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SACRAMENTO – Four
Seismic Hazard Zone maps – affecting
Oakland, Hayward, Union City, Fremont,
Castro Valley, San Lorenzo, Newark and
other communities -- became official
today. The maps, issued by the
Department of Conservation’s California
Geological Survey, impact planners,
developers, property sellers and real
estate agents.
If property is
located in a Zone of Required
Investigation, where liquefaction or
earthquake-induced landslides could
occur during a large earthquake, 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.
“These maps help
improve public safety by ensuring these
earthquake hazards are taken into
account during new construction,” DOC
Director Darryl Young said.
While strong shaking
causes most of the damage during an
earthquake, liquefaction and landslides
are an additional danger. Seismic Hazard
Zone maps show areas at risk of these
hazards during an earthquake.
The 1989 Loma Prieta
earthquake caused extensive
liquefaction-related damage in San
Francisco’s Marina District.
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. Loma Prieta also caused
landslides that blocked two lanes of
Highway 17 and damaged residences in the
Santa Cruz Mountains.
Retrofitting existing
structures to minimize the impacts of
liquefaction or landslides generally
isn’t as cost-effective as building in
safety features at the design stage.
Thus, these maps are aimed primarily at
new construction. They require that
design changes -- such as deep
foundations in liquefaction zones and
slope stabilization in landslide zones –
be made in the planning stage.
“It easier and less
expensive – not to mention better for
public safety -- to institute design
changes as a precaution in the
construction phase than to rebuild after
liquefaction or landslide damage,” Young
said.
With these new maps,
the California Geological Survey has
issued 89 official Seismic Hazard Zone
Maps, 72 for Southern California and 17
for the Bay Area. Another 17 maps are in
various stages of public review.
Each map covers about
60 square miles. Here's a closer look at
what each new Alameda County map shows:
♦ Hayward quadrangle
– There are widespread landslide zones
in the hilly regions east of the Hayward
Fault, covering about 22 percent of the
quadrangle. There are liquefaction zones
covering most of the low-lying areas in
the southwestern quarter of the
quadrangle. A small segment of this
quadrangle located in Contra Costa
County was not zoned. Some or all of the
communities of Oakland, Hayward,
Ashland, San Lorenzo and Castro Valley
are in this quadrangle.
♦ Mountain View
quadrangle – A liquefaction Zone of
Required Investigation covers most of
the mapped portion of this quadrangle.
There are no landslide zones on the map.
The portion of this quadrangle located
in Santa Clara County was included in an
official map released last year. The new
map now incorporates the Alameda County
portion. The cities of Fremont and
Newark fall within the Alameda County
portion.
♦ Newark quadrangle –
Most of the quadrangle consists of
flatlands, tidal marshes and salt
evaporation ponds along the eastern
margin of San Francisco Bay. The map
area includes the communities of Newark,
Hayward, Union City and Fremont. The
Hayward Hills extend into the
northeastern corner. The liquefaction
zone is spread across most of the
lowland terrain, including most of the
land surrounding Alameda Creek. There
are small landslide zones in the Hayward
and Coyote hills.
♦ Redwood Point
quadrangle – Only about five square
miles of Alameda County land – including
parts of Hayward and Fremont -- are
included in this map, which also covers
part of San Francisco Bay and San Mateo
County. The liquefaction zone covers all
of the land in the Alameda County
portion of the quadrangle; there is no
hilly terrain and thus no landslide
zone.
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. Mapping work
is ongoing in several Bay Area locales,
including Fremont, Morgan Hill, San
Jose, Palo Alto, Portola Valley and
Menlo Park.
Color copies of
official maps can be purchased through
DOC's California Geological Survey (415)
904-7707 or (916) 445-5716. The maps
also can be viewed and downloaded on the
Web at http://gmw.consrv.ca.gov/shmp.
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