Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 147

Other hazards associated with earthquakes

There are three main natural impacts resulting from earthquakes: liq-
uefaction, landslides and tsunami. Liquefaction occurs where the pore pressure
of water equals or exceeds the weight of the overlying soil. Soils behave as a
solid when the pore pressure (pressure in voids or spaces between individual
particles or grains) is lower than the weight of the soil (Bryant, 2005). But pore
pressure increases substantially during earthquakes resulting in a loss of grain
to grain contact within the soil. The increased pore water pressure suspends the
particles creating a dense slurry and the soil behaves as a liquid. The liquefac-
tion potential of a soil is dependent on the grain size of the sediments. Greater
cohesion between silt and clay particles prevents pore water pressure from hav-
ing the potential to equal the weight of the soil. Liquefaction, therefore, usually
occurs in medium- to fine-grained sands typical of alluvial and marine sediments
(Bryant, 2005).
Earthquakes cause landslides by weakening the structural integrity of natural
slopes. Rock fall is the most common form of slope failure but snow avalanches
have also been known to occur. A mainly rock and snow avalanche from a cliff
overhang in the Nevados Huascaran mountains (Peru) following a magnitude
7. 7earthquake caused more than 50 million m 3 of mud and boulders to travel
at more than 250 km h − 1 as a 30 m high surge. More than 18 000 people died
when the avalanche buried a number of towns (Chapman, 1999).
Earthquakes greater than magnitude 6.5 on the Richter scale have the poten-
tial to produce tsunamis (see Chapter 5). Earthquakes that produce tsunami are
often shallow earthquakes that occur at depths between 0 and 40 km (Bryant,
2005). Earthquake-generated tsunamis are usually associated with subsea fault
movements causing a vertical offset of the seabed. The peak energy period of a
tsunami is a function of the magnitude of the earthquake and can be described
using the following equation,

T t = 0 . 57 M − 2 . 85 (6.2)

T t describes the period of the tsunami while M describes the magnitude of
theearthquake on the Richter scale. Although the waves are relatively low at
sea (generally <1 m), they shoal and reach extreme heights in shallow water
close to the coast. Because shore lines are often relatively steep compared to
the wavelength of the tsunami, the wave does not break but surges up over the
foreshore. The run-up height of the wave also depends on the configuration of
theshore, diffraction of the approaching wave, and the characteristics of the
wave and resonance (Table 6.4). Tsunamis travel at speeds of up to 800 km h − 1
in the deepest parts of the ocean and 300 km h − 1 across the continental shelf.