe, Pakistan. Without landslides, life would have a difficult time establishing a foothold in such vertical terrain. The author has been
studying this region for nearly two decades and has determined that many of the landforms previously thought to be of glacial origin are actually the result of slides that post date the last ice age. In the process, he also learned that this came as no surprise to the region’s inhabitants.
(Photographs by the author unless otherwise noted.)
moving debris can climb 500 meters
or more up opposing slopes. When
moving down narrow canyons it responds to curves and valley-side spurs
by swinging from side to side or rising
and falling in caroming flow. At valley
junctions the debris can split, sending
separate lobes far up or down the valleys, or blocking tributaries. A great
diversity of plan forms and surface
features results (see figure 5).
Landslide-Interrupted Rivers
These landslides are short-lived events,
rare on human time scales or at any one
location. Their detachment scars on
rock walls and masses of debris below,
however, can survive and influence
landscape developments for millennia. Rock avalanches that travel across
river valleys will usually dam them.
Unstable, short-lived dams and floods
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from sudden breaching are of singular
concern, but along the Indus streams
remnants of lake bed deposits record
dozens of former landslide impoundments that lasted decades or centuries.
More than 150 landslide barriers remain incompletely cut through today,
continuing to act as local base levels
for valley development. Their composition, and the shock of the sudden
halting of the rock avalanche, make
for a highly compacted mass, able to
form strong, impermeable landslide
dams. Where stalled against opposing
slopes, barriers can be hundreds of meters high, and debris lobes spreading
far up and down valleys resist erosion
and add strength to the dam.
When landslides interrupt streams
two interdependent episodes follow.
At first sediment is trapped above the
dam, the aggradation phase. This con-
tinues until breaching occurs bringing
the second, degradation phase, as impounded sediments are trenched and
removed. It sounds straightforward
but both phases are complex, owing
partly to diverse landslide configurations and partly to the response of high
energy mountain processes. Lake sediments are mainly thin layers of finegrained material: clay, silt and sand
washed into the lake and settling out
slowly. Coarser material is dumped
quickly around the shores. Streams
build deltas or alluvial fans. The latter, an important landform first named
from this region by Frederick Drew
(1872), is typical where tributaries enter landslide-interrupted valleys. The
larger ones, several kilometers across,
usually comprise a mix of stream, torrent and debris flow materials. Smaller,
steeper ones are built mainly by debris
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2010 September–October
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flows. Aggradation in the landslide interruption produces complex buildups
with interfingering and overlapping of
many different sediments.
In the degradation phase, systems
of stream terraces develop as rivers incise into the sediment pile. Terrace sequences relate to dam history, whereas
their slope, width and other properties differ in up-valley sediments compared to those through the landslide
and in downstream areas. Tributary
channels become incised in the fans,
whose outer parts are truncated by the
main stream. Rock avalanche and debris fan materials, or bedrock outcrops,
create “defended” terraces whose geometry reflects the locations of resistant materials. Because rivers crossing
the landslide interruption need not
follow the pre-landslide course, they
are let down on bedrock spurs or in
Figure 3. In areas presently unaffected by
landslides, human habitation is somewhere
between difficult and impossible. Rivers cut
steep gorges into the uplifted terrain, offering scant horizontal ground. This mid-Indus
gorge is typical of river sections unaffected
by landslides.
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Figure 2. The study area, two-thirds of which
lies above 3,500 meters in altitude, too high for
human settlement. However, almost all of the
habitable valleys of the Hindu Kush, Karakoram and northwestern Himalaya mountains
are strongly affected by large landslides. Glaciers cover about 20,000 square kilometers of
the region.
new valley floor sections and may cut
secondary or “epigenetic” gorges, the
most common type of bedrock gorge
in the region. Such complications, once
recognized, help distinguish landslidegenerated terraces, fans and canyons
from features where sediment transport and stream incision respond
directly to climate and hydrological
changes, or tectonics.
On average, one cross-valley rock
avalanche was found for every 12 to 14
kilometers of upper Indus streams surveyed. The landslide interruptions are
encountered in every possible stage
of development, depending on age,
size and local conditions. Each case
is affected by others, some older and
some younger. Intact dams upstream
will starve downstream impoundments of sediment. Stream flows are
moderated by passing through a lake
or over flatter, aggraded areas above
dams. Conversely, degrading interruptions can send huge pulses of sediment
down the valley. Since there are many
overlapping sequences of aggradation
and degradation from landslide interruptions in different stages of development, they modify and complicate
sediment delivery throughout the river
system.
Ghoro Cho: Two Landslide Stories
Ghoro Cho (“heap of great stones”)
comprises a series of boulder-covered
ridges across the floor of Shigar Valley
(see Figures 6 and 8). Early in my investigations, I stayed there in the shepherds’ hut at Kor-Kor-Tsok-Boo. One
evening, having heard of my interest,
Apo (grandfather) Haji Ali elected to
tell me the “real” story of the great
stones. It was after supper and we
sat around the open fire, the animals
milked and settling down in the corrals, Haji Ali’s face alternately lit by the
flames or obscured by the smoke.
Long ago, he said, where Ghoro Cho
now lies there was a great and prosperous city. A traveling holy man came
there and asked the ruler, the Rajah,
for food and shelter. He was turned
away, and by other wealthy folk. At
last a poor woman gave him shelter
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and shared her food. Next morning he
told her to climb to some springs high
above the valley floor. He went to the
opposite side and smote the rock with
his staff. A great part of the mountain
came down, burying the city, its wealth
and pride.
Haji Ali’s story resembles morality
tales about disasters in many places,
including other Karakoram landslides.
It may seem more myth than fact. Yet
the details are singular, and whoever
crafted the story was a fair geologist or
landscape detective. Next morning the
storyteller walked with me, pointing
out the huge scar on the mountain side
struck by holy man. Opposite, he identified Mango village and its springs
where the old woman remained safe
above the cataclysm. He showed me
many features among the boulders of
Ghoro Cho that proved valuable in reconstructing its geological as well as
human story. Visiting the scar on the
mountain side, I found green crystalline bedrock identical to Ghoro Cho
boulders. Just 100 meters below Mango
village, I found the outermost boulders
and tell-tale greenish deposits marking
the landslide rim.
However, this is also important in
relation to another story—one from
modern geoscience. For over a century
Ghoro Cho was interpreted by earth
scientists as moraine: debris dumped
by glaciers during the Ice Age. I had
passed it many times assuming the
ridges were moraines, an easy mistake
to make. But Haji Ali had the answer
to definitively separate landslides from
moraines—through rock type. The
boulders and other visible fragments of
Ghoro Cho are 100 percent that green
crystalline rock of the source slope.
Major mineral and major element analyses of samples showed that even the
finest dust is of identical composition.
Glaciers had indeed flowed into the
valley, but carried quite different rock
types, and of great variety—much as
the rivers do today. Meanwhile, rock
avalanche particles, from smallest to
largest, have the tell-tale angular or
“chink stone” character of fractured
and crushed material. Moraines of
large glaciers have diverse particle
shapes including stones rounded in
meltwater streams.
Of course, rock type analysis is
bread-and-butter for any trained geoscientist, particle shape for the sedimentologist, but in this case, no one
thought it necessary to check them out.
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Figure 4. Most sections of upper Indus streams do not flow directly on bedrock but instead
flow over thick, broad valley-fill sediments, such as the Skardu basin as seen from Katzarah.
Investigators have long recognized that geomorphological events must overwhelm the rate
of stream incision in order to form such plains, but the mechanisms have not been well understood. The notion that many such sediments in the upper Indus Basin formed when rock
avalanches blocked rivers valleys is a recent one.
The most influential studies from the
1850s to the 1980s offered compelling
interpretations of Ghoro Cho as glacial. Indeed, Quaternary geologists believed these and some other landslide
deposits represented the same three
or four Ice Age glaciations described
around the European Alps and in
Scandinavia. The author of Haji Ali’s
story knew better, but the moraine hypothesis prevailed until the 1990s.
Ghoro Cho can be a formidable
place, dark and forbidding from a distance, mostly barren and swept daily
by dust storms. Yet, every square inch
is known to local people. Too far from
water for cultivation, the area serves
mainly as grazing land. Troglodyte
homes for shepherd families and their
animals lie under the largest boulders,
and the residents have names for every feature. Many other stories are
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2010 September–October
413
simple (unconstrained) runout
tributary valley
source
valley wall source
minor impact-slope effect
tributary valley
source
valley wall source
strong impact slope effects
valley wall source
extreme impact slope effects
valley wall source
tributary valley
source
Figure 5. Landslide patterns depend on the topography of both the rock source terrain and the
valley below. They may climb the opposite walls of narrow valleys by as much as 500 meters
vertically, and the debris may spread down valley, caroming off walls in back-and-forth patterns.
Topography creates a few characteristic shapes as shown here, but the reality is endless variety.
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told, recalling the wolves, bears and
Siberian tigers that once sought refuge
among the boulders; dangerous djinns
or “fairies”; and heroes from the Kesar
Saga, the great oral epic of Shigar valley. Among these narratives are solid
grains of empirical and practical truth,
useful as environmental knowledge or
contributing to a sense of place, history
and calamity.
Landslide Villages
The village of Gol on the Indus is a
typical oasis of human-made greenery amid bare rock walls, closely
adapted to features developed after
a landslide descended from nearby
mountain walls. In fact, debris from
two megaslides blocks the Indus here
for 11 kilometers. Most of the way it
forms barren, boulder-covered terrain used, much like Ghoro Cho, for
seasonal grazing and hunting, and
where shepherds gather great bales of
Artemesia for kindling. Gol itself is on
a rock avalanche lobe that travelled 5
to 6 kilometers up the valley. Bouldercovered mounds support homes and
schools, the mosque and dispensary.
Arable land sits on flats between, once
the floor of the landslide lake. It was
500 meters deep at Gol and at least
90 kilometers long. Farms and fields
are also scattered over river terraces
recording the sequence of incision by
the river.
Of course, the hands of local farmers and their wives have made the terraced fields, fertile soil and channels
to carry irrigation water. Stone from
the landslide is used for terracing and
other construction, broken from boulders left conveniently on the valley
floor. The villagers at Gol have their
stories too, telling of a former lake
and boats tied up at places far above
today’s river; of the cataclysm that created the lake; and of a flood from upvalley that destroyed it. I suspect this
also reflects an eye for landscape more
than remembered history. At least, our
age determinations put the landslide at
4,300 years ago (+/-170 years)—long
before permanent settlement!
Many villages like Gol occupy
landforms developed through landslide interruptions. Skardu and Khapalu, the main towns of Baltistan, are
spread over rock avalanche areas. At
Skardu and Gilgit the airports lie on
terrace levels related to former landslide dams. Taxiing from the runway
at Skardu you pass bluffs in fine-
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grained yellow sediments laid down
in a vast lake dammed by the landslide
at Katzarah. One can only marvel at
the extraordinary range of resources
and uses people have found for the
landslides.
Highly Valued Places
Some of the earliest records of human
presence, rock carvings and inscriptions, are found on rock avalanche
boulders or outcrops polished by the
rivers cutting through them. This includes Ghoro Cho and Gol. It seems
that indigenous peoples and ancient
travelers identified these as special
localities, often as sacred places. The
greatest concentration of petroglyphs,
possibly up to 5,000 years old, lies
between Rakhiot Bridge and Shatial
along the Indus River below Nanga
Parbat (8,129 meters in elevation). Perhaps there are exceptional numbers
of suitable sites, or the landscape inspired a special awe in early residents
and travelers. Either way, every part
of the Indus valley here is shaped by
large landslides and their interruption
cycles. Thanks to them, land suitable
for cultivation supports quite heavy
settlement today, although plans for a
major hydroelectric dam would drown
much of it and the petroglyphs. Proponents evidently are unconcerned by
the danger of great landslides!
From the 1st to the 10th century a.d.,
Buddhist pilgrims traveling between
the subcontinent, China or Tibet followed the Karakoram Indus valleys,
and marked the way with images of
the Buddha, stupas, hand prints and
inscriptions. Best known, perhaps, are
carvings and inscriptions immediately
south of Skardu—on a boulder of the
landslide that dammed Satpara Lake.
Another famous site, the Sacred
Rock of Hunza, is covered in pre-,
and post-Buddhist petroglyphs spanning thousands of years. It is located
where a 1.5 cubic kilometer rock avalanche descended from the walls of
Ultar Glacier. Sediments, formed in
the lake it impounded, are quarried
at the “Pakistan-China” brick factory, just upstream of the Sacred Rock.
When the river was let down on the
rock spur below Altit Fort, it cut a rock
gorge, leaving a mid-valley salient, the
Sacred Rock, between this and the prelandslide channel. Some 500 meters
up the slope above, the rock avalanche
provides the foundations of Baltit
palace, thought to be over 700 years
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Figure 6. Dams formed when rockslides block rivers may occasionally be breached quickly,
but more often these materials have considerable integrity and are only gradually reduced.
In these cases, alluvial plains, terraces and other water-affected landforms develop, as here
at Ghoro Cho, near the confluence of the Braldu and Basha rivers. The photographic vantage
point is from the west.
Figure 7. For more than a century, earth scientists assumed that the landscapes found in upper
Indus streams were the result of glacial retreat from the last ice age. All the while, storytellers
knew better—for example, Apo Haji Ali seen here at Ghoro Choh Shang, or “long caves” (see
Figure 8). Although the stories he and other elders tell include considerable mythology, they
also demonstrate the residents’ understanding of how their topography was formed and show
some geological sophistication.
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2010 September–October
415
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Figure 8. Ghoro Cho is home mainly to shepherds, with small agricultural plots in river sediments. The residents have developed a detailed
nomenclature based on the land types and uses for their community.
Atabad Rockslide
O
n January 4, 2010, a rockslide
swept away part of the village of
Atabad in the Hunza Valley of northern Pakistan (upper left of photograph below). Seventeen people were killed and
twenty-six homes were destroyed. The
slide also blocked the Hunza River,
creating a lake that inundated several
other villages and submerged three
miles of the Karakoram Highway.
N
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American Scientist, Volume 98
By July 7, the date of this image taken by the Advanced Land Imager on
NASA’s Earth Observing-1 satellite, the
lake was nearly 22 kilometers long and
about 115 meters deep, had displaced
about 250 households, and had cut off
from the outside world roughly 25,000
people located in valleys above the lake.
The “worst case” disaster would
be for the dam to fail catastrophically,
sending a great flood wave downstream threatening land, settlements
and infrastructure. To be safe, the government has resettled about 20,000 people downstream in temporary camps
above the likely catastrophic flood
height. Meanwhile, workers began digging a spillway in the dam by late January, but the going was slow, the reduction in dam height modest. However,
the dam did not erode quicky when
the lake finally overflowed in late May,
and since early July the lake level has
stabilized. The spillway stream from
the lake can be seen as a white line in
the upper left of the image.
Unfortunately, if the dam continues
to hold, it still presents considerable,
if less extreme, concerns. Decisions
must be made about the displaced
residents, as well as the relocation of
the Karakoram Higway. There are also
strong pressures to lower the dam,
which would be costly and require
great care to be done safely. The crisis
is far from over.
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old and recently restored as a World
Heritage Site. In general, the classic
landscapes of Hunza are adapted to
features controlled by this and other
landslides—the countless terraces, the
fruit tree blossoms of spring, bright
green crops in summer and orange expanses of apricots in Fall.
Nowadays the Sacred Rock is
reached along the Karakoram Highway (KKH) linking Pakistan and China. It follows gorges cut through the
Nanga Parbat and Karakoram Ranges,
and the roadbed lies over terraces and
fans generated by landslide interruptions. Bridging points exploit narrows
and foundations they have created.
Over the 275 kilometers from Sazin
to Sost, KKH crosses more than 40
catastrophic landslides. Some sections
where collapses or blockages happen
repeatedly are excavated through old
landslide deposits.
Risky Landscapes
The gravest hazards arise at the time
of the landslides and later if landslide dams collapse releasing sudden
floods. The greatest historical floods
on the upper Indus came from shortlived landslide dams in 1841 and 1858.
Temporary losses arose when the lakes
inundated settlements and severed
communications, as again above Atabad in 2010.
Yet landslide-related hazards are not
confined to these short-lived extremes.
The aggradation phase can become
increasingly hazardous. As sediment
builds up in channels the more frequent
regional hazards—debris flows, rainstorm and glacier-melt floods—will
change paths across fans and deltas,
threatening arable land and even village
sites. Degraded interruption complexes
tend to be safer because old lake bed,
terrace and fan surfaces sit well above
incised channels down which floods
and debris flows travel. Even so, rock
avalanche material mobilized by heavy
rains or flood scour can feed damaging debris flows, as happened at Gol in
1996. Fields were left buried in rubble,
and boulders acted like battering rams
destroying walls and homes.
Local communities do observe strategies to reduce or prevent harm from
these more common mountain hazards
through land use, watching places or
weather conditions that are dangerous,
or coming together to repair property
after a damaging event. However, the
landslides themselves are dangers of
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Figure 9. Gol was once the site of a rockslide-entrapped lake 500 meters deep and at least 90
kilometers long. Residents here grow hardy crops on small areas of arable ground left by the
landslide lake and graze animals over the mounds of landslide debris. In 1996 a rockslide triggered by heavy rains buried many fields in rubble and damaged buildings.
a quite different order. The evidence
outlined here raises singularly disturbing questions about what triggers them,
and the marked disparity between their
prehistoric and historical numbers.
“The Big Ones”?
Because nearly all 340-plus rock avalanches affecting the rivers descended
across ice-free valleys, they post-date
the last major glaciation (LMG). This
Figure 10. The World Heritage Site Baltit palace has as its foundation the top of a 1.5 cubic
kilometer rockslide that descended from precipitous mountain walls some 2,000 meters above,
and on down to where Sacred Rock lies beside the Hunza River. In fact, although the region is
nearly as difficult to cross as it is to inhabit, it has been the route for pilgrims moving between
the subcontinent, China and Tibet for at least 2,000 years.
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2010 September–October
417
Figure 11. Carvings of Buddha along with inscriptions are common along the Indus valleys that
made up the pilgrimage route through the Karakoram, and some may date to 5,000 years ago.
But these examples, etched into a boulder that is part of the rockslide that formed Satpara Lake,
immediately south of Skardu, are probably the best known.
brackets the time frame involved and, if
the LMG here followed the most common Northern Hemisphere pattern,
these valleys became ice-free about
12,500 to 10,000 years ago. The 11 events
with numerically-determined ages, and
some 30 related to them through the
sedimentation record, happened between 2,000 and 8,000 years ago. Some
temporal clustering of events is indicated, but there is no clear decrease (or
increase) in incidence over the period.
In geological, rather than human terms,
the landslides are very young. Mean-
Figure 12. River terraces left when landslide dammed lakes gradually drain, as here along the
Braldu River, have been exploited by indigenous peoples for many centuries to grow crops
in the short agricultural season. Unfortunately, such communities remain vulnerable to new
rockslides that are as impossible to predict as they are to withstand, let alone prevent. Seismicresistant construction may offer some protection, but the push to form larger, more concentrated
settlements may overwhelm such efforts.
418
American Scientist, Volume 98
while, surveys to date cover barely 25
percent of the basin. If these are at all
representative, four times as many may
yet be discovered over a mountainous
area of about 100,000 square kilometers.
The record of recent centuries offers no
precedent for their numbers, or their
magnitudes.
Immediate triggers of large landslides are usually earthquakes or
weather extremes. Stormy weather
has been associated with several cases,
but larger Karakoram events seem too
deep-seated. They were most likely
co-seismic. The 1841 landslides were
triggered by an earthquake, and instabilities leading to the 2010 event are
traced to a 2002 earthquake. However,
these and the 1858 event, though very
destructive, are among the smaller
rock avalanches; one reason their dams
failed so quickly! Many prehistoric
examples are one to three orders of
magnitude larger, and seem to require
equally powerful earthquakes.
Another puzzling fact, however, is
that the Karakoram appears relatively
“quiet” seismically. Modern records for
the Skardu-Shigar basins show only minor, scattered seismic sources, although
some 16 massive rock slope failures are
known, including 3 megaslides. Most
are younger than about 8,000 years,
and at least 9 are between 4,000 and
2,000 years old. How, therefore, do we
explain the larger landslides—and so
many more occurring prior to historical
records? Larger earthquakes seem required even than those that caused major disasters in surrounding areas, such
as at Quetta in 1935 (magnitude 7.7) or
Kashmir in 2005 (magnitude 7.6).
The landslides evidence seems increasingly to support the hypothesis of
Peter Molnar, Roger Bilham and others
about missing great Himalayan earthquakes. To bring surface strain features
into conformity with plate tectonic
stresses would, apparently, require intermittent megaearthquakes (magnitude
8 or 9). Their near absence from written
records suggests they recur on very long
intervals—500 to 1,000 years at least—
implying equivalent “seismic gaps” in
time and space across the Himalayan
arc. The northwest Himalaya, according
to seismic and geodetic studies, seem
overdue for a mega-earthquake—what
Californians call “the Big One.”
Life in a Catastrophic Landscape
If the larger landslides are indeed triggered by megaearthquakes, it means
© 2010 Sigma Xi, The Scientific Research Society. Reproduction
with permission only. Contact [email protected].
that rare, seismo-tectonic events punctuate, reset and, ultimately, control the
landscape agenda. The landslide-related features along the Indus streams
support such a view. They comprise
indirect and drawn-out, post-landslide
responses, or “relaxation” phases, following landslide shocks. They develop
on millennial time scales and, coincidentally, create conditions favorable to
generations of human habitation.
More sobering, however, are the
rare but cataclysmic hazards implied:
events that combine great earthquakes
with multiple large landslides, river valleys inundated above, and starved of
sediment and water below, for centuries at least. And no capacity presently
exists to predict, let alone prevent,
events on such scales. Certainly, more
could and should be done to adopt
and enforce higher levels of structural
safety for buildings, which could at
least save lives away from megaearthquake epicenters and landslide run-out
zones. Regional preparedness, if more
effective than in recent earthquake disasters, could improve survival and
recovery possibilities. For the moment,
however, expanding towns and cities,
along with large-scale infrastructure,
notably big dams and highways, are
increasing the concentrations of people
and structures at risk.
Through most of history land use in
the Himalaya was organized around
widely scattered villages and smallish centers. Today, such an approach is
commonly treated as outmoded—a less
efficient form of existence—and in many
cases traditional risk-averting arrangements and environmental knowledge
are being undermined or lost. On the
other hand, the inhabitants may carry a
deep structure of historical understanding about collective security for living
with a catastrophically generated land
base. Their landslide stories may be
more realistic than fatalistic in coming
to terms with such an environment.
Finally, although the Karakoram has
exceptional numbers, landslides and
landslide-related developments clearly exist in other mountains. Examples
are known, and the list grows rapidly,
throughout High Asia, in the western
cordilleras of the Americas, the European and New Zealand Alps, and
the Caucasus. In some of these, too,
indications of rare megaearthquakes
are being investigated. The situation
supports calls for a revised concept of
the drivers of landform development
www.americanscientist.org
in high mountains, and a rethinking of
responses to the geohazards involved.
Online Slideshow
http://amsci.org/slide-show-hewitt
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