LAS VEGAS - March 10, 2009 - UNLV researchers have completed the most detailed three-dimensional model to date showing how shallow sediments of the Las Vegas Valley will respond during an earthquake. The model can be used to design safer buildings and highway bridges and is the latest step toward the team's development of a comprehensive seismic hazard map for Las Vegas.
Nevada ranks third in the nation for large-magnitude earthquakes. In Southern Nevada, active faults have been identified that are capable of producing an earthquake of magnitude 6.0 or greater.
Howard R. Hughes College of Engineering Professor Barbara Luke and an interdisciplinary team of UNLV faculty and students performed site surveys throughout the Las Vegas Valley over the last two years to measure profiles of shear-wave velocity. In other words, they were measuring the speed at which certain stresses move through the various sediments that make up the Valley floor.
"The shear-wave velocity model is important because it will tell us how the different parts of the Valley would react to an earthquake," said Luke. "Areas with overall lower shear wave velocity would be expected to undergo more severe shaking at lower frequencies, lasting for a longer time, than those having higher velocity. These differences translate to different requirements for structural design across the Valley."
Shear wave velocity varies according to the type of material (rock, clay, sand, gravel, etc.) and the condition of the material, notably density. It helps describe the "stiffness" of a soil in shear, providing an indication of how the ground will respond during an earthquake. Knowing this, engineers can customize their structural designs for a site.
By combining results of direct field testing with analysis of more than 160 seismic site classifications filed by others with local government agencies, the UNLV research team compiled a database of 230 shear-wave velocity measurements. Using the database, along with interpretations of nearly 1,600 geologic well logs, the team created the shear-wave velocity model for the Valley to depths of hundreds of feet.
The final product for the research team, a seismic hazard map, will be used by local officials in disaster relief planning, land use planning and assessment of existing infrastructure in the event of an earthquake.
"Newcomers to our area are sometimes surprised to learn that the earthquake ground-shaking hazard here is significant," said Luke. "A major goal of our project is to inform local residents of our seismic risk so they can be prepared."
Luke and colleagues Aly Said (engineering) and Wanda Taylor (sciences) were awarded a multi-year grant from the U.S. Department of Energy, titled Earthquakes in Southern Nevada, to in part determine which areas of the basin would be most susceptible should a major earthquake occur.
A more detailed explanation of the project appeared in the November 2008 issue of The Leading Edge, an official publication of the Society of Exploration Geophysicists.
