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Listen to an Earthquake Expert Explain the Recent Rollers

Did you feel the December 20th 6.4 earthquake? For those of us in Humboldt, we rattled and rolled our way into the new year. EPIC’s Tom Wheeler talked with Dr. Lori Dengler, professor emeritus of geology at CalPoly Humboldt, about the quake for the EcoNews Report. Subscribe to the podcast on Apple Podcasts or Spotify for weekly environmental news roundups!

More than Magnitude: Get to Know Peak Ground Acceleration

After the earthquake, many of us wondered: how was that only a 6.4? (I have been in a 6.8, my own personal record, which felt far more tame than the recent quake.) Seismic magnitude is a measure of energy released during an earthquake, and is a poor indicator of how an earthquake is experienced.

For that, one metric we can turn to is “peak ground acceleration”—a scientific description of the “shakiness” of an earthquake. Peak ground acceleration measures, in g-force, what its name suggests: the change in acceleration at ground level, or the change in velocity measured over a period of time. Think of it like being in a car. From a stop, if you get to 60 mph gradually, you won’t feel much. If you gun it, you’ll feel a sudden rush of gravitational forces sucking you into your car seat.

The December 20th earthquake released its energy suddenly, so its energy was quickly released in a short burst of shaking.

Smaller Earthquakes Don’t Reduce the Odds of “the Big One”

A common misconception is that smaller earthquakes release pent up energy, making the odds of a “big one” less likely. While comforting, these two kinds of earthquakes are unrelated. The December 20th earthquake was a “slip/strike” earthquake, the standard fare for earthquakes in our region. These are when two almost vertical faults move horizontally. The “big one”—a subduction earthquake capable of magnitudes in excess of 9.0—is a wholly different kind of earthquake, where one tectonic plate is forced under another.

Earthquakes beget earthquakes, as Humboldt has felt, with over 270 aftershocks recorded since December 20th. Slowly, plates stabilize and the odds of aftershocks drop until we return to a kind of “normal.”

Why Did Rio Dell Suffer So Much Damage (When Other Areas Were Closer to the Epicenter)?

Simply, Rio Dell suffered the largest impacts from the earthquake because Rio Dell experienced the worst shaking—1.46 gs of peak ground acceleration compared to a little under 1 g in nearby Fortuna. While the epicenter for the December 20th earthquake was closer to Ferndale, the earthquake moved along a fault that points towards Rio Dell. Think of it like a rock hitting your windshield, causing a crack. Where the rock struck is the epicenter, but the force of the impact proceeds to spread elsewhere, cracking the windshield. In this rough analogy, Rio Dell was in the path of that “crack.” Rio Dell is also prone to quite rolly quakes, as the city sits on a large bed of sediment that can help to amplify earthquake waves.

The Econews Report podcast is a weekly environmental news roundup produced in Arcata, California by the Green Gang: Tom Wheeler/Environmental Protection Information Center, Caroline Griffith and Larry Glass/Northcoast Environmental Center, Alicia Hamann and Scott Greacen/Friends of the Eel River, and Jen Kalt/Humboldt Baykeeper.


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