Landslides - Extreme Events: A Physical Reconstruction and Risk Assessment - page 182

earthquake estimated to measure about 7.0 on the Richter scale. The slide mea-
sured 2.5 billion m 3 or 2.5 km 3 and consisted mainly of soil and shattered rock.
It buried the town of Usoy and its inhabitants, blocked the valley and dammed
the Murgab River resulting in the formation of a large lake. The lake level soon
rose, overflowed the landslide barrier and flooded Sarez, a village downstream
(Bolt et al ., 1977).
The 1964 Alaskan earthquake also triggered large landslides. This earthquake
measured 9.4 on the Richter scale. One of the resulting landslides, the Sherman
Glacier slide, occurred about 140 km from the earthquake epicentre where one
of the mountains, now called Shatter Peak, was shaken sufficiently well for
23 million m 3 of rock to break free from its crest. The falling blocks divided,
gaining velocity as they fell and landed on a glacier forming a flat, lobe-shaped
deposit only a few metres thick but which spread kilometres across the valley.
It is possible that an aircushion may have formed and supported this landslide
allowing it to travel the considerable distance across the valley. The aircushion
could have formed when the slide material hit the glacier and a transverse
oriented ridge at the bottom of the mountain that caused the slide debris to
be launched into the air forming a relatively flat sheet of flying debris. As this
sheet hit the snow and ice of the glacier, air could have been trapped between it
and the glacier surface allowing the slide to move more easily than would have
been otherwise expected. There were no casualties from the Sherman Glacier
slide as the area was unpopulated (Bolt et al ., 1977).
About 200 000 people died in Kansu, China in 1920 when a large earthquake
caused a massive landslide from the surrounding hills composed of loess (fine-
grained wind-transported sediments often derived from glacial terrains). The
earthquake broke the shearing resistance between the loess soil particles. The
low permeability of this material resulted in air being trapped within the mass
causing it to liquefy. Liquefaction is a common cause of landslides but it is usu-
ally water that becomes trapped within the material rather than air as occurred
in the Kansu event (Bolt et al ., 1977).
The Portugese Bend landslide in California, USA was a relatively slow moving
event. Urbanisation of the area during the 1940s and 1950s spread over former
landslide deposits. Heavy rainfall in 1956 caused about 1 km 2 of this urbanised
area to start sliding almost 20 m overaperiodofafewmonthsonaslopeof
only 6.5 ◦ . The mass continued to move at a rate of about three metres a year
and by 1980, 150 homes had been damaged or destroyed. The highway also had
to be relocated resulting in an estimated total economic loss of $10 million. The
LA County was held responsible for some of the damage as the area should have
been left unestablished. The county had to pay about $1 million in damage (Bolt
et al. 1977; Murck et al ., 1996).