I recently received some interesting information from Carol Dunn, professional “Disaster Guru,” about the risks we face in the Green Lake neighborhood should an earthquake strike.
On Solid Ground?
Carol sent the following geological map from the Pacific Northwest Center for Geologic Mapping Studies:
The purple areas of the map represent land that is fairly solid, leading to rocky soil for gardeners, but more stable ground in the event of an earthquake.
The blue areas are solid, but not as solid as the purple areas. If there is going to be a landslide, it will probably be in a blue area.
The brown areas illustrate sediment from Green Lake and historic water paths. These areas are, Carol says, “definitely prone to liquefaction,” meaning that, in an earthquake, the ground may act more liquid than solid, making it possible for the ground to spread, buildings to sink down, or underground pipes to move up.
This phenomenon is illustrated in some jarring pictures of the south side of Green Lake following a 1949 earthquake. The pictures can be viewed on page 386 of Assessing Earthquake Hazards and Reducing Risk in the Pacific Northwest, Vol. 2, available in PDF format here.
Liquefaction can also be witnessed in this video, taken during the Niigata earthquake of 1964:
If you would like to learn more about the ground under your house, check out GeoMapNW, which is made available by the Pacific Northwest Center for Geological Mapping Studies. Zoom in on the map until you see little blue dots. Change the mode to “identify” and click on a blue dot near your house. You will now have access to a copy of a geo-engineering report for the area.
The Water’s Path
Carol also took a look at a LIDAR (Light Detection and Ranging) view of Green Lake. LIDAR is an optical remote sensing technology that measures the exact altitude of an area and then visually strips away all of the trees and buildings. LIDAR maps for most of King County are available from the Puget Sound Lidar Consortium.
Carol overlayed a Google Earth image onto the LIDAR view of Green Lake and got this result:
Carol writes:
You can see where there might be the path of water from the East to the lake (which is supported by the geological findings). I wouldn’t be surprised if houses in that area have more problems during severe rain events.
A lot of the houses there are significantly higher than the sidewalk—the ones with rockeries and retaining walls need to be sure that they are designed properly—I’ve noticed as I’ve driven through the area that some are clearly cracked and slumping—those homes definitely face higher risks in ground shaking, if the wall gives way it isn’t hard to imagine the ground giving way as well.
Preparing Your Older Green Lake Home
I asked Carol about the many houses in the Green Lake neighborhood that are 100+ years old. She reports the following:
Having older houses does not automatically mean you will face greater problems in an earthquake if it is a wood frame house, but usually not without some level of retrofitting. Seattle has a guide on how to tell if you need to bring a professional in to help retrofit, and what things you can probably do yourself. Brick houses do face more risks in earthquakes.
Older houses definitely face a higher fire risk—particularly if they are made with a balloon frame. Those houses can be entirely consumed in flame in less than 2 minutes, so it is worth finding out if you do, and taking action to reduce the risks of fire, including adding fire blocks between floors. We have a residential fire in our area every 10 hours. Adding fire blocks can be expensive, but worth it.
Update, Monday June 21:
Carol sent us an additional graphic. This image overlays a map from seattlepi.com that illustrates the location of buildings damaged from the Nisqually earthquake with the LIDAR view of Green Lake:
Green dots represent structures with minor damage, yellow dots represent structures tagged “restricted use,” and red dots indicate damage so severe that the building was illegal to enter. Note the red dot at the the Green Lake Small Craft Center‘s Massart Shellhouse, which also sustained damage from Seattle’s 1965 earthquake.
Carol notes that factors that can influence how much damage is caused by an earthquake include a building’s construction style and materials, ground type, and the intensity of the shaking.
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Notes:
- If you would like more information about the ground beneath your house, consult a professional geologist or geotechnical engineer. The King County Department of Development and Environmental Services provides an information bulletin on selecting a geotechnical consultant, and a list of local firms can be found on the Seattle Daily Journal of Commerce website.
- For information about emergency preparedness in Green Lake, or to get involved in the effort to build an emergency preparedness plan for Green Lake, contact Paul Kostek at pkostek [at] aol.com.
Our sincere thanks to Carol Dunn for taking the time to share this information with the Green Lake neighborhood!
