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SACRAMENTO – The
California Geological Survey (CGS) today
released three preliminary maps showing
areas in and around the South Bay with
potential for earthquake-induced
landslides and liquefaction. These
Seismic Hazard Zone maps designate areas
where new development and construction
must implement special precautions to
protect life and property in the event
of a large quake. The maps will become
official after a 90-day public review
period.
“Once we define the
areas with the highest potential for
these hazards, it’s relatively simple
and cost-effective to take
pre-construction measures to minimize
the risk to life and property,” said
State Geologist John Parrish, head of
CGS, a branch of the state’s Department
of Conservation.
Communities covered
by the maps include Morgan Hill, Menlo
Park, Palo Alto, and Redwood City. Each
map covers an area about 62 square miles
in size known as a “quadrangle.”
Earthquakes of
magnitude 5.5 or greater can trigger
liquefaction or landslides. Liquefaction
occurs when water-saturated, sandy soil
is violently shaken and temporarily
loses its ability to support structures.
Liquefaction caused underground gas
pipes to rupture in San Francisco’s
Marina District during the 1989 Loma
Prieta earthquake; the leaking gas
fueled a big, 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.
Seismic Hazard Zone
maps are aimed at new development, as
beginning construction with safety
features in mind is usually easier and
more cost effective than retrofitting an
existing building for the impacts of
liquefaction or landslides. The maps
create Zones of Required Investigation.
For development within a zone, the local
building department must require --
before permits are issued -- that a
registered geologist investigate the
site for evidence of liquefaction or
landslide potential. If such evidence is
found, design modifications must be made
in the planning stage. Examples of these
modifications include deep foundations
in liquefaction zones or slope
stabilization in landslide zones.
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
zone.
“As the centennial
anniversary of the Great San Francisco
Earthquake reminds us, big quakes are
inevitable in California,” said Chuck
Real, the Supervising Geologist who
heads the Seismic Hazards Zonation
Program for CGS. “The good news is that
new construction within our zones will
be better prepared for the next ‘big
one’ than it would otherwise have been.”
For those in existing
homes or structures, the Governor’s
Office of Emergency Services, American
Red Cross and the Federal Emergency
Management Agency have all released
publications about becoming more
earthquake-ready, which should be
available at local public libraries.
The Seismic Hazard
Zonation Program has identified 345
California communities as high-risk
areas for liquefaction and/or
landslides; 167 have been zoned. There
are now 112 Seismic Hazard Zone maps
covering all or portions of eight
counties. Upon the official release of
the newest three maps, CGS will have
completed 22 seismic hazard zone maps in
the greater Bay Area.
A closer look at
what the new maps show:
Mt. Sizer
Quadrangle: Covers southeastern
Santa Clara County and consists almost
entirely of the rugged terrain of the
Diablo Range, except for a few square
miles of flat land in the southwest
corner partially occupied the city of
Morgan Hill. The rest of the quadrangle
consists mainly of sparsely populated
land.
The map designates
Zones of Required Investigation for
liquefaction and earthquake-induced
landslides zones only for the
southwestern half of the quadrangle
because mountainous terrain and
projected land use in the northeastern
half make urbanization unlikely. Zones
for the liquefaction hazard generally
are limited to a few narrow canyon
bottoms and stream valleys, such as
Packwood and Hoover valleys. Zones for
earthquake-induced landslides, however,
cover most of the area subject to
seismic hazard zonation because of the
predominance of steep slopes combined
with relative low rock strength.
Palo Alto
Quadrangle: Covers eastern San Mateo
County and northwestern Santa Clara
County. Most of the lower half of the
map area consists of rough terrain of
the Santa Cruz Mountains while most of
the northern half is occupied by Santa
Clara Valley and salt evaporation ponds
at the south edge of San Francisco Bay.
Included in the study area are the
cities of Atherton, East Palo Alto, Los
Altos, Los Altos Hills, Menlo Park, Palo
Alto, Portola Valley, Redwood City, San
Carlos, and Woodside.
Most of the area
covered by zones for liquefaction lies
within Santa Clara Valley in the
northern third and along the
northeastern margin of the quadrangle.
The zone encompasses large parts of the
cities of Palo Alto, East Palo Alto, and
Redwood City. Less extensive zones for
liquefaction encompass all of Portola
Valley as well as most of Francisquito,
Los Trancos, Madera, and lesser stream
canyons and valleys in the Santa Cruz
Mountains. Earthquake-induced landslide
zones affect only a small fraction of
land within the quadrangle. The largest
of these covers much of the steep-sloped
highlands southwest of Portola Valley.
Most of the remaining areas consist of
relatively small, isolated zones in the
hilly terrain between Portola Valley and
Highway 280.
Mountain View
Quadrangle: This is a revised
release that addresses a previously
unevaluated area of about one square
mile within San Mateo County. The entire
area -- low-lying marine wetland along
the shore of San Francisco Bay -- is a
potential liquefaction zone. Previously
released and remaining in official form
are those areas of the map covering
Santa Clara (June, 2002) and Alameda
(July 2003) counties.
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