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Note: To view
the liquefaction map for parts of
Oakland and Berkeley,
click here.
SACRAMENTO – Six
Seismic Hazard Zone maps – affecting the
communities of Oakland, Berkeley,
Hayward, Alameda, San Leandro,
Emeryville, Piedmont and Albany --
became official today. The maps, issued
by the Department of Conservation’s
California Geological Survey, impact
planners, developers, property sellers
and real estate agents.
One preliminary map
for part of San Jose and Santa Clara
County also was released. The Santa
Teresa Hills map will become official
after a six-month review.
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.
“The release of these
maps will increase awareness of these
earthquake-related hazards and prompt
better construction practices, thus
improving public safety,” DOC Director
Darryl Young said.
Strong shaking causes
most of the damage during an earthquake.
Seismic Hazard Zone maps, produced by
DOC's California Geological Survey, show
areas at risk from landslides and
liquefaction during an earthquake of
magnitude 6.0 or greater.
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 sand 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.
“These proactive
changes can lessen the impact of seismic
hazards and better protect life and
property during future earthquakes,”
Young said.
Each map covers about
60 square miles. Here's a closer look at
what each shows:
♦ San Leandro
quadrangle: The San Francisco Bay
occupies about half of the area on this
map. However, parts of the cities of
Oakland, Alameda, San Leandro and
Hayward are shown, as well as some
unincorporated portions of Alameda
County. Liquefaction zones cover most of
the mapped land area, from the Oakland
Coliseum south to the Salt Evaporators,
and from the east bay coastline east to
the MacArthur Freeway (580). The
northeastern corner of the quadrangle
includes some landslide zones on the
East Bay Hills.
♦ Richmond
quadrangle: The Richmond map zones were
only done for Alameda County, covering
the southern 20 percent of the map. A
liquefaction zone covers a small area in
Albany from the Golden Gate Fields area
east to San Pablo Avenue, from Cedar
Street north to about Solano Avenue.
Also, a narrow zone follows a creek bed
easterly from San Pablo Avenue to
Hopkins Street. Broad landslide zones
occur in the Grizzly Peak areas of
Cragmont, and some areas in the Thousand
Oaks areas. Small landslide areas occur
on the slopes of Albany Hill along
Interstate 80.
♦ Oakland East and
Oakland West: Liquefaction zones cover
all of Alameda and western Oakland from
the bay coastline to a few blocks east
of Highway 24. Liquefaction zones have
been mapped along the Nimitz Freeway
across from Alameda, spreading out
southeast to the Oakland Coliseum. The
zones extend along the beds of Samsal
Creek and Peralta Creek. Landslide zones
have been mapped in the Oakland and
Berkeley hills. The zones extend from
the UC Berkeley campus northeast of
Ashby Avenue to a two-mile wide region
between Skyline Blvd and the MacArthur
Freeway southeasterly through Knowland
Park to San Leandro.
♦ Briones Valley: No
Zones of Required Investigation for
liquefaction were mapped in the Alameda
County portion of this quadrangle.
However, historically the combination of
dissected terrain and weak rocks has
produced abundant landslides. A
landslide zone covers about 55 percent
of the Alameda County land area in the
quadrangle.
♦ Hunters Point: No
Zones of Required Investigation for
landslides were mapped in the Alameda
County portion of this quadrangle. While
the Hunters Point Quadrangle covers
approximately 60 square miles in
Alameda, San Francisco, and San Mateo
counties, San Francisco Bay occupies
most of the area. An official Seismic
Hazard Zone map covers the portions of
the City and County of San Francisco in
this quadrangle. The Alameda County land
shown on the new map consists of
approximately half a square mile at the
north end of Bay Farm Island, which
includes a southern section of the City
of Alameda. The liquefaction zone covers
the entire land area of Bay Farm Island.
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 Palo Alto, Morgan Hill, north
Hayward and Gilroy.
Black and white
copies of preliminary maps can be
purchased from BPS Reprographic Services
in San Francisco, (415) 495-8700. 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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