SEATTLE - At the University of Washington's Seismology Lab, professor emeritus Steve Malone points out a cluster of red dots on a computer map. The 15 dots in the vicinity of Olympia are windows on a small earthquake that happened deep underground on Tuesday.
Since scientists installed eight new arrays of seismometers on the Olympic Peninsula last summer, these kinds of "slow-motion earthquakes" called tremors are showing up in abundance. But unlike the shaking events we normally think of as earthquakes, nobody feels anything.
It was only about 10 years ago when scientists first discovered another kind of tremor, a deep tremor also called an ETS, standing for Episodic Tremor and Slip.
Scientists like Malone aren't sure if these are evidence of the Juan De Fuca plate, a large piece of the earth's ocean floor crust slipping under our part of the North American tectonic plate, or if the tremors are the result of water causing the phenomenon 30 to 50 miles below our feet.
What scientists learned about the ETS events is that they are predictable, occurring every 14 to 15 months. The last one started in April and ended on May 24. Like their smaller cousins, nobody feels them, no crockery falls off the shelves. But the energy they release is on the scale of a large earthquake, about a magnitude 6.5.
Again, it's the fact that this earthquake takes so long to happen dissipates that energy with nobody knowing. The newly discovered small quakes are magnitude threes and can last from a few hours to a day.
If nobody notices them what's the big deal? The catch is that scientists believe there's a connection between the tremors and the "big one." Because as the Juan De Fuca plate is forced underneath the North American plate, the two plates are locked together, giving way every 300 to 500 years. It's feared one or more of these silent tremors could add enough extra strain to trigger a catastrophe on the scale of a magnitude nine.
"The locked zone, when that slips, that goes in a not so silent earthquake." said Malone. "A big mega earthquake. And the relationship between the two of those we don't quite understand. But we're getting closer."