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SACRAMENTO – On the
eve of the anniversary of Loma Prieta,
which killed 63 people and caused an
estimated $6 billion in damage,
Department of Conservation’s California
Geological Survey released three new
official Seismic Hazard Zone maps. These
maps show the risk of liquefaction and
landslides in portions of Alameda and
Santa Clara counties.
Speaker pro Tempore
Leland Yee, PhD (D-San Francisco/Daly
City), has taken an active interest in
the CGS mapping program and seismic
safety in general.
“The Seismic Hazard
Mapping Program is a good example of the
state’s commitment to earthquake
preparedness,” he said. “It is vitally
important that the legislature be
committed to taking every step possible
to ensure public safety in the event of
an earthquake.”
The Seismic Hazard
Mapping Program is also working in three
other areas of Santa Clara County, as
well as three in Alameda County. The
program recently began its first work in
San Mateo County, and is in the process
of producing maps that establish zones
in the communities of Atherton, Menlo
Park, Palo Alto, Portola Valley and
South San Francisco.
“These maps improve
public safety by ensuring that new
construction takes into account not only
that we’re in earthquake country, but
also that there are concerns other than
shaking,” said Michael Reichle, Acting
State Geologist and head of the
California Geological Survey. “By
identifying the areas where liquefaction
and landslides are more likely to occur,
local planners and the building
community can take steps to minimize the
danger.”
Liquefaction and
landslides are potential side effects of
earthquakes in the magnitude 5.5 or
greater range that can cause destruction
over and above the damage done by
shaking.
Liquefaction – which
occurs when water-saturated, sandy soil
is violently shaken and temporarily
loses its ability to support structures
– caused underground gas pipes to
rupture in San Francisco’s Marina
District during the Loma Prieta
earthquake. Leaking gas fueled a
spectacular, hard-to-extinguish fire.
The temblor also produced landslides
that blocked Highway 17 for days. The
Seismic Hazards Mapping Act was passed
the year after Loma Prieta.
“Loma Prieta is
sometimes referred to as the
`geologists’ earthquake’ because it
demonstrated the effect poor soils can
have on the level of earthquake damage,”
said Charles Real, the Supervising
Geologist who oversees the mapping
program at the Department of
Conservation.
Cities and counties
use the maps to regulate development. If
property is located in a Seismic Hazard
Zone, where liquefaction or
earthquake-induced landslides are deemed
more likely to occur, local government
must withhold development permits until
the level of hazard has been determined
by a detailed geotechnical investigation
at the construction site. If hazards are
present, mitigation measures are
incorporated into development plans.
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 program has
identified about 345 California
communities as high-risk areas for
liquefaction and/or landslides; 159 have
been zoned. To establish its priority
list, the program looks at the level of
seismic hazard in each locale as well as
the amount of new development going on.
Work is planned in Marin, Contra Costa,
Sonoma and Solano counties once Alameda,
Santa Clara and San Mateo counties are
complete.
Geographic
Information System technology is used to
integrate the information into a
three-dimensional view of the ground and
shallow subsurface. Here is a closer
look at what is shown on the latest
Seismic Hazard Zone maps:
Niles Quadrangle,
Alameda County: Part of the city of
Fremont covers the western half of the
quadrangle, and the map also covers
portions of Newark, Union City and
Pleasanton as well as the unincorporated
community of Sunol. The liquefaction
zone is concentrated in the southwestern
quarter and central part of the
quadrangle. An earthquake-induced
landslide zone covers about 36 percent
of the quadrangle, where the combination
of steep slopes and weak rocks in the
East Bay Hills have produced widespread
landslides.
Morgan Hill
Quadrangle, Santa Clara County: The
southeastern end of Santa Clara Valley
and parts of the cities of Morgan Hill
and San Jose are located in this
quadrangle. The liquefaction zone of
required investigation covers the valley
floor, the lowlands along Las Animas
Creek and San Felipe Creek, and the
bottoms of other creek canyons such as
Llagas Creek. Approximately 35 percent
of the quadrangle – confined to hills
and mountains, with virtually none in
the valley -- lies within the landslide
zone.
Milpitas Quadrangle,
Alameda County: The Santa Clara portion
of this quadrangle was mapped in 2001.
This quadrangle lies along the
southeastern margin of San Francisco Bay
and includes portions of the cities of
Fremont, Milpitas, Sunnyvale, Santa
Clara and San Jose. The boundary between
Alameda and Santa Clara counties crosses
the northern half of the quadrangle,
along Scott Creek and Coyote Creek.
Nearly the entire area west of
Interstate 680 is designated as a
liquefaction hazard zone. Some of the
hilly areas in the northeastern part of
the quadrangle are zoned for
earthquake-induced landslides.
The Seismic Hazard
Mapping Program released its first maps
for review in October 1996. At the
moment there are 108 official maps
covering more than 160 communities.
Mapping has been completed in Los
Angeles, Orange, Ventura and San
Francisco counties, and the program’s
emphasis has shifted from Southern
California to the Bay Area.
In addition to
studying and mapping earthquakes and
other geologic phenomena, the Department
of Conservation regulates oil, gas and
geothermal wells; ensures reclamation of
land used for mining; administers
agricultural and open-space land
conservation programs; and promotes
beverage container recycling.
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