Geoscience Reference
In-Depth Information
breaking into fragments and then continues as a
slide or flow. Such mass movements have several
common characteristics:
1
landslides occur in unstable, faulted and
jointed rock masses;
2
a trigger mechanism is usually involved in
setting them off;
3
flow within the landslide is complex - often
involving several stages or modes of movement;
4
once deposited the mass is often still un-
stable and undergoes modification by other slope
processes;
5
these are very rapid mass movements.
Seismically triggered slope instabilities have
many of these features (see Case Study 2.1). For
example, the Sherman Glacier rock avalanche
in Alaska was triggered by the 1964 Alaska
Good Friday earthquake (McSaveney 1978). The
bedrock of this area consists of highly deformed
slightly metamorphosed sedimentary sequences,
which are heavily faulted and the fault planes are
steeply dipping. The rock avalanche fell 600 m
and then spread 5 km across the Sherman Glacier,
depositing a blanket 3 - 6 m thick. The avalanche
flowed as a large lobate mass with the particles
behaving like loose aggregates, spreading as it
Case study 2.1 Huascarán, Yungay, Peru 1970
The greatest mountain landslide disaster recorded to date occurred in Peru following an offshore
earthquake (magnitude 7.7) on 31 May 1970. The seismic event triggered a catastrophic rock
and snow avalanche from the summit ice-cap of Huascarán Mountain (6654 m), the highest peak
in the Peruvian Andes. The displaced mass fell vertically to the glacier below, where it generated
a gigantic debris flow surge that travelled down-valley at a speed of over 70 -100 m s
−1
entrain-
ing sediment in its path (Case Fig. 2.1). On reaching the meltwater river of the Rio Sacsha the
flow had changed into a mudflow of over 1 km wide, carrying gigantic boulders, some as great
as 15 m
3
. Eyewitnesses described the flow as a huge wave at least 80 m high (Whittow 1980).
The flow travelled 15 km down to the confluence with the Rio Santa in just a few minutes.
The mountain has had a history of devastating landslides. A mudflow had swept down the Rio
Sacsha in 1962 killing 4000 people and depositing 13
10
6
m
3
of material (Smith 2001). The
1970 mudflow was of much greater magnitude and completely overran the towns of Yungay and
Ranrahirca, leaving 18,000 people dead or missing. It is estimated that 50 -100
×
10
6
m
3
of
material was involved, burying some settlements up to 10 m deep in sediment (Case Table 2.1).
This is not the only catastrophic mass movement event identified in this valley. A smaller
avalanche also took place on 10 January 1962 and an earlier (Pre-Columbian) event, larger than
the 1970 avalanche, is identified from deposits preserved on the valley floor (Case Table 2.1).
×
Case Table 2.1
Summary characteristics of selected data from three of the largest avalanches observed from Nevados
Huascarán, Peru. The Pre-Columbian event is identified from deposits preserved outside the limits of the 1970 event.
(Source: Plafker & Ericksen 1978.)
Pre-Columbian
10 January 1962
31 May 1970
Area covered (km
2
)
>
30
6
22.5
Volume (106 m
3
)
100-200*
>
13
50-100
Average velocity (km h
−1
)
315-355†
170
280
Runup height at Rio Santa (m)
123
30
83
Velocity at Rio Santa‡ (km h
−1
)
>
140
>
60
>
120
*Estimated from the extent and nature of deposits.
†Derived by comparison with historic avalanches.
‡Calculated from runup height assuming avalanche front thicknesses of 15 m (1962), 30 m (1970) and 45 m (Pre-Columbian).
