The recent earthquake in the San Francisco area may have shaken northern Californians, but it also sent shock waves toward a lot of other states. Like Utah.
And the quake has even piqued the interest of Congress.In testimony Friday, Arch Johnston, director of the Center for Earthquake Research and Information at Memphis State University, told a joint hearing of U.S. House science subcommittees that earthquake potential in the eastern United States presents only a moderate hazard compared to regions within the Western states of California, Nevada, Utah, Washington and Oregon."
That was nothing new to Utah experts.
They have been looking at data gathered from California's recent killer earthquake - officially named the Loma Prieta quake - to see how similar Utah's circumstances are to those in California.
What they have found is that the Wasatch Front, while it has different kinds of faults, has close parallels with the San Francisco area, seismically speaking.
"The preliminary studies postulate that if we were to have the same level of ground shaking here as what was experienced in California, the Applied Technology Council (a group of structural engineers organized to apply engineering research) estimates the damage would be 20 to 40 percent greater," said Susan Olig, a geologist with the Utah Geological and Mineral Survey.
"That is because of our less stringent building practices."
Olig and Tony Popish, a geophysicist with the state Division of Comprehensive Emergency Management, recently attended an annual conference of the Western States Seismic Policy Council in Boise, and on hand were the experts from northern California who dealt directly with the Loma Prieta quake and its aftermath.
"We got to hear what happened from their standpoint, and a lot that information is directly applicable to Utah," Olig said.
Of particular interest is the fact that California had a lot of sensors in the area of the quake that plotted the actual ground shaking patterns. Preliminary results seem to indicate isolated pockets of damage related to whether the buildings were built on soft soils or on bedrock.
A good example of that can be seen at Yerba Buena and Treasure islands in the San Francisco Bay. Yerba Buena, a bedrock island in the middle of the bay, experienced a ground acceleration of 6 percent G (acceleration due to gravity).
Neighboring Treasure Island, which was created with soft artificial fill as a site for the Golden Gate Exposition of 1939, had peak ground acceleration 10 percent greater.
Another observation was that some pockets of damage were related to liquefaction, a situation where water-saturated fine-grained sediments actually liquefy and act like fluids when shaken. That causes extreme damage to building foundations, as was the case for many buildings in the Marina District of San Francisco.
"We have the same soil types in big portions of the Wasatch Front," Olig said. "There are a lot of old lake beds here with a high water table, particularly in the center of the valley."
Based on California's experience, Utah would probably see tremendous damage to homes and businesses built on the soft sediments of old lake beds.
Olig also noted that older structures in California built before the more stringent earthquake resistant building codes did not fare as well in the quake. But structures built to the newer code generally performed well.
Utah, which has less stringent building codes that the San Francisco area, would likely see heavy damage to buildings not meeting earthquake standards. Utah did not adopt the tougher national Uniform Building Code until July 1987 (though some local jurisdictions adopted the recommendationas earlier.
The code is an outline by a national conference of structural engineers and building officials. Each state is at liberty to reject or adopt the recommended code, which, among other things, looks at the earthquake potential and damage potential of each area and then rates each area on a scale of one to four.
Different zones are related to how much peak horizontal ground acceleration is expected to occur. Structural engineers use this information to determine the appropriate level of the earthquake resistant design of buildings. The higher the zone, the more resistant the building should be to structural damage from ground shaking.
The San Francisco area has a "four" rating, while the Wasatch Front has a "three" rating. Many Utah experts feel the Wasatch Front should carry the same "four" rating as northern California.
Popish noted that the heaviest damage to California buildings was to those built of unreinforced masonry - the same type of buildings Salt Lake City has many of in its downtown area.
Popish also pointed to what would be Utah's most basic problem in dealing with earthquakes, "We don't have the resources of California to respond once it happens. For example, we don't have the large number of police officers to secure large areas."
Popish also added that Utah should do more to retrofit buildings to meet the tougher building codes.
While Utah earthquake experts can learn a lot from what happened in California, they say they need more information about how Utah geology would react in a quake. "We have different faults and different geology," Olig said.
"There are applications we can learn from California, but there are still a lot of unanswered questions. We still don't know how much of that information from California is directly applicable to Utah."
Experts have been working with Sen. Craig Peterson, R-Orem, to draft legislation to add an earthquake instrumentation program to the University of Utah seismograph station.