This post has been updated

Six devastating earthquakes greater than magnitude 8 have occurred since 2004, causing

that earth has entered a period of increased seismic activity that could trigger megaquakes in vulnerable regions including the Pacific Northwest.

But a somewhat reassuring new study suggests otherwise. University of California researchers examined the timing of earthquakes worldwide from 1900 to the present. They found no evidence that any of the great earthquakes since 2004 triggered other great earthquakes. Furthermore, the recent run of megaquakes greater than magnitute 8 isn't unprecedented,

in Proceedings of the National Academy of Sciences:

...rates have been almost as high before, and the rate of smaller earthquakes is close to its historical average. Some features of the global catalog are improbable in retrospect, but so are some features of most random sequences–if the features are selected after looking at the data... Moreover, no plausible physical mechanism predicts real changes in the underlying global rate of large events. Together these facts suggest that the global risk of large earthquakes is no higher today than it has been in the past.



To be sure, earthquakes still pose a significant hazard for people in the Pacific Northwest. The region sits atop a fault zone that's nearly a mirror image of the deep fault that ruptured the sea floor off Japan in March, killing thousands. 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.

Geologists have found evidence of 19 massive Cascadia earthquakes in the past 10,000 years. The most recent struck in 1700. The chances a magnitude-9 earthquake will hit the Northwest within 50 years is about 10 to 15 percent, according to the most widely cited estimate. But Oregon State University geologist Chris Goldfinger says the picture is more complicated than that. "I don't think a single number does the job anymore," he

in a 2010 news story:

Goldfinger and others have reconstructed a 10,000-year history of major quakes along the Cascadia subduction zone by examining the remnants of undersea landslides. That history suggests that Cascadia has at least four segments that sometimes rupture independently of one another. Magnitude-9 ruptures affecting the entire subduction zone have occurred 19 times in the past 10,000 years. Over that time, shorter segments have ruptured farther south in Oregon and Northern California, producing magnitude-8 quakes.

The risks of a subduction zone quake differ from north to south. In the northern segment, Goldfinger's group also puts the odds at 10 to 15 percent during the next 50 years. Quakes originating there tend to rupture the full length of the subduction zone, he says. In southern Oregon and Northern California, quakes along the subduction zone appear to strike more frequently. Goldfinger and colleagues calculate a probability of 37 percent that another will strike within 50 years. By that time 360 years will have passed since the last major event, and records for the past 10,000 years show that four out of five big quakes in the south have occurred within 360 years of each other.

In the Northwest, earthquakes

until the 1980s. As a result, many hundreds of buildings, bridges, and other structures were built with no thought of standing up in an earthquake. As he Oregonian's Richard Read

, "collapse-prone schools containing 306,000 Oregon children could shake apart. Portland's Fremont Bridge and other spans and their approaches will likely fall. Landslides will probably bury highways, cutting off Oregon's coast. Towers suspending high-voltage lines over the Columbia River may topple, blocking ships needed for emergency supplies."

Portland residents have a chance to help scientists track earthquakes by having a seismograph placed in their home or business. In a citizen science project called

, the U.S. Geological Survey says it's trying to build networks with more dense and uniformly spaced seismographs in select urban areas to provide better measurements of ground motion during earthquakes.

--