Earthquakes and Tsunami as Hazards - Natural Hazards - page 212

Earthquake-induced shear stress (P-wave)

Compression

Dilation

Pore water pressure

Point of
liquefaction

Liquid limit

Pre-earthquake pressure

Time

Cumulative effect of compressional P-waves upon pore water pressure leading to liquefaction (after Bolt et al., 1975).

Fig. 10.9

during earthquakes. It occurs on slope angles ranging
from 0.3-3.0 degrees, with horizontal movements of
3-5 m. The longer the duration of ground shaking, the
greater the lateral movement and the more the surface
layers will tend to fracture, forming fissures and scarps.
During the 1964 Alaskan earthquake, floodplain sedi-
ments underwent this process, and lateral spreading
destroyed virtually every bridge in the affected zone.
Flow failures involve fluidized soils moving downslope,
either as a slurry or as surface blocks overtop slurry at
depth. These flows take place on slopes greater than
3 degrees. Some of these flows can move tens of kilo-
metres at speeds in excess of 15 km hr -1 . More
commonly, these flows can occur in marine sediment
under water. Loss of bearing strength simply represents
the transference of the ground load from grain-to-grain
contacts to the pore water. Any object that has been
sitting on this material and using it for support is then
liable to collapse or sink. At present, the exact condi-
tions responsible for earthquake-induced liquefaction
have not been determined. Evidence indicates that the
acceleration and number of shock waves control the
process; however, prior history of the sediment cannot
be ignored. Only mapping of subsurface material in
earthquake-prone areas will delineate the area of risk
from this process during earthquakes.
The effects of earthquake-induced liquefaction can
be spectacular. For example, the Japanese earthquake
at Niigata of 16 June 1964 registered 7.5 in magnitude

capillary water tension between particles is greater.
This cohesion is so large with very fine silt and clays
that the pore water pressure cannot reach the point of
liquefaction. Medium- to fine-grained sands satisfy
both criteria for liquefaction to occur. They are fine
enough to inhibit rapid internal water movement, yet
coarse enough that capillary cohesion is no longer
relevant. Since these sand sizes are common in
recently deposited river or marine sediments (in the
last 10 000 years), liquefaction is an almost universal
feature of earthquakes. The younger and looser the
sediment deposit and the higher the watertable, the
more susceptible the deposit is to liquefaction.
The build-up of pore water pressure can cause water
to vent from fissures in the ground resulting in surface
sand boils or mud fountains. Following the 7.6 M s
earthquake on 26 January 2001 in the Rann of
Kachchh, India, the magnitude of groundwater
evulsion was so great that it reactivated ancient river
channels and formed shallow lakes over a 60 000 km 2
area. If pore water pressure approaches the weight of
the overlying soil, then the soil at depth starts to behave
like a fluid. Objects of any density will easily sink into
this quicksand because the subsurface has virtually no
bearing strength . Thus, liquefaction causes three types
of failure: lateral spreads, flow failures, and loss of
bearing strength. Lateral spreads involve the lateral
movement of large blocks of soil as the result of lique-
faction in subsurface layers, due to ground shaking