Posted: Thursday, May 3, 2007 12:00 am
OSU News Service and Gazette-Times
An aerial-mapping technology is allowing geologists to pinpoint the location of "surface ruptures" - the lines where an underlying fault breaks through the Earth's surface - a development that an Oregon State University scientist says has implications for building codes throughout the western United States.
The technology, LiDAR (Light Detection and Ranging systems), reveals what landforms would look like if buildings and vegetation were stripped away. Robert Yeats, a professor emeritus of geosciences at OSU and an earthquake expert, said the results could help boost public awareness of the issue and help spark efforts to modernize building codes.
Surface ruptures can send part of a building surging up or sideways, literally ripping a structure in two and causing massive destruction or loss of life. Earthquake codes in the Pacific Northwest don't take surface rupturing into account, Yeats said.
"Most earthquake codes in Washington and Oregon consider and prepare for the damage that earthquakes can cause from ground motion or soil liquefaction," Yeats said. "In many areas, that's adequate. But the discovery of a large earthquake about a thousand years ago on the Seattle fault, which extends beneath downtown Seattle, changed all that."
"Now, our recent LiDAR studies have made it clear that surface faulting has been common in many parts of Puget Sound and other places in the Pacific Northwest," he said. "We have no codes that take this into account, no real awareness of the potential problems. We are building structures, including a wastewater treatment plant north of Seattle, which literally straddle these fault lines and could be completely destroyed or heavily damaged by ground ruptures during a future earthquake."
Yeats will outline these problems this weekend in a presentation at a regional meeting of the Geological Society of America at Western Washington University in Bellingham.
Yeats says that he sees this as the third major step in the public and policy approach to earthquakes in the Pacific Northwest - a region once thought to be largely devoid of major earthquake potential. The first step, begun in the 1980s and led by the U.S. Geological Survey, was the realization that the entire area is vulnerable to massive Cascadia Subduction Zone earthquakes along a line from northern California to British Columbia.
"The second step we took during the 1990s was to restructure our building codes to recognize the potential for earthquake shaking, and start to build in ways that could save lives," Yeats said. "Now, we need to take the final step, which is to realize that we can often identify the actual surface faults and better protect ourselves from ground movements directly on the fault line."
But he said, progress on this issue is very slow, due to a lack of public awareness, government inaction and the need for new building policies.
"Seattle and other parts of Puget Sound clearly have some of the most pressing concerns, because the U.S. Geological Survey has identified more surface faults that are active," Yeats said. "But there are issues in Oregon, too," he said, including Corvallis' Crescent Valley High School, which straddles a fault line, the Corvallis fault, "that may or may not be active."
Of particular interest, he said, is the Portland Hills Fault which runs through downtown Portland. It's not known for sure whether or not the fault is active or has caused surface disruption. Depending on the exact location of the fault, many downtown structures and the new multi-million-dollar tram may be near or actually straddle it, Yeats said.
Yeats said avoiding construction on a surface fault line is the easiest and most obvious solution.
But modern building and engineering techniques being developed by Jonathan Bray at the University of California, Berkeley may create other options. It may be possible, Yeats said, to design structures with special techniques that would reduce loss of life, if not completely protect a building.