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SACRAMENTO – As the rainy season
approaches, the California Geological
Survey (CGS) is releasing five landslide
maps intended to protect public safety
by guiding future development.
“Wet weather is just
around the corner, so people need to be
aware of the potential for landslides,”
said Supervising Geologist Chuck Real,
head of CGS’ Seismic Hazards Zonation
Program. “The most likely places for
landslides to occur are the places where
they’ve happened in the past. By
providing that information, these maps
will help raise awareness of the need to
prepare and to plan.”
These are some of the
most detailed landslide maps produced by
CGS, showing slides that can be
triggered either by earthquakes or by
heavy rain. In the coming years, CGS
hopes to release a total of about 100 of
these “landslide inventory” maps. Three
of the first five cover parts of the Bay
Area – in and around Morgan Hill and San
Jose -- while two cover areas in rural
Los Angeles County east of the rapidly
developing Santa Clarita area.
Although the maps are
non-regulatory, they provide important
information to local planners,
decision-makers and geotechnical
consultants: whether a landslide is
considered active or dormant, the
direction of movement, the type of
movement involved (some landslides are
more destructive than others), as well
as CGS’ confidence level in their
interpretation of the data. CGS, a
branch of the California Department of
Conservation, produces the maps by
incorporating previous mapping work with
a detailed review of aerial photography
and geologic fieldwork.
“Big earthquakes tend
to be more damaging than individual
landslides, but landslides are a more
frequent phenomena and cause a
remarkable amount of damage in
California,” said State Geologist Dr.
John Parrish, head of CGS. “The
potential for landslides can sometimes
be mitigated. However, in most cases, it
is best to avoid building on or near
them. The first steps are determining
where they are, how they move, and how
often they move. That’s where these maps
are helpful.”
A landslide is any
mass of earth or rock that slides, flows
and/or falls downhill. Landslides can
affect land from a few square yards to
hundreds of acres in area. They can be a
few feet to hundreds of feet thick.
In January of 2005, a
landslide claimed 10 lives in the
Ventura County community of La Conchita.
In June of 2005, a landslide in Laguna
Beach’s Bluebird Canyon caused no
fatalities but forced the evacuation of
more than 1,000 people from about 350
residences. Fifteen homes were destroyed
and several others severely damaged.
Large, slow-moving
landslides composed of bedrock can cause
extensive property damage but usually do
not result in loss of life. A debris
flow (commonly called a mudslide) is a
more dangerous type of slope failure
because it is fast moving and can cause
both property damage and injuries. Mud,
rock and debris caught by these rapid
flows can travel faster than 10 mph – in
rare cases, up to 100 mph.
Those living on or
below slopes are advised to know the
signs of impending landslides, to check
for them, and to contact local
authorities if they are concerned about
the potential for landslides.
“If it rains heavily
for several consecutive days, it’s
prudent to watch and listen for
location-specific landslide warnings
from federal, state and local agencies,”
Parrish advised.
CGS provides
technical information and advice about
landslides, erosion, sedimentation, and
other geologic hazards to the public,
local governments, agencies and
industries that make land-use decisions
in California. More information about
landslides, links to landslide maps, and
mitigation steps can be found on CGS
pages
here and United States Geological
Survey pages
here.
“In recent years,
there have been several cases of
landslides occurring in areas where
summer wildfires have burned off the
ground cover, making the soil more
vulnerable to flowing after heavy
rains,” Parrish said. “Many factors can
contribute to the formation of
landslides aside from rainfall,
including improper construction or
grading, earthquakes, weak or loose rock
and soil, and steep slopes.”
Property owners are
advised to consult a licensed geologist
before taking any steps intended to
mitigate potential risks or harm
associated with landslides.
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