Official City of Seattle landslide map

The awful landslide tragedy near Oso, about an hour’s drive north of Seattle, is getting the far-flung attention it should. The loss of life is horrendous–25 known to be dead at this writing with as many as 90 still missing and presumed deceased.

But almost as bad is the fact that various government agencies have known for decades this scenic stretch of Washington State along State Highway 530 on the edge of the North Cascades was particularly susceptible to a traumatic landslide off Skaglund Hill. Written reports detailed the danger to a clutch of homes from unusual shifting soil and a meandering Stillaguamish River that cut into the bottom of the face of the hill. Yet officials approved building permits for new housing and did little to warn the residents. Some survivors now say they had no idea of their documented peril.

Why am I, New To Seattle, writing about this? Take a look at this 16-year-old map of Seattle, produced for a municipal agency. Each tiny colored dot represents a documented landslide within the city limits back to 1890.

There are more than 1,500 tiny colored dots.

Clearly, hundreds and homes and other buildings in Seattle have been damaged by landslides, but I can’t quickly find any hard data about the cumulative loss of life from these earth movements. Over 125 years, though, the casualty total seems less than what happened in a few seconds last Saturday morning near Oso.

That may be partly because Seattle communicates and regulates a little better. The map comes from the something called the Seattle Landslide Report, published in 2000 by the city’s Department of Planning and Development. And right now, there’s this stark warning up on the web page of Seattle’s Office of Emergency Management under the one-word heading of “Landslide”:

Landslides are common in Seattle. Late winter and early spring are the most common times for slides, with most of the documented slides in Seattle occurring in January. Nearly all landslides in Seattle result when excess water is involved, and the majority of landslides also involve human factors of some type.

The probability of a slide event rises after a wet, cold winter, especially if a freeze occurs in late winter and early spring. The ground becomes saturated over the winter, and then porous following a freeze, so a subsequent rain will penetrate the surface while the high water table will prevent the ground from absorbing it. The water increases the slope stress by adding weight and increasing pore pressure within the soil.

Over 1,511 landslides have occurred in Seattle dating back to 1890. Seattle’s landslide prone areas have been studied. They comprise 8.4% of the City.

Decades ago, Seattle adopted an ordinance prohibiting construction in what is called a “steep slope area” without extensive geotechnical review and city approval.

Having moved around the country for the past four decades–and as a journalist covering plenty of natural disasters–I have developed an acute awareness of what Mother Nature can do, and how terrain can be everything when it comes to weathering her wrath.  Early on in my travels, I was told never to live on a street that has the word “canyon” in its name, advice I have followed faithfully. (There aren’t many canyons in my native New Jersey, whose highest point is just 1,803 feet above sea level–a mere one-eighth the height of Washington State’s 14,411-foot Mount Rainier.)

I live now in a close-in Seattle neighborhood, Magnolia, that, owing to its high bluffs and steep drops, has had more than its share of slides. (On the map above, Magnolia is the peninsula sticking out toward the top left into Puget Sound; the peninsula’s bottom perimeter is festooned with tiny colored dots).  I can tell you that before buying a home in 2011 I poured over this map and the landslide report and flood maps and every other such document I could get my hands on. Hopefully, I’m far enough back from the edge in case something bad happens, which can be at the top, near where I live; at the bottom, where there also are homes–or both.

As can be said about most of Washington State, Seattle is a beautiful place set amid breathtaking terrain. But the same reasons that make the views awe-inspiring–the juxtaposition of steep mountains, hills and cliffs with water of all kind–also makes the place fraught with peril. This is especially so given the persistent rain that can seep into and weaken everything. Not surprisingly, most of those dots on the map are along the city’s edge near the bluffs overlooking Puget Sound and Lake Washington.

Damage from landslides is not covered in the standard homeowners insurance policy. It’s possible to buy this coverage, but it’s costly–maybe $1,000 a year, I am told–with very high deductibles. Mortgage lenders don’t require it. I heard on the radio yesterday that in Washington, a state with nearly 3 million housing units, only 1,800 are so insured.

There are almost that many tiny colored dots just in Seattle alone.

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