Geoscience Reference
In-Depth Information
on the M
s
scale. The earthquake was located 60 km
north of the city and, at Niigata, produced maximum
ground accelerations of 0.16 per cent g - values that
cannot be considered excessive. The city itself had
expanded since the Second World War onto reclaimed
land on the Shinano River floodplain. The earthquake
shock waves did not destroy buildings. Rather, they liq-
uefied unconsolidated, floodplain sediments, suddenly
reducing their bearing strength. Large apartment
blocks toppled over or sank undamaged into the ground
at all angles (Figure 10.10). Afterwards, many buildings
were jacked back upright, underpinned with supports,
and reused.
The San Francisco earthquake of 1906 produced
some evidence of liquefaction around the city. The
breaking of water mains, which hampered fire sup-
pression, was generated by liquefaction-induced
lateral spreads, mainly near the harbor. Much of the
development of the city since has occurred on top of
estuarine muds with high water content. In addition,
much of the harborside construction in southern
California occurs on landfill, which can easily undergo
liquefaction if water-saturated. The San Fernando
earthquake in 1971 produced liquefaction in many
soils and caused landslides on unstable slopes. Similar
liquefaction problems were experienced during the
October 1989 San Francisco earthquake. When a large
earthquake strikes this city, as it inevitably will, lique-
faction will be the major cause of property damage.
The intensity of seismic waves does not have to be
great to induce liquefaction - as illustrated in Figure
10.10, where the bearing capacity of soils underlying the
buildings failed but the buildings remained virtually
undamaged. However, the intensity of compressional
shock waves triggering liquefaction may be consider-
ably lower than the values experienced at Niigata.
For instance, the Meckering, Western Australia,
earthquake of 1968 induced liquefaction of sands
underlying many parts of the expressway system in
Perth, 100 km away from the epicenter. At Perth, the
earthquake registered only 3 on the Mercalli scale. Not
only did the edges of roadways sink 4 cm within hours
of this seismic event, but the problem also persisted for
weeks afterwards, eventually resulting in widespread
damage to road verges.
The liquefaction process usually involves water
movement in clay-free sand and silt deposits. In
the loess deposits of China, it can involve air as the
suspending medium. The Kansu earthquake of
16 December 1920 broke down the
shearing
resistance
between soil particles in loess hills. The low
perme-
ability
of the material prevented air from escaping
from the loess, which subsequently liquefied; this
resulted in large landslides that swept as flow failures
over numerous towns and villages, killing over 200 000
people. The 1556 earthquake in Shensi Province may
have generated similar loess failure, giving rise to a
death toll in excess of 800 000.
Waterlogged muds can also amplify the long
period components of shock waves. Earthquakes at
Newcastle, Australia, in 1989 and in Mexico in 1985
provided clear evidence of this process at work. At
Newcastle, the seismic wave from a weak tremor
registering only 5.5 on the M
s
scale amplified in river
clays beneath the city. As a result, what was really only
a small tremor still damaged over 10 000 buildings,
creating a damage bill of $A1000 million. With the
Mexican earthquake, damage was even more exten-
sive. The Aztecs founded Mexico City in 1325 AD
upon the deep, weathered ash deposits of a former
lake bed, Lake Texcoco, later drained by the
Spaniards. These deposits have weathered to form
montmorillonitic clays with the ability to absorb water
into their internal structure, thus increasing their
water content by 350-400 per cent. From a geo-
physical point of view, these clays can be effectively
modelled as water, even though they have a solid
Liquefaction following the 16 June 1964, Niigata,
Japan, earthquake. These workers' apartments suffered
little damage from the earthquake, but toppled over
because of liquefaction of unevenly distributed sand and
silt lenses on the Shinano floodplain. Horizontal
accelerations were as low as 0.16 per cent g. The
buildings subsequently were jacked up and re-occupied
(photograph courtesy of the United States Geological
Survey, Catalogue of Disasters #B64F16-003).
Fig. 10.10
