CORVALLIS, Ore. – Much of Puget Sound and some other parts of the Pacific Northwest are vulnerable to “surface faulting” from earthquakes, researchers say, based on studies done with LiDAR – an aerial mapping technology that reveals what landforms would look like if buildings and vegetation were stripped away.
Surface ruptures, aside from the ground shaking associated with earthquakes, are the actual lines where an underlying fault breaks through the Earth’s surface. Surface faulting can send part of a building surging up or sideways, literally ripping a structure in two and causing massive destruction or loss of life.
Until the development of this powerful new technology by the U.S. Geological Survey, it had not been possible to locate surface ruptures precisely and learn how frequently they take place. The evidence of past surface faults is often hidden by dense vegetation or urbanization. But the ability to locate surface ruptures accurately and take steps to mitigate damage should prompt a new public awareness and building codes to deal with the issue, say researchers from Oregon State University.
“Most earthquake codes in Washington and Oregon consider and prepare for the damage that earthquakes can cause from ground motion or soil liquefaction,” said Robert Yeats, a professor emeritus of geosciences at OSU, and one of the nation’s leading experts on earthquakes. “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. “This includes the Seattle Fault system, Tacoma Fault and Southern Whidbey Island Fault between Seattle and Everett. 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.”
Not all earthquakes manifest themselves with faults that cause ruptures in the Earth’s surface, Yeats said.
But some do. And when the surface is affected, the amount of ground shaking near it is even stronger, not to mention that the ground may move in two different directions beneath a building. On a thrust fault the movement might be up and down – one part of a building would get lifted while the other collapsed. On a strike-slip fault, the movement would be more sideways, literally tearing the building apart. In either case, the destruction is often catastrophic.
Awareness of this concern developed in California as far back as 1971 during the San Fernando Valley earthquake, leading that state to pass the Alquist-Priolo Act. It requires special geological studies prior to placement of certain structures on earthquake surface faults that have moved within the last 11,000 years. Oregon and Washington have no such laws or regulations.
A key in this evolving story, Yeats said, has been the growing use of LiDAR, or Light Detection and Ranging laser systems. This sophisticated airborne sensing technique can view the land from above and use technology to see the bare ground while “removing” the overlying vegetation or buildings. With it, surface faults can often be clearly seen on a map, and then ground trenching used to verify the fault and its dates of movement – similar to investigations at an archaeological site.
This technology was used, among other places, to discover the previously unknown Toe Jam Hill Fault on Bainbridge Island in Puget Sound. Trenching by the U.S. Geological Survey revealed it had several episodes of surface rupture between 3,500 and 1,000 years ago. Significant earthquakes on this, or seven other Seattle-area surface faults that have been identified, could cause tens of billions of dollars in damages, Yeats said, and large loss of life.
“At this point, we still argue a little bit over the details, but this is not new science,” Yeats said. “It’s proven technology, and we can use it anywhere we want to outline risks from surface faults and do something about it, either by avoidance or earthquake engineering.”
However, Yeats said, progress on this issue is very slow, due to a lack of public awareness, government inaction and the need for new building policies.
“It may seem like common sense that you would not want to build a public school or hospital or other large building on top of an active fault that could split it in two,” Yeats said. “Unfortunately, history suggests that money will trump common sense every time.”
In places where such risks are taken far more seriously, Yeats said, building codes have been adapted and changes are already under way. Several school buildings at San Bernardino College in California are now being torn down and replaced, at a cost of hundreds of millions of dollars, because it’s become clear that they straddle the San Jacinto Fault.
“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. There are active surface faults in Klamath Falls that are very visible, anyone could see them. Crescent Valley High School in Corvallis straddles a fault line 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.
According to Yeats, 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.
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