Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 152

commonly depressed as a result of the displacement of the subsurface sand and
water.
Liquefaction of the sand layer takes place when shear waves propagate
through saturated granular layers. These waves cause collapse of the granular
structure, and in so doing can cause a significant increase in the intergranular
pore pressure if drainage is impeded (Li et al ., 1996). When the pore pressure
equals the weight of the impenetrable overlying sediment, the granular layer
liquefies. This means that the sand behaves like a viscous liquid rather than
asolid. The liquefied sand and water can then vent through fractures to the
surface (Li et al ., 1996). Sims and Garvin (1996)propose a model to describe the
formation of sand blows. First, as a function of the strong ground shaking, grain-
to-grain disruption and suspension occurs. This is then followed by a pore-water
pressure increase, a redeposition of the suspended sediment, fissure develop-
ment and finally eruption of sediment and water at the ground surface. Matter
from below the ground surface can often be found in both the sand blows and in
thedikes themselves, and this material can show the direction of the flow of the
vented sand (Li et al ., 1996). When the liquefied sand does not reach the ground
surface, a liquefied zone at depth can be identified when that layer exhibits
little or no shear strength and increased pore-water pressure. Critical to the liq-
uefaction process is the increase in pore-water pressure. Measurements of in situ
pore-water pressure during modern earthquakes, such as the 1987 Superstition
Hills (USA) earthquake, show that the pore-water pressure increases gradually to
equal the pressure of the overlying sediment, but does not peak until cessation
of the earthquake (Sims and Garvin, 1995).
Sand blows have been observed and documented in relation to earth-
quakes worldwide. On a regional basis, the areal extent and concentration of
liquefaction-induced features are proportional to the energy released by the
earthquake. The location of individual sand blows, however, is determined by
the age and distribution of potential sources of sand, the presence of an imper-
meable layer above the liquefied sand, the thickness of the impermeable layer,
and the morphology of contact between the source layers and the impermeable
layers. The development of liquefaction features is dependent on several factors
such as the shaking intensity of the earthquake and the presence of liquefiable
sediments. The size of sand blows is assumed to be related to the intensity of
shaking during the earthquake. Structures formed at higher intensity shaking
tend to be larger than features deformed during lower intensity events. Their
preservation is largely dependent on the environment in which they are found
(Sims and Garvin 1995).
Sand blows can be used as indicators of prehistoric earthquakes in areas that
are seismically active but lacking clear surficial faulting. However, distinguishing