The "fanquakes" generated at Seattle Seahawks games may be a symptom of the poor soil strength of the area and the stadium's unique construction moreso than a symptom of extraordinarily passionate fans

I was watching the Seahawks-Saints game the other night and the commentators were talking about the "fanquakes" when something hit me: the "fanquakes" might only be observed because of the poor soil strength in the area and the unique construction of the stadium.

For background, CenturyLink Stadium(I'm calling it that and not Lumen Field), where the Seattle Seahawks play, has been known to generate 1.0 to 2.0M earthquakes when the fans move a lot in response to a big play or event. These quakes are measured by a seismometer across the parking lot from the stadium.

CenturyLink Stadium is built on some of the worst soil in the area(right above the "SODO" label). This land used to be tidal mud flats, but, in the early 1900s, large portions of Seattle were regraded. This dirt was conveyors away to fill the mud flats and reclaim the land. This land was then used to build the SoDo district and Harbor Island.

Almost 100 years later, a new stadium was built on the soil. To account for the weakness of the soil in a seismic event, about 2,000 pilings extending 50 to 70 feet under the stadium were added.

When fans get moving at these games, the stadium starts to really shake. The stadium being on weak soil alone would have generated some load stresses to create waves in the soil, but with the pilings helping by essentially acting almost as sub-surface antennae to effectively propagate the waves, the stresses are propagated so well in the poor soil that it ends up creating a measurable earthquake a couple hundred feet away.

I'd love to hear input on this from anyone with a background in geophysics, soil, or seismology! It might not be entirely fleshed out, but I think the general concept has some merit to it. I may be overlooking something, though. Maybe the pilings are at a length that demonstrates mechanical resonance with seismic waves in the soil that adds to it? I'm not sure.

Either way, I'm starting to wonder if the "fanquakes" should be more of a warning sign of the soil quality of the area than a thing to celebrate.

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