|
Note: The maps
can be viewed
here.
SACRAMENTO – The
Department of Conservation’s California
Geological Survey today released two new
regulatory Seismic Hazard Zone maps.
These maps show the risk of liquefaction
and landslides in portions of Santa
Clara County and – for the first time --
San Mateo County.
“These Seismic Hazard
Zone maps are good examples of the
state’s commitment to earthquake
preparedness,” said Mike Chrisman,
Secretary for Resources. “They are
important tools in the effort to protect
public safety.”
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
“These maps identify
the areas where liquefaction and
landslides are more likely to occur,”
said State Geologist John Parrish, head
of the California Geological Survey.
“With that knowledge, local planners and
the building community can take steps to
minimize the danger 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.”
The Mindego Hill
Quadrangle map encompasses about 59
square miles of mainly mountainous
terrain partly within and to the south
of the communities of Portola Valley in
San Mateo County and Palo Alto and Los
Altos Hills in Santa Clara County.
Most of the area
shown on the map remains undeveloped,
and a substantial part of it is
parkland. About 64 percent of the area
covered is zoned for earthquake-induced
landslide potential. Only a fraction of
the area covered – the channels and
narrow floodplains of Sausal, Los
Trancos, Adobe and other creeks – is
zoned for liquefaction potential.
The Castle Rock Ridge
Quadrangle map encompasses about 60
square miles of mainly rugged terrain in
the Santa Cruz Mountains; however, the
zoned area is limited to one-third of
the map, about 20 square miles of land
in Santa Clara County.
Of the zoned area –
including land partly within and
southwest of Saratoga and Monte Sereno
-- about 65 percent is designated for
landslide potential because of the
widespread occurrence of steep slopes,
low rock strength and expected high
ground acceleration in the event of a
strong earthquake. Only a small amount
of the area – along Saratoga, Lyndon and
San Tomas Aquinas creeks -- is
designated as a liquefaction zone.
Liquefaction – which
occurs when water-saturated, sandy soil
is shaken violently 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.
If the site-specific
geotechnical studies conducted prior to
development reveal that liquefaction or
landslide hazards are present, design
changes to better protect life and
property during future earthquakes --
such as deep foundations in liquefaction
zones and slope stabilization in
landslide zones – are required before
new developments are approved and built.
The program has
identified about 345 California
communities as high-risk areas for
liquefaction and/or landslides; 160 have
been zoned. There are now 109 Seismic
Hazard Zone maps covering all or
portions of eight counties.
To establish its
priority list, the California Geological
Survey looks at the level of seismic
hazard in each locale as well as the
amount of new development going on. The
program is continuing to work in Santa
Clara and Alameda counties, and also is
in the process of producing maps that
establish zones in the communities of
Atherton, Menlo Park, Palo Alto, and
South San Francisco.
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.
###
|