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NR 2006- 07
March 29, 2006

Contact: Ed Wilson
Mark Oldfield
Don Drysdale
(916) 323-1886

EDITORS: The 100th anniversary of the Great San Francisco Earthquake is April 18. The California Geological Survey (CGS) is regarded as the primary source of geological and seismological products and services for decision-making by California's government agencies, its businesses and the public. The following is a feature about the state’s earthquake history. It is the second of several releases related to California seismology and CGS’ work we will send in the weeks prior to the centennial anniversary.

Information about CGS’ earthquake-related programs, as well as background that may be helpful in your coverage of the 1906 earthquake anniversary, is available on the CGS Web site at: http://www.consrv.ca.gov/CGS/index.htm.

For more information, or to arrange interviews with CGS scientists about earthquake-related stories, please call (916) 323-1886.

LIKE THE REST OF THE STATE, SACRAMENTO MUST BE PREPARED

SACRAMENTO – Before the 54-story Towers on Capitol Mall soar into Sacramento’s skyline, pilings will be driven deep into the ground to ensure the buildings’ earthquake stability. Like many of Sacramento’s structures, these $500 million, 615-foot behemoths will need to be built to withstand the effects of a strong earthquake along any large faults near the capital city.

While Sacramento doesn’t face the same level of earthquake threat as the San Francisco Bay Area or the greater Los Angeles metropolitan area, it has felt the effects of earthquakes past. How Sacramento, with its relatively new big-city skyline, will fare in the next Northern California temblor is unknown. In fact, structural engineers sometimes refer to the “Sacramento problem” when discussing how tall buildings will respond in earthquakes.

“Sacramento doesn’t have any known faults running under it,” said State Geologist Dr. John Parrish, who heads the California Geological Survey (CGS), a branch of the Department of Conservation. “However, there are major faults near enough that give us a couple of significant earthquake concerns. One concern is the potential impact of liquefaction, particularly on the levees that surround much of the community. The second is the performance of high-rise buildings during strong ground shaking.”

Liquefaction occurs when the grains of poorly consolidated, sandy soils are shaken apart and the ground water is squeezed into the open spaces between the grains. That forms a substance much like quicksand that no longer has the strength to support buildings or other heavy structures. Liquefied soil can allow the ground to crack and move, resulting in damage to structures such as buildings and levees, buried pipelines and utilities.

"We're in an area where much of the soil is sedimentary river deposits, and there's a high water table because we're near the confluence of two rivers,” said CGS Supervising Engineering Geologist Charles Real. “Those are two of the major ingredients that create liquefaction.

“The only missing ingredient for liquefaction here is earthquake shaking, and that's why mapping Sacramento for the hazard of liquefaction hasn't been as high a priority as mapping other areas. The likelihood of damaging earthquake shaking here is not as high as it is for the Bay Area, but Sacramento has experienced shaking in the past. If San Francisco had another large earthquake, something of 1906 proportions, we can't say with certainty what that would do to Sacramento. There's a lot of development pushing up against our levees, and there are concerns about how well many of those old levees would hold up in a large earthquake."

Real heads the Seismic Hazards Zonation Program for CGS. The program has produced more than 100 regulatory maps covering the Bay Area and Southern California. The maps show zones where there’s a relatively higher potential for liquefaction and landslides during large earthquakes.

“We know that liquefaction can occur at a considerable distance from an earthquake's epicenter; look at what happened in San Francisco’s Marina District during the 1989 Loma Prieta earthquake,” Real said. "Earthquakes produce waves of energy. We’ve learned that although the rapid violent vibrations die out as you move farther away from an earthquake's epicenter, the lower frequency seismic waves travel farther and can affect high-rise structures. Although less violent, shaking from long period waves lasts longer and can still cause sandy saturated soils to liquefy.

“Certainly the 1906 San Francisco earthquake was felt in Sacramento, and the 1892 Winters earthquake, which was estimated to be magnitude 6.6, damaged the Capitol and other buildings here. There hasn't been a large earthquake centered near Sacramento since the construction of high-rise buildings. So the question of how our tall buildings, as well as our levees, would react is an important one.”

As the Supervising Geologist for CGS’ Strong Motion Instrumentation Program (SMIP), Dr. Anthony Shakal works closely with the engineering community in the development of building codes and construction practices. The nearly 1,000 recording stations that SMIP has placed around the state – including a handful in Sacramento at the Capitol and Cal-EPA building -- record how structures such as buildings, bridges and dams respond to ground shaking.

“In structural engineering, they refer to the ‘Sacramento problem,’ ” Shakal said. “What happens when you combine tall buildings, deep valley sedimentary soil, and a large earthquake at a distance? For high-rises here in Sacramento, the earthquake underfoot isn’t as much a concern as the earthquake in the next county or two counties away because of those long seismic waves.”

Shakal pointed out that during the 1989 Loma Prieta earthquake, centered more than 100 miles away from Sacramento just north of Santa Cruz, Department of Conservation personnel located in the Resources Building in downtown Sacramento felt significant shaking. In 1992, DOC employees on the upper floors of the 28-story Renaissance Tower (now known as 801 K Street) reported noticeable swaying after a magnitude 7.2 earthquake over 200 hundred miles away offshore of Humboldt County.

Several faults thought capable of producing large earthquakes are within 60 miles of Sacramento: the Calaveras, Hayward, Greenville, Concord-Green Valley and the Foothills system.

The giant Towers on Capitol Mall are being built under the Uniform Building Code. The code addresses a building’s response to seismic shaking, and varies depending on the building’s height. Strong motion recordings, like those made by Shakal’s group, directly establish the formulas used in design for a building’s period – the amount of time in takes to sway back and forth in its natural condition.

“We can estimate what the building’s motion will be before the first piles are driven,” Shakal said. “If you’re in a house in Sacramento, you might not feel the motion from an earthquake on the Calaveras or Hayward fault. But if you were way up in a skyscraper, you’d almost certainly feel it.”


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