The September 3, 2000 Yountville Earthquake
A magnitude (M) 5.2 earthquake occurred at 1:36 AM on September
3, 2000 in the mountains 10 miles northwest of the town of Napa,
near Yountville, CA. Although seismologists initially thought that
the earthquake occurred on the West Napa fault, accurate locations
now indicate that the earthquake occurred about 3 miles west of
this fault.
Earthquakes of this size can occur anywhere throughout the Bay
Area on deep faults that cannot be observed through geologic mapping
of the Earth's surface. For example, a M 5.7 earthquake occurred
on March 3, 1986 in the East Bay hills, about 10 miles northeast
of San Jose, near Mt Lewis. This earthquake generated nearly 2000
aftershocks over a period of a year and a half, yet geologists have
been unable to identify any fault at the surface that can be related
to these earthquakes at depth.
The Yountville quake occurred on a northwest oriented fault, and
rupture occurred through right-lateral strike-slip motion (Yountville focal mechanism),typical of events in
central California. As of 10AM 09/11/2000 PDT, there have been only
18 aftershocks within 5 miles (8 km) of the epicenter, ranging in
magnitude from 1.2 to 2.6. It is unlikely that any of these aftershocks
have been felt.
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| Yellow dots indicate historical seismicity, from 1970 - Aug.
31, 2000. Red dots indicdate seismicity from Sept. 1 - Sept.
8, 2000, including the Sept. 3rd M 5.2 mainshock. |
Strong motion instruments recorded unusually high levels of shaking
in the city of Napa as a result of the September 3rd
Magnitude-5.2 earthquake near Yountville, California. Recordings
of strong shaking by stations operated throughout the region by
the U.S. Geological Survey (USGS), the California Department of
Conservation's Division of Mines and Geology (CDMG), and UC Berkeley
demonstrate that peak shaking levels in the city of Napa were amplified
five to eight times relative to a station located in the mountains
less than a mile from the earthquake epicenter (example of accelerograms). Both the high levels and
local amplification help explain the surprising concentration of
earthquake damage throughout the city, according to scientists from
the three agencies.
The highest recorded level of shaking came from an instrument located
in a fire station north of the city of Napa, approximately six miles
from the epicenter. The shaking there was 50 percent of the force
of gravity. This means that buildings at the site were subjected
to a horizontal force that was 50 percent of the building’s weight.
A station located south of town, near Napa College, recorded a peak
shaking level nearly as high. Both recordings are substantially
higher than expected for a magnitude 5.2 earthquake and are consistent
with the significant damage that the city suffered.
While earthquake shaking levels depend on the distance from the
earthquake source, the high level of ground shaking in Napa also
appears to be the result of two other factors: first, the amplification
of shaking by young sediments along the Napa River (map
of Quaternary surface deposits), and second, the focusing of
strong motion to the southeast, the direction the earthquake rupture
propagated. A regional map of shaking levels (ShakeMap)
can be viewed on the web. The offset of the strongest shaking to
the southeast from the epicenter, and the amplification within the
basin of sediments underlying Napa and along the northern shore
of San Pablo Bay are also clear and can best be viewed in the web
map of instrumental intensity.
The M5.2 event is the largest earthquake in this area since 1969
when two earthquakes, magnitude 5.6 and 5.7, struck Santa Rosa about
80 minutes apart. These earthquakes are associated with the Rodgers
Creek fault system (historical
seismicity)
In fact, these 3 events are the only earthquakes with magnitude
greater than 5.0 to hit the region 50-60 mi north of San Francisco
Bay since 1906. However, in the past year there have been several
magnitude 4 or greater events in this same region
1) A magnitude 4.7 event near Bolinas on 8/17/99 (ShakeMap)
2) A magnitude 4.3 event south of Santa Rosa on 9/22/99 (ShakeMap)
3) A series of 3 earthquakes, magnitudes 4.0, 4.2, 4.0 near Cloverdale
between January 10-18, 2000 (ShakeMaps for M4.0,
M4.2,
M4.0)
It is not clear whether the increase in seismicity in this region
reflects the end of a long period of seismic quiescence. If so,
then this would indicate that the levels of stress are perhaps sufficiently
high that larger quakes could be expected soon for the region. A
similar seismicity pattern was observed in the 50-year period preceding
the 1906 earthquake (seismicity time line).
However, there is too little data available on which to alter the
formal probabilities for the seismic hazard in this region (Earthquake Probability Study for the San Francisco
Region). Clearly, had such an argument been put forth after
the two earthquakes that rocked Santa Rosa in 1969, the subsequent
30 years of earthquake activity in this region would have proven
the argument to be flawed.
This earthquake injured 25 people, including 2 people critically.
70 people sought shelter at Red Cross facilities. Damage estimates
are rising and range from $10 to $50M. On Wed, Sept 6, Governor
Davis declared a state of emergency in the Napa Valley. A request
has been made for federal funds. As of September 8, the City of
Napa building inspectors have issued 168 "yellow tags"
and 16 "red tags".
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