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SAN FRANCISCO -- If
historical trends are an indication, the
San Francisco Bay Area -- relatively
quiet since the 1989 Loma Prieta
earthquake -- is likely to experience
potentially damaging earthquakes in the
next four to nine years, according to
research conducted by the California
Department of Conservation's Division of
Mines and Geology.
In a paper presented
Wednesday at the Seismological Society
of America meeting, seismologist Tousson
Toppozada, an expert on California
earthquake history, noted that there has
been a distinctive pattern associated
with the four major earthquakes in the
Bay Area since 1800. That pattern
indicates the possibility of increased
activity 15-20 years after the magnitude
7.0 Loma Prieta earthquake.
"We must be clear:
This is not a prediction of a specific
earthquake," Toppozada said. "At the
same time, it is prudent to realize that
we will experience activity in the
magnitude 5.5 to 6 range again sometime
in the near future.
"We really don't know
how long the current quiescence in the
San Francisco Bay Area can last, but we
know it can't last forever. If the past
is any indication of the future, we can
expect some potentially damaging
earthquakes within the decade, possibly
by 2004."
Toppozada led a team
that researched the pre-instrumental
(generally, before 1942) earthquake
history of California using newspaper
accounts, documents from Spanish
missions, letters and diaries. By
analyzing and mapping the effects of an
earthquake as recorded in those
documents, Toppozada and his coworkers
were able to compare the historic
temblors to earthquakes recorded by
modern instruments and determine the
approximate epicenters and magnitudes.
Major earthquakes
occurred in the Bay Area in 1838
(magnitude 7.4 on the San Andreas
fault), in 1868 (magnitude 7.0 on the
Hayward fault) and 1906 (the magnitude
7.8 San Francisco earthquake on the San
Andreas fault) as well as 1989. Each
major earthquake was preceded by a
seismically active period at the
magnitude 5.5 to 6.5 level and was
followed by a period of low seismic
activity. The larger the earthquake, the
more likely it is to relieve regional
stress, leading to longer seismically
quiet periods. There were 18 years of
low seismicity after the 1838 earthquake
and 50 relatively quiet years after the
San Francisco earthquake.
Toppozada points out
that the 1868 Hayward and 1989 Loma
Prieta earthquakes are the most similar
of the four major temblors. They were
about the same size and the activity
preceding them was similar -- 10
magnitude 5.5-6.5 temblors in the 13
years preceding the 1868 event, seven in
the 20 years leading up to Loma Prieta.
The 1868 earthquake
was followed by about 13 to 15
relatively quiet years. There have been
only two magnitude 5.1-5.4 earthquakes
in the greater Bay Area since Loma
Prieta, south of San Juan Bautista in
1998 and in Napa last year. According to
Toppozada, the similarities between the
1868 earthquake and Loma Prieta suggest
an increased likelihood for Bay Area
seismic activity in the magnitude
5.5-6.5 range within the next few years.
Toppozada also pointed out that there
are differences between Loma Prieta and
the 1868 temblor, most notably that they
occurred on different faults.
"The pattern is
evident, but certainly not definitive,"
he said.
"This report is not a
signal to panic, but to prepare," DOC
Director Darryl Young said. "Earthquakes
are simply a fact of life in California,
and the increased likelihood of
potentially damaging earthquakes in
light of this research emphasizes the
importance of preparedness and emergency
response activities in this region."
Young encouraged
local governments to review their
earthquake response plans and the
strengthening of buildings that are
vulnerable to earthquake damage.
In addition to its
program to identify and map seismic
hazards, the Department of Conservation
manages California's earth resources
through its programs that safeguard
farmland and open space; oversee oil,
gas and geothermal wells; ensure mined
land reclamation; study earthquakes; and
promote beverage container recycling.
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