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EDITORS: The 100th
anniversary of the Great San Francisco
Earthquake is April 18. The California
Geological Survey (CGS) is regarded as
the primary source of geological and
seismological products and services for
decision-making by California's
government agencies, its businesses and
the public. The following is a feature
about the state’s earthquake history. It
is the second of several releases
related to California seismology and CGS’
work we will send in the weeks prior to
the centennial anniversary.
Information about CGS’
earthquake-related programs, as well as
background that may be helpful in your
coverage of the 1906 earthquake
anniversary, is available on the CGS Web
site at: http://www.consrv.ca.gov/CGS/index.htm.
For more information, or to arrange
interviews with CGS scientists about
earthquake-related stories, please call
(916) 323-1886.
SACRAMENTO – Before
the 54-story Towers on Capitol Mall soar
into Sacramento’s skyline, pilings will
be driven deep into the ground to ensure
the buildings’ earthquake stability.
Like many of Sacramento’s structures,
these $500 million, 615-foot behemoths
will need to be built to withstand the
effects of a strong earthquake along any
large faults near the capital city.
While Sacramento
doesn’t face the same level of
earthquake threat as the San Francisco
Bay Area or the greater Los Angeles
metropolitan area, it has felt the
effects of earthquakes past. How
Sacramento, with its relatively new
big-city skyline, will fare in the next
Northern California temblor is unknown.
In fact, structural engineers sometimes
refer to the “Sacramento problem” when
discussing how tall buildings will
respond in earthquakes.
“Sacramento doesn’t
have any known faults running under it,”
said State Geologist Dr. John Parrish,
who heads the California Geological
Survey (CGS), a branch of the Department
of Conservation. “However, there are
major faults near enough that give us a
couple of significant earthquake
concerns. One concern is the potential
impact of liquefaction, particularly on
the levees that surround much of the
community. The second is the performance
of high-rise buildings during strong
ground shaking.”
Liquefaction occurs
when the grains of poorly consolidated,
sandy soils are shaken apart and the
ground water is squeezed into the open
spaces between the grains. That forms a
substance much like quicksand that no
longer has the strength to support
buildings or other heavy structures.
Liquefied soil can allow the ground to
crack and move, resulting in damage to
structures such as buildings and levees,
buried pipelines and utilities.
"We're in an area
where much of the soil is sedimentary
river deposits, and there's a high water
table because we're near the confluence
of two rivers,” said CGS Supervising
Engineering Geologist Charles Real.
“Those are two of the major ingredients
that create liquefaction.
“The only missing
ingredient for liquefaction here is
earthquake shaking, and that's why
mapping Sacramento for the hazard of
liquefaction hasn't been as high a
priority as mapping other areas. The
likelihood of damaging earthquake
shaking here is not as high as it is for
the Bay Area, but Sacramento has
experienced shaking in the past. If San
Francisco had another large earthquake,
something of 1906 proportions, we can't
say with certainty what that would do to
Sacramento. There's a lot of development
pushing up against our levees, and there
are concerns about how well many of
those old levees would hold up in a
large earthquake."
Real heads the
Seismic Hazards Zonation Program for CGS.
The program has produced more than 100
regulatory maps covering the Bay Area
and Southern California. The maps show
zones where there’s a relatively higher
potential for liquefaction and
landslides during large earthquakes.
“We know that
liquefaction can occur at a considerable
distance from an earthquake's epicenter;
look at what happened in San Francisco’s
Marina District during the 1989 Loma
Prieta earthquake,” Real said.
"Earthquakes produce waves of energy.
We’ve learned that although the rapid
violent vibrations die out as you move
farther away from an earthquake's
epicenter, the lower frequency seismic
waves travel farther and can affect
high-rise structures. Although less
violent, shaking from long period waves
lasts longer and can still cause sandy
saturated soils to liquefy.
“Certainly the 1906
San Francisco earthquake was felt in
Sacramento, and the 1892 Winters
earthquake, which was estimated to be
magnitude 6.6, damaged the Capitol and
other buildings here. There hasn't been
a large earthquake centered near
Sacramento since the construction of
high-rise buildings. So the question of
how our tall buildings, as well as our
levees, would react is an important
one.”
As the Supervising
Geologist for CGS’ Strong Motion
Instrumentation Program (SMIP), Dr.
Anthony Shakal works closely with the
engineering community in the development
of building codes and construction
practices. The nearly 1,000 recording
stations that SMIP has placed around the
state – including a handful in
Sacramento at the Capitol and Cal-EPA
building -- record how structures such
as buildings, bridges and dams respond
to ground shaking.
“In structural
engineering, they refer to the
‘Sacramento problem,’ ” Shakal said.
“What happens when you combine tall
buildings, deep valley sedimentary soil,
and a large earthquake at a distance?
For high-rises here in Sacramento, the
earthquake underfoot isn’t as much a
concern as the earthquake in the next
county or two counties away because of
those long seismic waves.”
Shakal pointed out
that during the 1989 Loma Prieta
earthquake, centered more than 100 miles
away from Sacramento just north of Santa
Cruz, Department of Conservation
personnel located in the Resources
Building in downtown Sacramento felt
significant shaking. In 1992, DOC
employees on the upper floors of the
28-story Renaissance Tower (now known as
801 K Street) reported noticeable
swaying after a magnitude 7.2 earthquake
over 200 hundred miles away offshore of
Humboldt County.
Several faults
thought capable of producing large
earthquakes are within 60 miles of
Sacramento: the Calaveras, Hayward,
Greenville, Concord-Green Valley and the
Foothills system.
The giant Towers on
Capitol Mall are being built under the
Uniform Building Code. The code
addresses a building’s response to
seismic shaking, and varies depending on
the building’s height. Strong motion
recordings, like those made by Shakal’s
group, directly establish the formulas
used in design for a building’s period –
the amount of time in takes to sway back
and forth in its natural condition.
“We can estimate what
the building’s motion will be before the
first piles are driven,” Shakal said.
“If you’re in a house in Sacramento, you
might not feel the motion from an
earthquake on the Calaveras or Hayward
fault. But if you were way up in a
skyscraper, you’d almost certainly feel
it.”
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