M A S T E R P A G E
Teacher Background Reading
Liquefaction
2.4a
How Liquefaction Occurs during Quakes
Liquefaction happens during an earthquake when vibrations cause the pressures to build up in the ground water
that occupies the pore spaces between the grains of sand, silt, or loess. The longer the duration of the earthquake,
the more likely that liquefaction will be induced. The only solid strength of such a deposit is provided by the
friction between grains touching each other. When the pressure in the water that fills the pore space between the
grains is sufficient to spread them apart, the solid nature of the sand, silt, or loess deposit is changed into that of a
viscous liquid: “quicksand” or “quickclay.”
Because it takes time for the pressures that produce liquefaction to build up underground, and because quicksand
is a heavy, thick fluid that moves slowly, conditions of liquefaction, sand boiling, and associated phenomena may
not he apparent during the shaking. In fact, they often do not manifest until after the shaking has already passed,
sometimes not until 10-20 minutes later. The quick conditions or boiling of the sand can persist for hours or even
days after the quake, sometimes as much as a week.
How Big Does It Take & How Near to the Quake?
A natural question regarding seismically induced liquefaction is how big an earthquake is required to induce
quick conditions and how close it has to be for such effects to be possible. With regard to size, several technical
publications suggest that liquefaction does not occur for earthquakes less than Richter magnitudes of 5.2.
However, minor liquefaction effects in areas underlaid by particularly ideal predisposing conditions (loose sand
deposits saturated with a near-surface water table) have been observed for earthquakes as small as 4.7 on the
Richter scale in the New Madrid Seismic Zone. Minor damage to vulnerable structures has occurred in such areas.
With regard to distance, an earthquake in June of 1987 of magnitude 5.2 in southeastern Illinois caused
liquefaction phenomena near Bell City, Missouri, 150 miles (240 km) from the epicenter. A swimming pool, two
large grain bins, a carport, and three houses were damaged (one severely). There was also fissuring and lateral
spreading. At the same time, points nearer the epicenter of that quake did not experience such ground failures.
Three years later in 1990 this same area experienced no liquefaction phenomena when a 4.7 earthquake struck
only 20 miles (32 km) away.
Nearness to the epicenter implies greater amplitudes of ground motion, but distance implies a longer duration of
shaking, since the wave train consists of many waves traveling at a variety of speeds. The epicenter of the
magnitude 8.1 earthquake that struck Mexico City in 1985 was 240 miles (384 km) away and induced
liquefaction that severely damaged some buildings. Although lasting less than a minute at its distant source, that
quake lasted several minutes in Mexico City. Ground shaking amplitudes within the city, were never large, yet
400 buildings collapsed, resonating with the long-lasting wave train (or sequence of waves) amplified by
underlying clays.
The New Madrid earthquakes of 1811-12 induced extreme examples of liquefaction, manifesting as sand boils
and explosion cratering in the area of St. Louis, Missouri, and across the river in the flood plain of Illinois.
Liquefaction also occurred from those quakes as far as Cincinnati, Ohio, more than 300 miles (480 km) away.
Three Ways to Induce Liquefaction
Liquefaction in soils can be stimulated three ways: seismically, mechanically, and hydrologically. Seismically-
induced liquefaction is caused by seismic waves. Mechanically-induced liquefaction is caused by vibrations that
come from railroad trains, motor vehicles, tractors, and other mechanical sources of vibratory ground motion.
Hydrologically-induced liquefaction occurs when ground-water pressures increase due to rising stream levels
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