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5. Understand How Earthquake Risk Varies by Location
Earthquake damage is typically concentrated in locations that can
be identified in advance:
- Areas nearest to the fault segments that are likely to move.
- Areas of soft soils where shaking is increased.
- Areas where the ground may settle or slide.
Failure of the ground during an earthquake can happen in many
ways. Cracks commonly rupture the ground near the slipped segment
of the earthquake fault. Landslides are likely on steep slopes,
especially if an earthquake hits during the rainy season. Soft ground
-- like that around the margins of San Francisco Bay -- may settle
during shaking. This settling will add to the hazard already posed
by shaking.
Damage in the Marina District of San Francisco during the Loma
Prieta earthquake of October 17, 1989, illustrates well the problems
aggravated by soft soil. This area sustained major damage even though
it was more than 50 miles north of the fault segment that slipped
in the Santa Cruz Mountains. The damage was unusually high primarily
because the shaking was increased by the soft soil, and the ground
failed. Furthermore, many buildings in the Marina District had ÒsoftÓ
first stories and other design details that are hazardous during
earthquakes. During the great San Francisco earthquake of 1906,
the shores along the lagoon that later became the Marina District
experienced some of the strongest shaking observed in San Francisco.
Shaking during the Loma Prieta earthquake was 3 to 4 times stronger
in the Marina District than on bedrock at Fort Mason, just a few
blocks to the east, because the Marina is underlain by mud nearly
100 feet thick.
But there is another reason for the severe damage in the Marina
District. In 1912, the original lagoon was filled with sand to prepare
for the Panama-Pacific International Exposition. Sand was an unfortunate
choice because, when wet, it can flow like a liquid during the shaking
of an earthquake. This process, called liquefaction, deforms streets,
sidewalks, pipelines, and buildings. The filled ground in the Marina
settled during the Loma Prieta earthquake as much as 5 inches. Seventy-three
percent of the buildings in the Marina District that became unsafe
for occupancy or entry after the quake were located on the filled
land. Techniques have been developed in the past few decades to
engineer landfills so as to reduce the chance of liquefaction and
ground failure during earthquakes.
This detailed map of the Marina District shows the areas underlain
by sandy fill and the distribution of damaged structures. Land that
liquefies during one earthquake has been observed to liquefy again
in subsequent earthquakes. Special engineering techniques are available
to minimize the effect of liquefaction, but they involve significant
costs.
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Earthquake hazards vary throughout your community depending
on the closeness to active faults, the type of soil,
the potential for ground failure, and the age and design
of structures. Recognizing these differences can provide
a basis for guiding future development to minimize earthquake
hazards. Clearly, new facilities such as hospitals and
fire stations would best be located in the safest sites,
and the most hazardous regions would best be designated
for parks or other low-density uses. Often, however,
even hazardous areas are too valuable not to be used,
and special design procedures are needed.
In the early 1970s, each California county and city
was required to develop a Seismic Safety Element for
its General Plan that included consideration of earthquake
hazards. Citizens interested in the future development
of their community may wish to consult this plan at
their local planning office and to encourage updates
of this plan in the near future.
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