|
SACRAMENTO – After an
earthquake with an estimated magnitude
of 6.6 that caused widespread damage
nearby, the local newspaper’s headline
read: “The Earth Dances a Lively
Hornpipe.”
The paper in question wasn’t the San
Francisco Chronicle, Los Angeles Times
or Eureka Times-Standard, all of which
have had their fair share of earthquake
headlines.
It was the Sacramento Bee.
While Sacramentans undoubtedly have less
to fear from earthquakes than many other
Californians, the area isn’t totally
immune from being shaken up at times.
The 97th anniversary of the Great San
Francisco Earthquake is April 18. But
there’s another anniversary of which
Sacramento-area residents should be
aware.
The “lively hornpipe” the Earth danced
that was felt so strongly in Sacramento
occurred on April 19, 1892. The
epicenter was estimated to be somewhere
in the heavily damaged area of
Vacaville, Dixon and Winters (“Main
street is a scene of desolation,” the
Bee reported of Winters). There was also
damage in the capital, Woodland, Chico,
Davis, Napa, and Fairfield, among other
places. A second destructive magnitude
6.4 quake occurred two days later,
followed on April 29 by a 5.6
aftershock.
Unlike Los Angeles and San Francisco,
Sacramento is not known to be underlain
by active faults. However, there are
many known faults all around, and
perhaps some unknown ones, too.
The Foothills Fault System – which
includes faults with names like
Cleveland Hill, Spenceville, Deadman,
Maidu, Prairie Creek and Swain Ravine –
passes just east of Folsom Lake and runs
through locales such as Auburn,
Placerville, El Dorado Hills and Shingle
Springs. The system runs from Mariposa
to the Chico area. The 1975 magnitude
6.1 Oroville earthquake was caused by
movement along the Cleveland Hills
fault.
There is a complex zone of faults on the
western side of the valley, many of
which do not reach the surface. A zone
of blind thrust faults is located along
the western margin of the Great Valley.
A segment of this fault zone may have
produced the 1892 Winters-Vacaville
earthquakes. Faults that are visible at
the surface include the Vaca fault and
the Montezuma Hills fault. Other faults
have been mapped in the eastern Coast
Ranges, but their earthquake potential
is unknown.
To the northwest of Sacramento is the
Dunnigan Hills fault, which has produced
some odd-looking geography just to the
west of Interstate 5. And to the south,
an earthquake estimated at magnitude 6.0
hit the delta in 1889, most heavily
damaging the area around Antioch, Rio
Vista and Collinsville.
There are also faults closer to the Bay
Area thought to be capable of generating
major earthquakes: the Calaveras,
Hayward, Greenville and Concord-Green
Valley. All are within 60 miles of
Sacramento as the crow flies; a large
enough quake on any one of them would
certainly be felt and could potentially
cause damage in the capital.
“The 1989 Loma Prieta earthquake caused
superficial damage in the Resources
Building in downtown Sacramento and
certainly was felt by many residents,”
pointed out Michael Reichle, assistant
director of the Department of
Conservation’s California Geological
Survey. “That was a magnitude 6.9 quake
centered more than 100 miles away, near
Santa Cruz. An earthquake of that size
or larger on the Calaveras or Hayward
fault would almost certainly cause some
damage in Sacramento.”
Sacramento’s skyline was rather modest
in 1989. While 14 years later no one
will confuse it with Manhattan, there
are several more skyscrapers. Tousson
Toppozada, a senior seismologist with
DOC’s California Geological Survey, said
it will be interesting to see how
Sacramento’s tallest buildings respond
the next time there is a magnitude 7 or
larger earthquake centered within 200
miles.
“Obviously, there were no 20-story
buildings in town at the time of the
Winters earthquake, and there were only
a few when Loma Prieta struck,”
Toppozada said. “There was a magnitude
7.0 earthquake 70 miles west of Cape
Mendocino on September 1, 1994. That’s
about 250 miles away from Sacramento,
yet the seismic waves caused a great
deal of motion in the top floors of our
headquarters building (at 8th and K
streets), and some people felt compelled
to exit the building.”
Toppozada explained that tall buildings
-- designed to flex in the event of
earthquakes -- are more likely to be
affected by the waves generated by
large, distant temblors, while shorter
structures are more impacted by closer
quakes.
For more than a century, the foothills
area was considered seismically
inactive. That changed with the 1975
Oroville earthquake. The temblor didn’t
cause much damage outside of the
sparsely populated Oroville area, but it
did have a major impact.
“The scientific community had to
reassess that large area as seismically
active,” Reichle said. “The foundations
for the Auburn Dam were being built at
the time, and for design purposes, we
were asked to estimate how large an
earthquake the system could generate. We
estimated a magnitude 6.5, capable of
displacing the dam’s foundation by about
three-quarters of a foot. That sent the
dam back to the drawing board. The cost
multiplied over time, and the dam was
never built.”
###
|