Brief Report on the September 3, 2000 Yountville/Napa, California Earthquake
By Eduardo Miranda and Hesam Aslani, John A. Blume Earthquake Engineering Center, Stanford University
The earthquake occurred at 01:36 AM local time (PDT) with a local magnitude (ML) of 5.2. Its epicenter was located by the United States Geological Survey (USGS) at 38.3770 N, 122.4137 W as 3 miles (5 km) WSW of Yountville, CA and 9 miles (14 km) NW of Napa on some small hills west of the Napa Valley.
The moment magnitude (Mw) of the event was 5.0 according to UC Berkeley's Seismological Laboratory. According to the USGS the earthquake occurred on an unmapped fault just west of the West Napa Fault although initial reports located the event erroneously on the West Napa fault. Its focal mechanism was that of a typical right-lateral strike-slip on a northwest striking fault at a depth of 9.4 km. It was felt throughout the northern Bay Area. The quake produced a few small aftershocks, the largest of which reached a magnitude of 2.4.
The largest ground acceleration was recorded in Station 1765 of the USGS located in the Fire Station No. 3 where a Kinemetrics Etna digital accelerograph recorded horizontal accelerations of 0.42g and 0.48g. Another instrument of the California Strong Motion Instrumentation Program (CSMIP) Station No. 68150 at the Napa College located at the south of Napa also recorded a very high horizontal acceleration. This station which was only recently upgraded with CalEMA/FEMA support as a TriNet North/CISN station was approximately 17 km from the epicenter and recorded a peak ground horizontal acceleration of 0.337g. Acceleration time histories recorded at this station downloaded from the CSMIP site are shown in Fig 1. It can be seen that the maximum acceleration in channel 3 is not associated with a high frequency spike. Maximum ground velocities and ground displacement in this component are 25.1 cm/s (10 in/s) and 2.1 cm (0.82 in), respectively.The closest station to the epicenter was a station from the UC Berkeley Seismological Laboratory located in the Camernet Winery on tuff, 5 km (3 miles) NW of the epicenter. The maximum acceleration at this site was 0.063g. This suggests a possible directivity effect towards the south of the fault (towards the city of Napa).
See Figure 1: Ground motion recorded at Napa College - CSMIP station.
The soil conditions in most of the valley correspond to alluvial deposits. Response spectra computed from ground motions recorded from CSMIP are shown in Fig. 2. It can be seen that the maximum absolute acceleration spectral ordinate for 5% damping is approximately 1.0g for a period of about 0.4s. This acceleration ordinate at 0.4 s corresponds to approximately a spectral displacement of 4.0 cm (1.57 in).
Peak ground accelerations recorded at these stations are larger than those computed from most recent attenuation relationships developed for California (Seismological Research Letters, Vol. 69, No. 1, Jan/Feb 1997). For a vertical strike-slip fault, M=5.2 and a soil site at 10 km (6 miles) from the rupture, attenuation relations by Abrahamson and Silva 1997, by Campbell 1997, by Boore and others 1997, by Sadigh and others 1997, by Spudich and others 1997, predict median ground accelerations smaller than 0.12g. Thus, even considering a coefficient of variation of about 0.6, ground accelerations recorded at the Napa Valley CSMIP station and at the Napa Fire Station USGS station are significantly higher than most of those recorded in other California earthquakes under similar conditions.
For a more detailed description of the ground motions and for background on the seismicity of the region it is recommended to see the University of California, Berkeley, Seismological Laboratory.
The largest city within the epicentral region is the City of Napa which has a population of about 70,000. The area is well known for grape growing and wineries. The first recorded exploration into Napa Valley was led by Padre Josť Altamira, founder of the mission at Sonoma, in 1823. Spanish and Mexican control remained until the Bear Flag Revolution, and the valley became one of the first in California to be settled by American farmers, who started arriving in the 1830s. The City of Napa was laid out in 1848 by Nathan Coombs on property he had received from Nicolas Higuerra, holder of the original Spanish Grant. The first business establishment was opened in the new city in 1849. (http://www.cityofnapa.org/).
Damage to historic buildings
The oldest building standing today in Napa was built in 1901 and houses the historic architectural records of the city. The building was closed so we could not learn from the city historian what was the performance of this structure during the 1906 earthquake. This is a two-story, renovated, stone-masonry building which experienced some cracks as a result of this earthquake above the windows of the second floor and the fall of one stone in the main facade. Fortunately the earthquake occurred at 1:30 in the morning and nobody was standing next to the building. A street lamp in front of the building shifted from its vertical position.
The oldest commercial building in Napa is the Pfeiffer building which was built in 1875 (building plate shown in Fig. 3). Located on Main street this is a two story stone masonry structure. It has a 2 ft. unreinforced stone masonry parapet above the roof level with minimal bracing. This parapet experienced only minimal diagonal tension cracking along the mortar (Fig. 4).
Another pre-1906 historical building in downtown Napa is located on 1st street. This building was built in 1901 (Fig. 5) and today it houses Napa's Historical Society. The building was closed so it was not possible to learn from the city historian what was the performance of this structure during the 1906 earthquake. This is a two-story, renovated, stone-masonry building which experienced some cracks as a result of this earthquake above the windows of the second floor and the fall of one stone in the main facade (Fig. 6). Fortunately, the earthquake occurred at 1:30 in the morning and nobody was standing next to the building. A street lamp in front of the building shifted from its vertical position (Fig. 7).
The opera house, located on Main street, is an unreinforced masonry building (URM) with a facade dated 1857. The rest of the building is probably newer than the date on the facade. This structure was being renovated at the time of the earthquake. It is likely that the large wooden diagonal bracing in the interior of the building played a role in preventing serious damage to the building. The side wall is approximately 25 m (80 ft) long by 8 m (26 ft.) tall of exposed URM. It only experienced some diagonal tension cracking in the upper left corner near the facade and the parapet (Fig. 8).
Undamaged retrofitted building in downtown Napa. This building is located on Main street next to the Opera house. Building appears to have been recently renovated with tubular steel bracing.
Detail of undamaged two-story URM. One other old URM along main street also had diagonal tension cracking, however other URM buildings appeared undamaged. One would have expected more damage to some of these URM buildings had the motions been more severe.
Damage to other buildings in downtown Napa
Downtown Napa (Fig. 9) has several blocks of commercial buildings. In general, the damage was relatively small. Most of these buildings were undamaged (Fig. 10). However, several commercial buildings suffered small cracks in their masonry facades (Fig. 11) and several had broken windows. The post office had approximately 14 broken windows along its main facade. (Fig. 12). Some of these broken windows could have resulted in serious injuries to people inside the post office if the earthquake would have occurred at an hour where the building was in service.
Damage to a two-story office building in downtown Napa.
Detail of the damage to the two-story office building in downtown Napa.
East of the post office there are several churches, some of them with relatively slender towers. No damage to these was visible from the exterior. The cinema was also undamaged (Fig. 13). An URM building located on Main street next to a parking lot had some damage near two steel beams that were probably installed to increase the size of the openings in the facade (Fig. 14). The tourist office is located in a new shopping mall on 1st street. The only visible sign of the occurrence of the earthquake in this building was a metallic ring separating a fire sprinkler head from the acoustic ceiling that fell during the earthquake (Fig. 15). A Christmas-tree-like decoration with wine bottles inside this tourist office was intact after the earthquake, which suggests that the motion was not very strong in downtown Napa (Fig. 16). However, other commercial buildings in downtown Napa (liquor stores, hardware stores) did suffer damage to contents that fell off the shelves where they were placed.
Near downtown there are several blocks of old residential construction. Most houses in this area are at least 60 years old and many of them have relatively tall crawl spaces (2 ft or more) on cripple walls similar to those that have been shown to be very vulnerable in previous earthquakes (Fig. 17). Nevertheless, most of them suffered no damage that was visible from the exterior. The most common type of damage to houses in this area was to URM chimneys (Fig. 18). However, damage to chimneys was by no means widespread. As a matter of fact several old and slender URM chimneys did not suffer visible damage.
The Napa College where the CSMIP station is located is approximately 1 mile south of downtown. Most buildings on campus are one-story reinforced concrete buildings with concrete walls and a precast concrete double T roof system. We found no damage on campus. We visited the bookstore on campus which besides textbooks has slender racks of CD's, calculators, pens, etc. on display. On one side of the store there are slender racks with fast food and some vertical refrigerators with window doors that store cold drinks. The store has a conventional, fairly large ceiling (about 25 x 60 ft) consisting of rectangular gypsum panels supported on hanging inverted T aluminum shapes.
None of the ceiling panels moved nor the hanging fluorescent light fixtures. Only a few textbooks fell off the shelves and one plastic bottle of Calistoga mineral water fell off a shelf in the storage room of the store. Several in-store, slender display racks, normally prone to overturning, provide evidence that the motion was not very severe on campus. In the Napa College we also visited the kitchen. No damage to ceiling, plates, glassware or of any other type was reported.
South of the city, near highway 29 and Imola Avenue, there are several two-story apartments that suffered some damage as a result of the earthquake. Damage consisted of fallen roof tiles (Fig. 19), cracking of stucco walls (Fig. 20) and Fig. 21), diagonal cracking in masonry walls produced by settlement, and some separation of chimneys. In the inside extensive damage to contents was reported by upstairs neighbors. Also some damage occurred to carport roof structures made of slender masonry columns supporting a wood roof (Fig. 22). The carports had been yellow tagged by the city.
Houses in the west of Napa consist of primarily newer and larger houses. No damage was visible in this area.
Yountville and other areas
No damage to wineries along highway 29 was visible from the outside. Closer to the epicenter is Yountville, a much smaller city than Napa. This city was founded by, George Calvert Yount in 1835, however current construction, in general, is much newer than in Napa. Other than damage to some chimneys we saw no other damage. The Domaine Chandon winery (sparkling wine) is located on the east side of highway 29. This is the winery located closest to the epicenter. It reported no damage. Not even broken bottles of sparkling wine. This is another indication that the motion was not very strong in this area because as part of the manufacturing process, the sparkling wine is stored in bottles placed in relatively slender steel racks where the bottles are manually rotated.
Near the Domaine Chandon winery and even closer to the epicenter, there is a relatively large veteran housing facility for retired military personnel. At this location there is also a medium-size hospital. No damage was observed from the exterior to any of the buildings.
This earthquake injured 25 people, including 2 people critically. 70 people sought shelter at Red Cross facilities. Damage estimates continue to rise slightly and range from $30 to $50M. On Wednesday, September 6, Governor Davis declared a state of emergency in the Napa Valley. This proclamation allowed local governments to seek reimbursement from the state under the Natural Disaster Assistance Act and allowed the state to apply for U.S. Small Business Administration loans. Governor Davis then requested President Clinton to provide further federal assistance for homeowners and businesses. As of September 8, the City of Napa building inspectors have issued 168 "yellow tags" and 16 "red tags" to structures affected by the earthquake. On September 14, President Clinton issued a major disaster declaration for California which made federal disaster aid available for earthquake victims in the Napa County.
Both empirical attenuation relationships from previous California earthquakes as well as the level of observed and unobserved damage suggest ground motions experienced by most structures in the City of Napa had relatively small intensity levels. However, ground motions recorded at the CSMIP station located in Napa College or those recorded at USGS station location at the Napa Fire Station No. 3 have a relatively high intensity. Many of the structures we visited, in particular URM masonry buildings with unbraced parapets in their facades and old wooden houses on tall crawl spaces supported by cripple walls, would have suffered more damage in our opinion if ground motions at these locations corresponded to spectral displacements of 4 cm or spectral accelerations near 1g. Thus, this earthquake should not be interpreted as an indication of adequate behavior of these types of constructions under 0.34g or 0.48g ground motions. On the contrary this earthquake should serve as a wakeup call for owners of these types of construction to undergo at least a small level of retrofitting of their constructions. In particular bracing and anchoring of URM masonry walls and parapets as well as lateral bracing and anchoring of cripple walls is needed.
The Rodgers Creek fault near the Napa valley is a significant seismic hazard in the North Bay. According to the 1999 report by the USGS, the Rodgers Creek-Hayward Fault system has a 32% probability of experiencing one magnitude 6.7 or greater earthquake in the next 30 years.
The whole economy of this region strongly depends on wine sales and on tourism to the wineries, both of which depend on the adequate performance of the wineries along the Napa Valley in future earthquakes. Hence, a careful evaluation of winery structures and their contents, in particular storage racks (See earlier tests done at the Pacific Earthquake Engineering Research (PEER) Center at the University of California, Berkeley) and storage tanks, is also recommended.
PEER researcher and Stanford University Professor Eduardo Miranda and Hesam Aslani visited the Yountville Area following the September 3, 2000 earthquake and prepared the report published here for the Pacific Earthquake Engineering Research (PEER) Center at Berkeley.
- Abrahamson, N. A. and Silva, W. J., Empirical response spectral attenuation relations for shallow crustal earthquakes, Seismological Research Letters, 68, 1, Jan.-Feb. 1997, pages 94-127
- Campbell, K. W., Empirical near-source attenuation relationships for horizontal and vertical components of peak ground acceleration, peak ground velocity, and pseudo-absolute acceleration response spectra, Seismological Research Letters, 68, 1, Jan.-Feb. 1997, pages 154-179
- Boore, D. M., Joyner, W. B. and Fumal, T. E., Equations for estimating horizontal response spectra and peak acceleration from western North American earthquakes: a summary of recent work, Seismological Research Letters, 68, 1, Jan.-Feb. 1997, pages 128-153
- Sadigh, K. et al., Attenuation relationships for shallow crustal earthquakes based on California strong motion data, Seismological Research Letters, 68, 1, Jan.-Feb. 1997, pages 180-189
- Spudich, P. et al., SEA96 -- a new predictive relation for earthquake ground motions in extensional tectonic regimes, Seismological Research Letters, 68, 1, Jan.-Feb. 1997, pages 190-198
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