Portions of the cities of Los Altos, Los
Altos Hills, Mountain View, Palo Alto,
San Jose, Santa Clara and Sunnyvale are
on ground susceptible to liquefaction
during a large earthquake, according to
a preliminary Seismic Hazard Zone map
released today by the California
Department of Conservation.
Strong shaking causes
most of the damage during an earthquake.
However, Seismic Hazard Zone maps,
produced by DOC's Division of Mines and
Geology, show areas at risk from
landslides and liquefaction during an
earthquake magnitude 6.0 or greater.
These secondary hazards can be dangerous
when poor soil conditions exist within
40 feet of the ground surface.
City planning and
building officials use the maps to
identify areas that require
site-specific geologic or soil
investigations before new development is
permitted. Design changes on new
development and large remodeling or
restoration jobs can lessen the impact
of seismic hazards and better protect
life and property in future earthquakes.
Generally, it is more cost effective to
build improvements into new structures
than to retrofit existing ones.
Marina District was hard hit by the
effects of liquefaction during the Loma
Prieta earthquake of 1989. Liquefaction
occurs when water-saturated sandy soil
is shaken and, much like quicksand,
temporarily cannot support the weight of
buildings or other heavy structures.
Loma Prieta also caused landslides that
blocked two lanes of Highway 17 and
damaged residences in the Santa Cruz
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.
The Mountain View
quadrangle map covers a 60-square mile
area that extends from the Los Altos
Hills and Cupertino in the south to the
San Francisco Bay shoreline. The "zone
of required investigation" for
liquefaction covers most of the flat
ground that lies east of the Foothill
Expressway and north from Highway 280 to
the bay, including the Naval Air Station
at Moffett Field.
There are narrow
bands of liquefaction zones along Adobe,
Hale, Permanente and Stevens creeks.
Landslide Hazard Zones occur along the
steep stream banks in the Los Altos
Hills area. For more detailed
information, contact your local building
Conservation geologists use computer
models as well as analyses of existing
geological mapping and numerous
engineering borings to produce the maps,
which are drawn on a scale where one
inch equals 2,000 feet.
The preliminary map
of the Mountain View quadrangle becomes
official after a six-month review
period. Once the map is official, new
building permits issued inside the zones
will require a geologic study to
determine the level of hazard. All
property sold in California requires
that a natural hazards disclosure be
provided to the buyer prior to sale.
Property inside a designated Seismic
Hazard Zone will be part of that
disclosure, as is the case for property
in designated flood or wildfire zones.
The Department of
Conservation has mapped more than 3,000
square miles in California, including
most of Los Angeles, Ventura and Orange
counties, and has provided this hazard
information to more than 130 cities for
their use in planning and issuing
construction permitting. Mapping in the
Bay Area is ongoing in Berkeley, San
Leandro and parts of Alameda and Santa
Color maps can be
purchased through DOC's Division of
Mines and Geology at (415) 904-7707 or