Despite an unusual run of giant earthquakes, the Northwest's risk likely unchanged

Published: Tuesday, December 20, 2011/ By Joe Rojas-Burke, The Oregonian

The Associated PressIn this March 11, 2011, photo, a tsunami floods over the breakwater protecting the coastal city of Miyako at Heigawa estuary area after northeastern Japan was hit by a powerful earthquake.

This post has been updated

Six devastating earthquakes greater than magnitude 8 have occurred since 2004, causing some experts to speculate 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, the researchers report 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 told The Oregonian 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 weren't considered much of a threat 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 recently noted, "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 NetQuakes, 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.

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Report sees widespread earthquake damage

A 7.5 temblor would cost $550 million, leave nearly 3,000 without water.

By Cory Hatch, Jackson Hole, Wyo.

December 21, 2011

A magnitude 7.5 earthquake on the Teton Fault could cost more than half a billion dollars and temporarily displace hundreds of households in Jackson Hole, a new report suggests.

The fault, which runs 35 miles along the base on the Teton Range, has been "ominously silent" since it last erupted in an earthquake nearly 5,000 years ago, said Wallace Ulrich, a Teton County resident and former state geologist. Most faults in the region undergo an earthquake every 800 to 1,000 years, researchers say.

Ulrich oversaw the 132-page report "Wyoming Earthquake Hazard and Risk Analysis." A Teton Fault quake would have the biggest economic impact of 16 earthquake scenarios the report examined across Wyoming, according to geologists with the Wyoming State Geological Survey who assembled the document.

"The Teton Fault is one of the largest potential faults on the state," study co-author and state project geologist Seth Wittke said in a telephone interview from Laramie. "So you'd expect a maximum-magnitude earthquake to create a lot of [economic] losses."

The results are estimates and not "set in stone," Wittke said (see box on 26A for methodology).

Such an earthquake would impact Teton, Lincoln, Park, Sublette and Fremont counties. Teton County would be hardest hit, with an economic loss of $494 million from damage to buildings alone.

"Moderate to heavy damage would be likely within 20 miles of the fault, including the communities of Moran, Wilson, Jackson and Moose," the report says. "Light damage is predicted to extend out to 35 miles from the epicenter, including Alta and the Rafter J area. [V]ery light damage can be expected up to 50 miles from the epicenter near West Thumb, Alpine and Bondurant."

Of the roughly 12,000 buildings in Jackson Hole and surrounding communities, the earthquake would damage nearly 4,000, including St. John's Medical Center.

There's a 72 percent chance the hospital would be damaged extensively, the report said.

The majority of the hospital would probably be closed for several weeks, study authors wrote.

"The Saint John's Medical Center would be less than 1 percent functional the day of the earthquake," the report said. "Functionality would increase to 3 percent by day seven, and 28 percent by day 30."

All of the region's highways and all but two of its 95 bridges would likely survive the earthquake intact. The report did not say which two bridges would be damaged.

Also, the report did not mention landslides and avalanches that could close roads. Nor did it discuss potential damage to the Jackson Hole Airport runway.

A substantial quake would cause $46 million to damage to roads, bridges and other transportation infrastructure in Teton and Sublette counties, the report predicts.

Only one of the region's three police stations and five of the region's 10 fire stations would remain open. Emergency services at Grand Teton National Park headquarters at Moose and Jackson Hole Airport, which would likely be close to the epicenter of the quake, would be especially hard hit, according to the report.

Of the fire departments, the one at the airport would sustain the most damage, followed by the Teton park fire department stationed at Moose. Next on the damage list would be both fire departments in Wilson, and the Jackson Hole Fire/EMS station in Jackson.

Of the police stations, the earthquake would damage the Jackson Police Department headquarters and the Teton County Sheriff's Office, where emergency dispatch is located. The damage would be so severe that operations would be only at half force after one week.

Only the police station in Alpine would escape mostly unscathed.

The quake would temporarily close all but five of the region's 16 schools. The Teton Science Schools' Journey School has the highest chance of extensive damage.

Next on the damage list would be by C-Bar-V Ranch and Davey Jackson Elementary School. Kelly Elementary, Alta and Bondurant elementary schools would experience little damage, according to the projections.

The hardest-hit fire departments and schools would be operating at only about 50 percent capacity after a month.

"Seeing how long it would take some of these emergency facilities to get back to even 50 percent surprised me a little bit," Wittke said.

The study authors did not include estimates of death and injury because they were too hard to calculate given Jackson Hole's highly variable tourist population, Wittke said. The Jackson Lake Dam, which some geologists say could rupture during a Teton Fault earthquake, also was not analyzed.

A Teton Fault quake would damage 803 households severely enough to make them uninhabitable. About 400 people would require shelter.

The quake would cut power to more than 800 households and cut off drinking water to 2,792 homes. It would cost $4.5 million to repair utilities such as power, water and sewer in Teton County.

The quake would create 174,000 tons of debris.

source