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SACRAMENTO – The San
Ramon area is experiencing a swarm of
minor earthquakes that while uncommon in
most of California is not unprecedented
in that area.
Between 6:54 Sunday morning and noon
Wednesday, 106 earthquakes had taken
place in the San Ramon area. Seven of
those were greater than magnitude 3 –
not likely to cause damage, but strong
enough to be felt. The largest was
magnitude 3.9.
Previous swarms near the Calaveras fault
in Contra Costa County have not heralded
a larger, damaging earthquake, and
scientists at the Department of
Conservation’s California Geological
Survey are hopeful that the current
phenomenon will follow that historic
trend.
“We can’t be 100 percent certain that
there will not be a larger event, but
that would not be consistent with what
has been recorded in the past,” State
Geologist Jim Davis said. “Most likely,
the swarm will persist for a few weeks,
as has been the case in the past. It
wouldn’t be surprising to see an event
as large as magnitude 4.5. We are
keeping an eye on what’s going on and
reporting the scientific data to the
Office of Emergency Services and
officials in the San Ramon area.”
Earthquakes generally occur in a series:
foreshocks, main shocks and aftershocks.
In a swarm, there are a series of
perhaps hundreds of earthquakes,
typically minor and generally of a
similar size, without an outstanding
main shock or largest event.
“Swarms don’t follow the regular pattern
of a main shock followed by a series of
aftershocks that decrease in magnitude
and frequency,” said Chris Wills, a
Department of Conservation supervising
geologist. “Typically, if there’s a
magnitude 3.5 earthquake, it’s followed
by something in the 2.5 range, and then
aftershocks of decreasing magnitude. In
the swarm we’re seeing now, there have
been several magnitude 3.5 to 3.9 events
with a number of magnitude 2.5 to 3.5
quakes. Although swarms do decrease in
activity over time, it is common to have
several earthquakes of similar size over
several days to weeks”
The swarm currently shaking the San
Ramon area was at first thought to be
associated with the Calaveras fault.
However, analysis shows that it actually
is occurring on a two-mile long fault
that is perpendicular to the Calaveras.
This fault has not been mapped at the
Earth’s surface, and it is possible it
is entirely a subsurface fault.
Earthquake swarms
occurred in the nearby Danville area in
1970 and 1976. The 1970 swarm included
353 earthquakes from May 22-June 19, the
largest being magnitude 4.3 and 4.2
temblors. In 1990, the Alamo area, 10
miles north of San Ramon, experienced
minor earthquakes over a six-week
period, including a magnitude 4.6
temblor and 13 quakes greater than
magnitude 3.0.
“There have been a number of swarms in
recent years in this area,” Wills said.
“There really isn’t a good model for
what’s happening here, but it seems as
though there’s a series of small faults
near the end of a major one (the
Calaveras) at work. These faults seem to
abut the Calaveras fault in an area
where the activity on the Calaveras is
decreasing from south to north. It’s a
fascinating geological area.”
However, the stress released by these
minor earthquakes is insignificant
compared to larger events that can occur
on faults in the bay area. According to
a 1999 study of earthquake probabilities
in the San Francisco Bay Area, there is
a 70 percent likelihood of a magnitude
6.7 or greater earthquake between 2000
and 2030. The probability for the
Calaveras fault producing such a temblor
was placed at 18 percent.
“Historic records tell us that we should
expect earthquakes in the magnitude 5.5
range every decade or so in the Bay
Area,” Department of Conservation
seismologist Tousson Toppozada said.
“There hasn’t been anything of that size
in the Bay Area since the magnitude 6.9
Loma Prieta earthquake in 1989.”
The northern section of the Calaveras
fault ruptured in a magnitude 5.5-6
earthquake in 1861. Surface cracking of
1.5 miles occurred, and there were
reports of minor damage in what was then
a sparsely populated area. That
earthquake reportedly was felt as far
away as Sacramento, Petaluma and Santa
Cruz. That 1861 earthquake was not part
of an earthquake swarm. Slightly larger
earthquakes have occurred on the
Calaveras fault further to the
southeast. The most recent was a
magnitude 6.2 earthquake in 1983 that
caused $8 million in damage in a
sparsely populated area of Morgan Hill.
The Department of Conservation’s
California Geological Survey studies
earthquakes to help Californians plan
and build earthquake-resistant
communities. CGS maintains a network of
strong ground motion seismographs that
combined with instruments maintained by
Caltech, UC Berkeley and the USGS forms
the California Integrated Seismic
Network (CISN). This newly expanded,
combined network allows scientists to
study the ground motions from
earthquakes, leading to advances that
result in more earthquake-resistant
structure.
CGS also studies the distribution of
historic earthquakes and evaluates
faults, combining that information to
prepare maps showing the potential for
ground shaking, fault rupture,
liquefaction and seismically induced
landsliding.
On December 20, CGS will release the
newest Seismic Hazard Zone maps for
Alameda County. These maps impact the
planning community, developers, property
sellers and real estate agents. If
property is located in a zone of
required investigation, where
liquefaction or landslides could occur
during a large earthquake, the local
building department must require
geologic studies before projects are
permitted. Also, property sellers and
real estate agents must inform potential
buyers if property they're selling is in
a zone, as is the case when property is
in a designated flood or wildfire zone.
In Contra Costa County, Alquist-Priolo
Earthquake Fault Zones were established
by CGS along the Hayward, Calaveras and
Concord faults in the 1970’s, and
revised in the last two decades. These
maps define zones to ensure that
structures for human habitation are set
back from known faults. Because of this
zoning, much of the development in the
San Ramon area has included detailed
studies so that buildings are not built
directly over the Calaveras fault. This
will greatly reduce the damage from a
major ground-rupturing earthquake.
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