PSU Magazine Winter 1993

shifts in fault lines, and they have the most surface visibility. Portland's largest recorded quake was a crustal quake. Oregon and the Northwest, how– ever, are susceptible to two other types of earthquakes in addition to the crus– tal type. The 1949 Seattle quake was an intraplate quake, caused by the bending of the Juan de Fuca plate beneath the North American plate. Scientists know relatively little about these quakes, Mabey exp lained, because there's no fault breakage on the surface when they occur. Oregon may have experienced an intraplate earthquake in 1873, when a tremor measuring about 6.3 throttled the Port Orford area on the coast. Of greater concern to Mabey and other earthquake specialist are the sub– duction zone earthquakes. These are the most powerful, reaching mag– nitudes above 8. A notable example was the devastating Anchorage earthquake of Good Friday 1964. The quake registered a 9.2 magnitude, caus– ing widespread structural damage, in– juries and deaths. This quake released perhaps 30 times as much energy a the notorious San Francisco quake of 1906. Subduction zone quakes occur when two moving land plate stick together. Stress builds before a sudden break. The resulting snapping activity cau es the world's largest earthquakes, which often are accompanied by tsunamis– seismic-caused sea waves. Also, these quakes bring the longest duration of surface shaking. 8 PSU In the Northwest, scientists are directing their attention to the Juan de Fuca plate off the Pacific Coast. The plate is being squeezed by the Pacific Ocean plate to its west. The plate also is trying to move east beneath the North American plate, which sits below Oregon and Washington. The building tension is what worries scientists the most. There are no recorded subduction zone quakes in Oregon, but stud ies of ground layers on the coast indicate that the region probably had such a quake about 300 years ago. Mabey and others say such quakes can occur once every 350 to 600 years, which-again– means that the prospect of such a quake in the Northwest during our lifetime is a possibility. Mabey likes to emphasize that the duration of shaking i what makes earthquakes so damaging. A quake measuring 6, for examp le, brings shak– ing that lasts about seven seconds. A quake measuring 8, however, triggers shaking that can last nearly one full minute. Anchorage's devastating 1964 quake shook the earth for four minutes. What does all this mean to Oregonians? And, specifically, to Portlanders? Plenty. Metropolitan Portland's location along two rivers and amid steep, muddy hills makes it particularly susceptible to earthquake damage. Much of downtown Portland, for example, is built on soft soil near the riverbank. During an earthquake, in a process called liquefaction, this river– bank soil behaves like a liquid. Heavy structures may sink and light struc– tures-such as septic tanks, fuel storage tanks and sewage treatment holding fac ilities-may float to the surface. Also, structural objects affected by liquefaction have a tendency to slide downslope, in a process called lateral spreading. Mabey says most of Portland's liquefaction zone is confined to areas along the river. But that's also where much of the city's industrial and com– mercial activ ity takes place. And not only are its structu res built on soft soil, but many buildings weren't designed with earthquakes in mind. The build– ings of greatest concern are those of medium height (four to fifteen stories) and those of brick. Visitors to downtown Seattle may notice that many cornices on older buildings have either been removed or reinforced as an aftermath of the power– fu l 1949 quake there. Similar buildings in Portland have never received such a shakeout. "When we (Portland) get a signi– ficant earthquake, you'll see a lot of parapets and cornices coming loose," ays Mabey. Many Portland buildings simply weren't designed with seismic activity in mind, even into the 1970s. "We have a big inventory of buildings that weren't built to high seismic design criteria," says Mabey. Modem high-rise build ings are less of a concern because they're already built to withstand high winds and thus are more resistant to earthquake shaking. Fortunately, the state is upgrading it building codes. The Oregon Build– ing Codes Agency passed a series of new design standards for most new buildings. The standards went into effect Jan. l. The codes do two things. First, they require engineers to de ign buildings that can re ist higher levels of shaking caused by earthquakes. Secondly, they require structures' columns and beams to be connected better, " o if an earthquake occur , the connections can withstand shaking for a longer period of time," says Mabey.