Warning: This earth will move

Seattle city officials always get a chuckle out of a certain "For sale by owner" sign that was photographed at the edge of a bluff in 1938: "NO SLIDES," the sign promises. "Civil engineer says . . . `Good condition to build on.' "

The site? Perkins Lane - a Magnolia neighborhood street where landslides have continued to gnaw away at the steep hillside ever since.

With landslides, history repeats itself, the experts say. Gravity will win. "If you buy property above or below a landslide area, or in the middle, the ground is probably going to move," says Cheryl Paston, project manager in the Seattle Public Utilities department.

That history will repeat itself was more than confirmed by an exhaustive study of the history of Seattle landslides, commissioned by the city from Shannon & Wilson, a geotechnical and environmental consulting company. The resulting report includes new maps of slide-prone areas and a review of all the methods that can be used to prevent slides or keep them from doing damage to property.

It's part of a more intensive new effort by city officials to educate property owners about landslide risks in the wake of the winter of 1996-97, when there were almost 300 landslides in Seattle alone. A series of public meetings is scheduled, beginning Tuesday, to share information from the just-released report. (See adjoining story for schedule and details.)

Of course, it may take hundreds of years for the ground to move. In some years - like this one - there will be few landslides because rainfall has been relatively scant and well-dispersed throughout the winter.

And landslides can be anticipated. Houses can be engineered so they are able to withstand them. Preventive measures can be taken to reduce their risks.

"All of the landsliding that goes on happens on 1 percent of the land (in the city) - the steep bluffs along the Sound and the sides of linear ridges," said Bill Laprade, vice president of Shannon & Wilson. "People who build or live in these areas need to be cognizant and take responsibility for the problems."

City officials don't expect the new maps to be a surprise to anyone; they had already identified most of the land that's prone to slide. But the new maps are more precise. They tap into an archive of 1,326 documented landslides dating back to 1890. That archive includes thousands of photos that show houses dangling off the edge of eroded cliffs, uprooted trees that have spilled down precipitous bluffs, fans of mud oozing downhill.

Regulations adopted after the 1996-97 slides already address most ways of designing new construction to reduce landslide risks, said Craig Ladiser, deputy director of operations for the city's Department of Design, Construction and Land Use. With the new study and maps, the city can be more precise about what land is truly slide-prone, and warn property owners early in the development process.

A few new areas were added to the list of landslide-prone spots on the map; most are city-owned ravines, such as the Interlake trail between the north end of Capitol Hill and the Montlake neighborhood.

Although the wearing down of steep hillsides is inevitable, people do have a bearing on where and if landslides occur. The Shannon & Wilson report shows that there was some factor of human influence in 84 percent of the landslides studied. Human influence includes such things as improperly routing a downspout down a steep hill, or stripping plants off a hillside.

Perhaps not surprisingly, the landslide study shows that most slides occur in January, after the heavy rains of November and December make the groundwater table rise. They're somewhat less common, but still possible, in December and February.

Four types of landslides occur in Seattle. The most common is called a shallow colluvial, or "skin slide," in which the shallow outer rind of a hillside slides rapidly. These slides also are often accompanied by a flow of debris downhill. About 68 percent of past landslides in the city fall under this category.

The largest and most destructive slides are deep-seated, caused by groundwater pressures within a hillside. About 20 percent of Seattle's landslides are of this type.

The study also examines "runout zones" - the area at the bottom of a landslide-prone hill that could be hit with sliding mud and other debris. People living in the path of a runout zone are at greater danger than those living atop a landslide, Laprade said. A house at the top of the hill "can go along for the ride," but property in the runout zone below can be crushed by the thousands of tons of debris coming downhill.

In January 1997, a family of four was killed when their home on Bainbridge Island was crushed by debris that slid down a steep hillside. It wasn't the first time a landslide claimed lives in the region; in February 1947, two children sleeping in their beds were killed by a Kirkland mudslide, and in 1897 four train workers were killed by a slide, according to newspaper accounts of the accidents.

Laprade said the new maps do a better job of highlighting runout zones, which in Seattle usually extend 50 to 100 feet beyond the toe of a hillside.

The Shannon & Wilson study identifies all types of remedial measures that can be used to prevent or reduce slide damage. Laprade calls it a kind of "tool kit" that can help people living on steep slopes decide what remedial steps to take.

One such remedial project is scheduled to be constructed this year in West Seattle. In a slide-prone area north of Bonair Drive Southwest, drains will be installed to keep the groundwater table low. The $4.2 million project is being paid for through a grant from the Federal Emergency Management Agency, money from the city and a local improvement district formed by 150 property owners.

"It's relatively old technology," said Paston, with Seattle Public Utilities. The project should help stabilize the upper bluff, but "it won't solve the lower bluff problems. The face of that bluff is going to continue to slough off."

Because, when it comes to landslides, history always repeats itself.