If the earthquake expected along the Wasatch Front waits long enough, we'll be ready for it.
Deputy State Geologist Bill Lund said if we can go 50, 60 or 70 years before we have a big earthquake, most buildings will have been built or remodeled to stand up to the stress.Utah's Uniform Building Code is written by engineers and the International Conference of Building Officials. Chapter 23 of the code outlines requirements for building in Seismic Zone 3.
Seismic Zone 3 includes all the Wasatch Front, but its boundaries are political rather than geological. For example, the northern border of Seismic Zone 3 is the Idaho border. The Intermountain Seismic Belt outlined by geologists runs from northern Arizona, all the way through the middle of Utah and extends as far north as Canada.
New construction and vacant older buildings in Utah must meet the code before the buildings can be occupied. However, existing occupied structures are not required to be brought up to the new seismic code.
Earthquakes can produce two primary effects - ground rupture and seismic waves. Ground rupture occurs most often along the fault. The Wasatch Fault is a dip-slip normal fault so the ground ruptures as one block moves up and another block moves down.
A seismic wave moves out from the epicenter of the earthquake, creating vibrations that put lateral stress on structures.
The building code outlines the type of stress a building must withstand from seismic waves. Salt Lake County Geologist Craig Nelson said the code specifies how much lateral acceleration a building must be able to take.
The Utah building code does not address ground rupture. Nelson said the code does not prohibit building across a fault, but Salt Lake County approved a Natural Hazards Ordinance in May 1989 that prohibits such building sites.
Utah County does not have an ordinance that specifically prohibits building on a fault, said Al Carlson, plans examiner for Provo City. "We have people whose houses were built in the '70s across the fault."
Provo now has a Sensitive Land Ordinance that applies primarily to hillside lots, he said. The ordinance requires an inspection by a geotechnical engineer before a building permit can be issued.
A geothechnical engineer is an expert at identifying geological hazards at a building site.
California's building code specifies that building must be at least 50 feet away from an active fault, Nelson said. Salt Lake City does not have a specific setback requirement.
The California setback requirement would not meet Utah's needs, said Nelson, because the California fault's movement is horizontal and the Wasatch Fault's movement is vertical. The area of deformation for lateral movement is not expected to exceed 50 feet.
The area of deformation on a vertical fault varies widely, said Nelson and each building site in Salt Lake County is evaluated separately. Some places in Salt Lake City have a required setback of only 25 feet and other areas have a setback requirement of 1,200 feet.
Building for Seismic Code 3 requires that a structure take about .18 G, said Nelson. One G is the gravity of earth. Less than .10 G is the force of an earthquake with little or no damage. Earthquakes of force .10 to .20 G cause moderate damage and earthquakes above .20 G cause major damage.
California is in Seismic Code 5 with a stricter building code for structures able to withstand greater stress. Nelson said in 1988 there was some lobbying to upgrade seismic requirements in Utah.
The Wasatch Front is in danger of experiencing a .20 G force earthquake and greater, so the current building code may not be adequate.
Building to the seismic code does not guarantee that a building will not sustain damage, Lund said, but building to code should mean that the building will maintain enough integrity to protect human life.