Geoscience Reference
In-Depth Information
of Los Angeles, where an earthquake with a
90 per cent probability of occurrence was forecast
between 1985 and 1993, less than 20 per cent of the
population purchased earthquake insurance and
fewer than 17 per cent made their homes more earth-
quake resistant, despite widespread publicity about
the likelihood of the event. Following the Loma
Prieta earthquake of 1989, the increase in insurance
in the most affected area was only 11 per cent, still
leaving over 50 per cent of the population uninsured.
The major populated segments of California lie in one
of the world's most unstable zones for earthquakes
and are poorly prepared for the consequences of
these events.
A major earthquake in southern California would be
catastrophic. Many past earthquakes in southern
California produced little damage because they
occurred in unpopulated areas. Many of these areas
are now densely settled. The San Fernando earthquake
of 1971, with an M
s
magnitude of only 6.6, caused the
collapse of public buildings, including two hospitals
built to earthquake building code specifications.
Sections of freeway overpasses built to the same
standards also failed to withstand the shock waves.
Subsequent to that earthquake, many public buildings
and roadways were reinforced or rebuilt. Building
codes have been tightened to account for the type of
damage produced by that relatively small earthquake.
Two dams in the area almost failed. One, the Lower
Van Norman earth-filled dam, came within inches of
collapsing and flooding 80 000 people in the lower
San Fernando Valley. There are 226 dams in the San
Francisco region alone with over 500 000 people living
downstream. Many of these dams, together with others
in southern California, have subsequently been
strengthened. Some of these remedial measures have
not worked. The 17 October 1989 Loma Prieta earth-
quake in San Francisco, measuring 7.1 on the M
s
scale,
collapsed a large section of the upper deck of the
Nimitz Freeway in Oakland, even though the supports
holding up this expressway were reinforced in the
1970s (Figure 10.7). Much of the housing construction
in the San Francisco area since 1906 has occurred on
unconsolidated landfill that could liquefy or amplify
shock waves up to ten times, causing total destruction
of buildings. This additional hazard was also brought
home in the October 1989 earthquake, which caused
the complete collapse of many houses in the Marina
district for this reason.
Aerial view of collapsed sections of the Cypress viaduct of
Interstate Highway 880 (H.G. Wilshire, U.S. Geological
Survey, <http://wrgis.wr.usgs.gov/dds/dds-29/>).
Fig. 10.7
Not all of the branches of the San Andreas Fault
have been mapped. For instance, the Diablo Canyon
nuclear plant north of Los Angeles was built to with-
stand an earthquake of 8.5 on the M
s
scale originating
from a distant fault. However, during construction a
major and active fault was discovered offshore within
5 km of the plant. While the plant's design was altered,
there is still doubt whether or not it could withstand a
major earthquake along the nearby fault. It is
inevitable that the next major earthquake in southern
California above 7 on the M
s
scale will bring untold
destruction. It has the potential to destroy large
sections of major cities and to start numerous fires,
which could turn into urban conflagrations similar to
the 1906 earthquake. Numerous dams could fail
causing major flooding and loss of life in downstream
valleys, and the process of ground liquefaction, espe-
cially in the San Francisco area, could flatten suburbs
in most of that city.
