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SACRAMENTO --
Significant portions of the San Jose
area could be susceptible to
liquefaction or landslides in the event
of an earthquake magnitude 6.0 or
greater, according to two Seismic Hazard
Zone maps released today by the
California Department of Conservation.
These preliminary
maps, each of which covers an area of
about 60 square miles, become official
after a six-month review and comment
period. Once they are official,
disclosure to the buyer must be made
before property in a Seismic Hazard Zone
is sold.
Shaking causes most
of the damage during an earthquake.
Seismic Hazard Zone Map maps, produced
by DOC's Division of Mines and Geology,
show areas at risk from landslides and
liquefaction, secondary hazards that
also can be dangerous.
Liquefaction was a
major cause of damage during the Loma
Prieta earthquake of 1989, especially in
San Francisco's Marina District. It
occurs when water-saturated sandy soil
within 40 feet of the surface is shaken
and temporarily acts like quicksand.
Loma Prieta also caused landslides that
blocked two lanes of Highway 17 and
damaged residences in the Santa Cruz
Mountains.
Planning officials
use the maps to identify areas that
require site-specific geologic or soil
investigations before new development is
permitted. Design changes on new
development and large remodeling or
restoration jobs can lessen the impact
of seismic hazards and better protect
life and property during future
earthquakes.
``These maps tell
local governments where they should take
extra steps to make people and buildings
safer," Department of Conservation
Director Darryl Young said. "It is much
less expensive to build in features to
minimize the potential damage of
liquefaction and landslides than to
retrofit."
The Milpitas
quadrangle map covers an area roughly
bounded by Highway 680 on the east to
the Alviso salt evaporator ponds, and
from the Alameda County line south to
the Bayshore Freeway. Most of this area
is zoned for liquefaction, the exception
being areas just east of Highway 680.
There are landslide hazard zones in the
hillsides east of Highway 680 from
Jacklin Road to the county line.
The Calaveras
Reservoir quadrangle map covers an area
roughly bounded by Highway 680 on the
west, seven miles east to the Cherry
Flat Reservoir, and from Alum Rock area
north to the Alameda County line.
There are
liquefaction zones noted in San Jose
just west of Highway 680 in Berryessa,
along Penintencia Creek through Alum
Rock Canyon, the low-lying area at
Tujarcitos Golf Course, Calaveras Valley
south of Calaveras Reservoir, and along
Arroyo Hondo Creek east of the
reservoir. Landslide zones occur
throughout hilly areas of northern San
Jose and eastern Milpitas from Hamilton
Road northwest to the hills north of El
Levin County Park. Landslide zones also
appear from Hamilton Road north to the
Alameda County line east of the
Calaveras Reservoir.
Department of
Conservation 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.
Fifty-one maps
covering more than 115 cities are now
official. The effort to identify
California's seismic hazards is ongoing.
Preliminary mapping is ongoing in the
West San Jose and Mountain View areas,
among others.
Black and white
copies of the completed maps are
available at cost through BPS
Reprographics Services in San Francisco,
telephone (415) 512-6550.
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