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SACRAMENTO -- In an
effort to make maps that impact land-use
decisions more accessible, the
California Department of Conservation is
offering its large inventory of
Alquist-Priolo Earthquake Fault Zone
maps for the Central Coast and
Northern/Eastern California as digital
images on two CD-ROMs.
Basically, the maps
from DOC's Division of Mines and Geology
are screening tools that determine
whether extra care should be taken to
ensure that new construction does not
take place on top of active faults.
Locating new structures away from active
faults can result in fewer injuries and
lower repair costs in the event of an
earthquake. By law, real estate agents
and sellers must inform buyers whether
property being sold is within one of the
zones shown on these maps.
The Northern/Eastern
CD contains 192 black-and-white images
of maps with Earthquake Fault Zones
highlighted in yellow. The digital
images in this CD show the zones in
Alpine, Butte, Inyo, Lassen, Mendocino,
Modoc, Shasta and Siskiyous counties.
The Central Coast CD contains 182
digital images including Alameda, Contra
Costa, Fresno, Kern, Lake, San Luis
Obispo, San Mateo, Santa Clara, Santa
Cruz, Solano, Sonoma, Stanislaus and
Yolo counties.
Viewing the digital
images requires Adobe Acrobat Reader
version 4.0, which is included in the CD
for Windows/NT, Macintosh and Unix
operating systems. The Earthquake Fault
Zones are indexed by county and city.
Users can select portions of map images
and copy them to word processing or
graphic software programs, where they
can be annotated and printed.
"The main benefits
are accessibility, cost and ease of
storage," Department of Conservation
Director Darryl Young said of the
digitized maps. "A paper copy generally
costs up to $7 per map. In this format,
all of the maps for Northern and Eastern
or Central California are available on
one CD for $30, and users can print out
whatever portion they need. You can move
from county to county and map to map by
clicking an arrow. It's as user-friendly
as possible."
For information about
purchasing either of the new CDs, call
DOC's Division of Mines and Geology at
(916) 445-5716. DOC's Division of Mines
and Geology previously released a CD
containing images of 235 Alquist-Priolo
maps for the Southern California region.
The Alquist-Priolo
Earthquake Fault Zoning Act addresses
the hazard of surface fault rupture,
which occurs when the movement on a
fault deep within the earth breaks
through to the surface. Not all
earthquakes result in surface rupture.
The earthquake must be large enough
(generally greater than magnitude 5) in
order to reach the surface. By
definition, blind thrust faults that
cause earthquakes, such as the
Northridge Earthquake of 1994, do not
break the ground surface and thus are
not shown on Alquist-Priolo maps.
The Act was passed in
1972 -- a direct result of the 1971 San
Fernando Earthquake, which caused
extensive surface fault rupture that
damaged numerous homes, commercial
buildings, and other structures. Surface
fault rupture -- generally the most
easily avoided seismic hazard -- causes
about five percent of the damage
resulting from earthquakes. Ground
shaking and related effects such as
liquefaction and landslides account for
95 percent of the damage.
Each Alquist-Priolo
map covers about 60 square miles, but
the actual zones generally are about
1,000 feet wide. Before development can
take place in those zones, the developer
must perform a geologic investigation.
Typically, a developer hires a
state-licensed geologist to determine
whether the project area is underlain by
active faults (those that have caused
ground displacement in the last 11,000
years). The developer submits the
geologic report to the local planning
department, which must agree with the
report’s findings before the development
can proceed. Structures for human
occupancy generally are set back 50 feet
from active faults.
There have been 27
earthquakes associated with surface
faulting in California since the first
Alquist-Priolo maps were issued, most
recently the magnitude 7.1 Hector Mine
quake last October. The last six of
those occurred mainly within established
A-P zones. Seven of those 27 quakes had
surface displacement of greater than a
foot.
In addition to
studying and mapping earthquakes and
other geologic phenomena, the Department
of Conservation maps and classifies
areas containing mineral deposits;
ensures the reclamation of land used for
mining; promotes beverage container
recycling; regulates oil, gas and
geothermal wells; and administers
agricultural and open-space land
conservation programs.
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