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SACRAMENTO – The
state today released two maps that
identify parts of Los Angeles County
that must take precautions to avoid the
potentially devastating affects of large
earthquakes.
The Department of
Conservation’s California Geological
Survey produced these Seismic Hazard
Zone maps, which improve public safety
and impact planners, developers and real
estate transactions.
The maps define zones
where there is evidence that
liquefaction or landslides are more
likely to take place during damaging
earthquakes, generally those with a
magnitude of 5.5 or greater.
Liquefaction occurs when water-saturated
sandy soil is shaken and, much like
quicksand, temporarily cannot support
buildings or other heavy structures.
Liquefied soil can cause the ground to
crack and move, resulting in damage to
structures, buried pipelines and
utilities.
One of the new maps
is of the Acton Quadrangle in central
Los Angeles County, centered about 20
miles east of the Santa Clarita Civic
Center and 27 miles north of the Los
Angeles Civic Center. The other is of
the Pacifico Mountain Quadrangle; the
center of the area is 10 miles south of
Palmdale and 30 miles north of downtown
Los Angeles.
If property is
located in a Zone of Required
Investigation, the local building
department must require geologic studies
before projects are issued permits.
Also, property sellers and real estate
agents must inform buyers if property
they're selling is in a Seismic Hazard
Zone, as is the case when property is in
a designated flood zone.
“Knowing where these
hazards are improves public safety,” DOC
Darryl Young said. “Once you define the
problem, you can take steps to minimize
the danger.”
In many cases, it is
cost effective to retrofit houses and
buildings to minimize the effects of
severe shaking, which causes most of the
damage in big earthquakes. Local public
libraries have a number of publications
that can be used as guides to making
homes more earthquake-ready.
It is generally not
as cost effective to retrofit an
existing building for the impacts of
liquefaction or landslides as it is to
build in safety features at the design
stage. Thus, design changes to better
protect life and property during future
earthquakes -- such as deep foundations
in liquefaction zones and slope
stabilization in landslide zones – are
required before new developments are
approved and built.
“It’s better for
public safety, as well as easier and
less expensive for builders, to
incorporate design changes and make
buildings more resistant to these
hazards in the construction phase than
to rebuild after liquefaction or
landslide damage,” Young said.
The Acton Quadrangle
map defines liquefaction zones in the
bottoms of Soledad Canyon and its major
tributary canyons, where significant
development is occurring. Landslide
zoning covers nearly a fifth of the
evaluated portion of the quadrangle
(land inside the Angeles National Forest
is not zoned). There are numerous steep
slopes where rock falls and slides could
be triggered by earthquakes.
Nearly the entire
Pacifico Mountain Quadrangle is in
rugged San Gabriel Mountain terrain.
Development at this time is limited to
ranching and rural residential areas,
although there is a great deal of
recreational use. The liquefaction zones
are limited to the bottoms of the area’s
numerous canyons. About one-fifth of the
evaluated area is in landslide zones.
With these new maps,
the California Geological Survey has
issued 91 official Seismic Hazard Zone
Maps, 75 for Southern California and 18
for the Bay Area. Another 13 maps,
including several for Los Angeles and
Ventura counties, are under review.
DOC/California
Geological Survey geologists use
computer models as well as analyses of
existing geological mapping and hundreds
of engineering borings to produce the
maps, which are drawn on a scale where
one inch equals 2,000 feet.
Color copies of
official maps can be purchased through
DOC's California Geological Survey at
(213) 239-0878, (916) 445-5716, or (415)
904-7707. The maps also can be viewed
and downloaded on the Web
here.
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