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I was appropriately beaten to the punch by @earthquakeguy for this #seismogramsaturday, and he's much more of an expert on this topic than I, so please look at his post first. I'll add a few of my own figures below.
Today we're talking about the tremor pulses that go hand-in-hand with slow slip during Episodic Tremor and Slip (ETS) events. These are fascinating events in subduction zones that are well described here: earthquakescanada.nrcan.gc.ca/pprs-pprp/pubs…
Some of you may have seen the tremor maps created by seismologist extraordinaire Aaron Wech, and shown on the @PNSN1 page at pnsn.org/tremor.
If you're not familiar with ETS tremor, these figures can be scary because it looks like we've had THOUSANDS of quakes in the Pacific NW. 
In actuality, these points don't really represent discrete quakes. They are tiny, tiny pulses within longer tremor sequences. They are so small that we have to use special techniques to even find them, and they certainly aren't large enough to be felt.
As an example, here's 20 minutes of data from yesterday, from a station on the Olympic Peninsula. The top panel shows background seismic noise--things like wind and surf and cars and all of the things that shake the ground on a normal, quake-free day.
The bottom panel shows the same signal, but with the noise filtered out. Do you see those tiny, tiny little pulses of energy (slightly higher amplitude) between 400-800 seconds? Those are the tremor pulses. They barely, barely shake the ground.
Here's a similar plot. This time the top panel shows a different day with a M2.3 quake that was ~80 km from the station. The bottom panel shows a zoomed-in version of the tremor. You can see from the vertical axis that the tremor pulses are about 100 x smaller than the M2.3.
For what it's worth, only 3 people in the greater Seattle area even reported feeling that M2.3, and they described it as very weak shaking. So those tremor pulses absolutely cannot be felt.
But the tremor is fascinating, as it helps us understand how our subduction zone works. Again, to learn more about ETS events and slow slip earthquakes, follow this link. earthquakescanada.nrcan.gc.ca/pprs-pprp/pubs…
Some research suggests that large earthquakes are preceded by ETS events. But in the PNW we get frequent (~annual) ETS events, and we haven't had a megathrust quake in 319 years. This means that MOST ETS events do not trigger large quakes.
So these events are not cause for worry. But as with all seismic activity, they remind us that we live on an active planet, and if you live in EQ country, it's always prudent to be prepared.
Happy #SeismogramSaturday, my friends. Post your questions here!
