Landslides bring added risks

At an emergency preparedness seminar Thursday night, Islanders learned about the hazards they could face during “landslide season” and the resulting damage, should an earthquake shake the Island.

“We’re definitely in an area with some real geological features based on earthquakes and landslides that happened long ago,” said Don Cole, a building official with the city of Mercer Island who led much of Thursday’s presentation.

He points to the Island’s submerged forests, thought to have been caused by earthquake-induced landslides nearly a 1,000 years ago.

The Island lies within the Seattle Fault Zone. The Vasa Park fault and Newcastle Hills fault, though their exact location isn’t well understood, also extend through the Island, in the north and south.

The city completed a project in 2009 to map landslide hazard zones and while those don’t directly overlap with MI’s seismic activity, the two are correlated. The slopes around the exterior of Mercer Island are most prone to landslides. Loose sediment, such as soft clays and sand, as well as soil saturation heighten that risk.

Cole explains at the top of MI, lake sediments never consolidated. When glaciers receded they deposited lake sediments that now constitute much of the Town Center. That looser material requires many of the Town Center’s buildings to have deep foundation systems to get through the loose sediment.

“We’re such a beautiful island that it’s pretty easy for us to forget some of the geological features that make it so beautiful, can also be dangerous,” says Cole.

Researchers with the University of Washington released an update earlier this year revealing that outdated information for the Seattle area undermined the damages potentially caused by an earthquake or landslide. Much information has been made available in the last four to five years because of technological advances.

“I’d say our information is still up-to-date,” says Cole, “but damage expectations do change.”

Winter through spring is landslide season and Mercer Island averages between 12 and 15 landslides annually, most of those minor, says Cole. About 85 percent of landslides are thought to be caused by a human factor, such as minor flooding, broken pipes, plugged storm drains or excavating in high-risk zones.

“With recent events in Japan and Chile, this is kind of in our face,” says Jennifer Franklin, an emergency preparedness officer with the MI police department, also present for Thursday night’s meeting. “We [can’t] bury our heads in the sand.”

The city of Mercer Island follows a strict procedure when a landslide is reported, coding the severity of the incident as “yellow,” “green” or “red.” The police and fire departments and the city’s maintenance office are notified and respond accordingly. Depending on the type of property damaged during the landslide, building structures are assessed to determine if they’re safe to inhabit.

Franklin encourages residents to take precautions of their own, despite having a sound emergency procedure. That includes preparing for three to seven days of total isolation, with enough food and water. She also advises that neighbors get to know one another, in case they need to depend on other residents during an emergency.

To view the city’s maps of seismic and landslide zones, follow the link:


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