Geological map shows more hazards for Mercer Island

More of Mercer Island may be vulnerable to hazards like landslides and earthquake damage than previously thought, according to a new geologic map of the island developed by a University of Washington team.

Overall, the map shows about 16 percent more hazardous ground than an older, less accurate geologic map drawn up in 1962 by the U.S. Geological Survey.

The map is so detailed that residents can zoom in on their block, and even on their house, to learn more about the geology underlying their home. It depicts soils, sediments and topography across the island. It shows areas of weak ground — such as soft clays and sand — as well as unstable swaths of landslide debris and permeable ground prone to slippage.

The new map could affect city decisions on everything from where roads and utilities are placed to how — and where — land is developed.

"These findings have definitely increased, overall, the amount of ground that's going to fall into hazard zones," said University of Washington geologist Kathy Troost, who led the mapping project.

The map places Mercer Island squarely within the Seattle Fault Zone, a band of ground riddled by several major faults.

Although the island has been known to fall within the fault zone since 2002, no earlier geologic map of Mercer Island displayed its position in the zone. The island's placement underscores the risk of earthquake damage for the city's residents.

Troost and city officials will present the map to the public Wednesday.

Mercer Island officials said the new map will provide a useful blueprint for city planners. Roads, sewers and water pipelines are all affected by regional geology, said city Maintenance Director Glenn Boettcher.

"Knowing as much as we can about the geology of Mercer Island underground, we can make better decisions about our infrastructure," he said.

Mercer Island, which contributed $150,000 to the project, is only the second city for which the university team has produced a complete map. In May, Troost's team released a similar map for Seattle, hailed as the most detailed geologic map for any city nationwide. Boettcher said planners will meet over the next year to discuss the impact of the newly mapped geology.

To create the map, Troost and another geologist compiled geologic data from nearly 3,000 boreholes and wells drilled by developers and the city during construction projects since the 1930s. They then trooped across the island to study protruding bedrock and soils — an effort they had to undertake in winter, because summer vegetation hides underlying geologic formations.

The team discovered, among other things, a series of muddy ancient lake shores. These deposits of mud and sand were left behind more than 10,000 years ago, when heavy glaciers depressed land surfaces so deeply that Lake Washington completely covered Mercer Island.

Today, the muddy deposits represent weaker ground, where buildings need stronger foundations to prevent dangerous shaking during earthquakes.

The new map doesn't pinpoint any specific large faults under the island, "which isn't to say they aren't there," said Troost. Roads, houses, parking lots and vegetation probably obscured visual evidence of faults, Troost said.

But by next year, the city of Mercer Island should learn still more about the geology under its feet. The U.S. Geological Survey has undertaken a project that will use seismic techniques to precisely map fault lines underlying the island.

Homeowners, developers and other residents can expect a new assessment of hazardous areas for the city based on the map, said Mercer Island Building Official Don Cole. Such zones are currently regulated under King County's "critical areas ordinance," which restricts development in geologically unstable areas.

Some plots governed by the ordinance will probably be dropped from the list of critical areas, Cole said, thanks to the map's precise geologic information.

"We'll be able to exclude areas that may have been mapped incorrectly," he said. "The old map was painted with a broader brush."

But other parts of the city could face stricter regulations, because previously unknown geological hazards will push new regions onto the critical-areas map, Troost said.

Residents and developers "are going to be forced to be aware of their geologic environment, forced to be aware of the potential hazards," she said.

For Mercer Island, as for much of the steep-sloped, rainy Puget Sound region, those hazards can cost. Last winter, for example, a major landslide prompted by heavy rain wrecked a portion of East Mercer Way, costing the city an estimated $200,000 for ongoing repairs.

Jason Koehler, project manager for the Mercer Island company RKK Construction, said an increase in regulated areas on Mercer Island would be "problematic" for the company, hampering its ability to build.

Still, he described the map's new information as a boon. The company could rely on mapped geology rather than hiring a geotechnical engineer for extensive surveys, he said, saving the company money.

Also, he said, "If something's deemed dangerous or critical ... that might steer us away before we even get into a project."

The map will also help owners of existing Mercer Island homes decide whether they need earthquake insurance, said Karl Newman, president of the Northwest Insurance Council, a nonprofit public education coalition of insurance companies.

Earthquake insurance approximately doubles the cost of homeowners' insurance, but for high-risk areas identified by the map, the risk may justify the expense, he said.