Published: Wednesday, February 23, 2011, 9:02 PM Updated: Friday, March 09, 2012, 6:25 PM
Joe Rojas-Burke, The Oregonian By Joe Rojas-Burke, The Oregonian
The earthquake in Christchurch, New Zealand rated only a magnitude 6.3 and yet killed at least 92 people and collapsed modern, seismically reinforced buildings because it erupted at shallow depth near the center of a city. Portland sits on shallow faults capable of similar destruction.
“The same characteristics that caused such destruction and so many deaths in Christchurch are similar to those facing Portland, Seattle, parts of the Bay Area and many other West Coast cities and towns,” says Robert Yeats, a professor emeritus of geology at Oregon State University. He asserts that the region needs to pay more attention to the many shallow faults hiding beneath population centers.
Disaster preparedness in Oregon has largely focused on a different kind of earthquakes: those from the collision of massive sections of the earth’s crust, called tectonic plates. From Northern California to British Columbia, an ocean-spanning slab called the Juan de Fuca Plate is plunging beneath the North American plate. In a complete rupture across this Cascadia subduction zone, geologists expect magnitude-9 ground-shaking to persist for several minutes across much of Oregon and Washington. They rupture about once every 450 years.
Most shallow crustal faults are far smaller than subduction zone faults — and haven’t gotten the attention. Shallow earthquakes erupt when one section of Earth’s crust slides past another to relieve pent up forces. Ground shaking lasts for seconds rather than minutes, and strikes in a smaller area than a subduction quake.
Geologists have located three shallow faults that cut beneath the most populated parts of Portland. The largest, the Portland Hills Fault, may pose the greatest risk. It stretches from Oregon City to Scappoose. Corvallis and Seattle also straddle active crustal faults.
Ian Madin, chief scientist with the Oregon Department of Geology and Mineral industries, has studied the Portland faults and says it’s hard to rate the hazard because they have never ruptured in historical time. Evidence in disturbed sediment layers reveals one or two strong earthquakes on Portland Hills Fault in the last 15,000 years. The next one, however, could happen tomorrow.
“All the evidence suggests our local faults have earthquakes very infrequently,” Madin says. “But because the faults are right underneath downtown Portland, if they move it’s going to shake very hard.”
Another risk factor Portland shares with Christchurch is its extensive development on water-saturated sediment along rivers. That kind of ground tends to lose stability during an earthquake and behave almost like a liquid, causing sturdy foundations to sink, tilt and collapse.
That puts a lot of critical infrastructure at risk, says Yumei Wang, a geotechnical engineer with the Oregon Department of Geology and Mineral Industries. Fuel pipelines, petroleum storage tanks, ports and electrical transmission lines are built on ground that will turn to Jell-O in an earthquake.
The strength of buildings may be the most decisive factor in how well a city survives an earthquake. Yeats says New Zealand has some of the most progressive building codes in the world and is better prepared for earthquakes than most U.S. cities. Schools in Christchurch appear to have stood up well.
Many Oregon schools would not. More than half are at a high risk of collapse from a quake, according to a 2007 report by the Oregon Department of Geology and Mineral Industries. Oregon recently awarded $15 million to help two dozen schools and emergency facilities strengthen buildings. State law requires all public safety buildings be upgraded by 2022 and public schools by 2032.
“Up until the 1980s, conventional wisdom held that we were not in highly seismic region,” says Scott Ashford, head of the school of civil and construction engineering at Oregon State University. As a result, he says. “We have an incredible amount of legacy systems, 100-year old water lines, and most of our bridges, built before modern seismic design came around.”
While planning for a subduction zone quake prepares cities for more localized crustal earthquakes, Yeats says additional steps are worth taking. Oregon and Washington urgently need to more carefully identify hidden faults and figure out how much risk they pose, he says.
One model, he says, is California’s longstanding Alquist-Priolo Earthquake Fault Zoning Act. It began detailed mapping of fault zones in the 1970s and prohibited new construction of houses within the zones unless geological studies show that the fault poses a minimal hazard to the proposed building.
“We need more due diligence to be sure that new developments are safe,” Yeats says.