An image demonstrating the extent of the last glacial maximum in our region. (Image: Ron Lewis via Ice Age Floods Institute)

Separating the interesting side of geologic hazards from the true, helpless terror they can represent is a difficult task.

This is why, when you read headlines about our region’s volcanoes and earthquakes, they are rarely serene. And rightly so. Swarms of earthquakes at Mt. Rainier and tsunami warnings from Russian tremors are fear inducing, regardless of hyperlocal impact.

However, having our heads in the sand — or the glacial till — isn’t going to help.

Capitol Hill is mostly a pile of sand and rocks deposited during the last glacial maximum around 16,000 years ago. This isn’t particularly unique across the Puget Sound landscape because the entire basin was covered by glaciers, and the deposits left by their expansions and recessions is part of our environmental heritage.

Below what geologists call Vashon Till (our pile of sand and rocks), are more layers displaying these peaks and troughs of glacial activity, readily visible on our cliffs and shorelines, most notably along the beach at Discovery Park. There is no escaping the impacts of glaciers or the rumblings of continents, even if we regrade hillsides, these layers are still there.

Still, the structural integrity of glacial till clearly hasn’t been a barrier to development in Seattle and the rest of Puget Sound. It also isn’t exactly a solid building surface. Because till is inherently heterogeneous, a mix of coarse and fine materials, it can be temperamental in some circumstances. Many of us have experienced what happens when you dislodge, or otherwise disturb a rock grounded in finer materials – it can create a mini landslide and collapse the surrounding structure. That’s the sort of stuff we worry about when we live within geologic spitting distance of several major fault lines. Steep slopes and areas that collect water can be serious hazards when combined with violently shaking ground and human structures.

Thankfully, the City of Seattle pays attention, and has pinned down such hazards, along with a slew of other significant features under the Environmentally Critical Area (ECA) codes. These places, earmarked for special consideration in land-use, include geologic hazard areas, steep slope erosion hazard areas, flood-prone areas, wetlands, fish and wildlife habitat conservation areas, and abandoned landfills. The goal of identifying these places is to both preserve valuable natural resources across Seattle and to avoid preventable damage to people and property. If you want or need to make changes to your house or landscape within these areas, you come face to face with ECAs and they directly influence what you can and can’t do.

A map showing the overlapping variety of ECAs in Seattle. They are numerous and frequently tied to places too hazardous to build, also making them of de facto conservation value. The pink in this map are Liquefaction prone areas. (Image: City of Seattle SDCI Property Information Map)

I think most of us are aware of the risk of steep slopes (the Oso tragedy being on the forefront of many minds), and can imagine houses perched over unstable banks slipping off in a landslide induced earthquake. But another geological hazard within the ECAs are called Liquefaction-prone areas and there are several on and near Capitol Hill. There is a chance you are reading this and live right within one of them and never knew it because most days, it’s not all that relevant.

Liquefaction is a well documented phenomenon that is most often noted during earthquakes. When loose, water saturated soils are violently shaken, they lose their strength and become unstable, liquidy messes. You can watch a cool video demonstrating the process here, but liquefaction doesn’t always look exactly the same and can result in water jetting to the surface or grade destabilization when the liquified soil isn’t on the surface. I would put it on the level with erupting volcanoes – cool, and also I’d rather not be too nearby when it happens.

Some of the liquefaction-prone zones across Seattle make sense.

Low-lying shorelines that flirt with the water table, like areas around South Lake Union. Or places that aren’t even glacial till, but instead fill from the city’s early garbage and regrading, like the Seattle Waterfront.

Others are less easily recognizable, particularly because with our layers of concrete and personal vehicles, subtleties in topography can be mostly ignored. However, if there’s glacial till and water collects there, even water that slowly seeps away, liquefaction is a concern.

A map showing Liquefaction prone areas (the yellow outline areas) on and nearby Capitol Hill. The red marker is the zone near Broadway, the blue marker is the areas stretching along 22nd and 23rd Ave, and the black marker is in Madison Valley. Data Credit: City of Seattle GIS. (Image: Brendan McGarry)

One Liquefaction-prone area on the Hill is a blob that stretches from Thomas to Mercer east of Broadway. The topography of this part of the Hill is subtle, but this little stretch is just low enough that it collects water from the surrounding high points before it slowly drains into Lake Union. This little dip is noticeable if you go looking on foot, but it’s easy to ignore driving by. In the last glacial recession, this was likely a small body of freshwater, dammed by ice and soil before it slowly drained away over thousands of years.

Another zone is present from just north of Meany Middle School, stretching in a narrow band adjacent to 22nd and 23rd Ave, south to Garfield High School. I tend to think of anything East of 17th Ave as straight downhill into Madison Valley (the largest Liquefaction-Prone area near the Hill and a known collection point for water, evidenced by the Madison Valley Stormwater Storage Facility) but it’s not exactly a straight luge. What topography demonstrates is that water hits a subtle little trough, a shoulder of land that collects water on the way to Lake Washington. You can notice it as you drive along 23rd if you pay attention. Just like the other section off Broadway, recessional lake deposits (left during the last glacial maximum) demonstrate that water probably sat above the surface here thousands of years ago.

If you live in one of these liquefaction zones, you might find this alarming. But such is life in this post glacial, fault adjacent landscape. We can be aware and make wise choices in our buildings and land use. The naturalist in me wants to know what these little spots were like before Seattle really got going.

Were there bog-loving plants? What would the air have held a unique mix of fresh and the pungently organic? How many more animal voices would have been audible than today? Aldo Leopold’s thinking feels apt here: “One of the penalties of an ecological education is that one lives alone in a world of wounds.” But I disagree with how Leopold continues with this idea: that being ecologically minded is a solitary endeavor. We are not alone, and despite the phrase being watered down by overuse, we are in community. How we choose to care for our local ecosystems and how we respond to natural disasters, when they inevitably come, should be communal. We’ve inherited a lot from the past, both unchangeable geologic truths and a highly altered surface filled with the joyful places and cultures that inhabit Capitol Hill. Neither can be ignored, and both are entirely relevant to our daily lives and our futures.

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