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Federal Way sits on earthquake fault line

About a dozen active fault lines — the source of earthquakes — cross Western Washington.

“These things are around in our neighborhood and we need to be aware of them,” said Brian Sherrod, Federal Way resident and research geologist with the U.S. Geological Survey.

Sherrod enlightened listeners Sept. 23 on what this means for residents. The good news: Science allows geologists to identify earthquake zones and predict the size of an earthquake along a known fault line. The bad news: Federal Way sits atop the active Tacoma fault line.

“If you really want to put yourself at ground zero, we’re sitting on it,” Sherrod said.

The Tacoma fault line crosses Kitsap County and the Hood Canal and ends in Federal Way. It is part of the Cascadia Subduction Zone, which stretches from Vancouver Island to Northern California, according to The Pacific Northwest Seismic Network Web site, www.pnsn.org. The zone is able to produce earthquakes up to a 9.0 magnitude, Sherrod said. The 2001 Nisqually earthquake registered on the Richter scale as a 6.8. China’s latest disaster registered 7.8 to 8.0 in magnitude.

“Take the Nisqually earthquake and instead of it being 30 seconds long, think four or five minutes,” Sherrod said.

The last big earthquake in the Cascadia zone happened about 1,100 years ago, Sherrod said. Around this time, both the Seattle and Tacoma fault lines produced a significant quake. Nearly every other fault line in the zone triggered an earthquake about the same time, he said. Geologists are still not sure what caused the series of ground-shaking disturbances, Sherrod said. But, he expects a series of smaller, but still dangerous, quakes to rock Western Washington soon.

“I would think we need to think about Cascadia going off in our lifetime,” he said.

The zone separates the Juan de Fuca and North American plates in the Pacific Ocean. When one plate gives under pressure, an earthquake results. An earthquake takes place when one plate rises vertically above and over part of another plate, or slides horizontally along the surface of the other plate.

“Normally, they’re locked, but when they pop and move, that’s when you get an earthquake,” Sherrod said.

The movement produces fault scarps — a change in the ground’s elevation or alignment. Scientists study the scarps in an effort to determine how large the earthquake was and when it took place. Overlapping layers of soil are clues to which plate moved and in which direction it traveled. Scientists use the length of the fault line, the amount of sediment displaced and a graphing system to determine the earthquake’s size.

The larger the earthquake, the more damage it could possibly do. If the Tacoma fault shifted, it could cause landslides or destroy dams that provide electric power to much of the region, Sherrod said. If an earthquake struck at sea, a tsunami could result.

As witnessed by an acquaintance of Sherrod’s in Denali, Alaska, in 2002, a large earthquake could bring odd sites such as a semi-truck bouncing across the terrain, Sherrod said. That disaster registered 7.9 in magnitude.

South King Fire and Rescue, Federal Way police and citizens should take that thought with them and prepare for when the next big one hits, he said.

Contact Jacinda Howard: jhoward@fedwaymirror.com or (253) 925-5565.

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To learn more about earthquakes in the Puget Sound, visit the USGS Web site at www.usgs.gov/.

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