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SACRAMENTO --
Significant portions of Alameda County
could be susceptible to landslides or
liquefaction in the event of a strong
earthquake, according to six new Seismic
Hazard Zone maps released today by the
California Department of Conservation.
The communities
affected include Oakland, Berkeley,
Hayward, Alameda, San Leandro,
Emeryville and Albany. Three Seismic
Hazard Zone maps covering the cities of
Oakland and Piedmont were released in
2000; the new maps incorporate new
geologic information.
The U.S. Geological
Survey also released two new
non-regulatory maps that complement the
Seismic Hazard Zone maps.
Strong shaking causes
most of the damage during an earthquake.
However, 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.
These secondary hazards can be dangerous
when poor soil conditions exist within
40 feet of the ground surface.
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.
Once they become
official after a six-month review, the
maps will impact local planners,
developers, property sellers and real
estate agents. If property is located in
a “zone of required investigation,”
where liquefaction or landslides could
occur, the local building department
must require geologic studies before
projects are issued permits. Also,
property sellers and real estate agents
must inform potential buyers if property
they're selling is in a Seismic Hazard
Zone, as is the case when property is in
a designated flood or wildfire zone. The
maps have had no demonstrable impact on
real estate prices.
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. Therefore, design changes are
required before new developments are
approved and constructed. Changes – such
as deep foundations in liquefaction
zones and slope stabilization in
landslide zones -- made during the
planning phase can lessen the impact and
better protect life and property during
future earthquakes. The new maps are
important tools that land developers
will use to ensure project feasibility.
“Knowing where
liquefaction and landslides are most
likely to occur means that local
officials can require special
engineering steps on new construction to
make people and buildings safer,"
Department of Conservation Director
Darryl 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. About a square mile
centered on Lafayette Square is excluded
from the zone. 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.
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.
One of the new maps
from the USGS shows the liquefaction
hazard in the communities of Alameda,
Berkeley, Emeryville, Oakland and
Piedmont for a magnitude 7.1 earthquake
on the Hayward Fault. USGS scientist
Thomas Holzer explained that this map
complements the new Seismic Hazard Zone
maps by showing the degree of
liquefaction hazard within the
liquefaction zone mapped by DOC’s
California Geological Survey.
The second USGS map
shows the capability of different soil
types to amplify earthquake shaking in
the communities of Alameda, Berkeley,
Emeryville, Oakland, and Piedmont.
Records from many earthquakes show that
ground conditions immediately beneath a
building affect how hard the building
shakes. For example, said Holzer,
buildings on soft clay soils tend to
shake more violently than those on rock.
In 1989, after the Loma Prieta
earthquake, much of the damage in San
Francisco and the east bay was a result
of this effect, Holzer said.
The USGS maps are
available for viewing at
http://quake.wr.usgs.gov (contains
link to printable maps); the spatial
database and accompanying report are
available for downloading at
http://geopubs.wr.usgs.gov/open-file/of02-296/
as Open-file Report 02-296,
"Liquefaction Hazard and Shaking
Amplification Maps of Alameda, Berkeley,
Emeryville, Oakland, and Piedmont,
California: A Digital Database."
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