A Perspective on Tremor Activity

A Perspective on Tremor Activity

January 7, 2016

by Aaron Wech

With all the tremor activity occurring in the Pacific Northwest recently, I thought it important to provide a little perspective.

First a few facts: 

  1. The tremors reported basically represent semi-continuous (very low-level) shaking that accompanies slow slip on the subduction zone fault.
  2. Because they are continuous in nature, they’re difficult to track using the same techniques as with earthquakes, and they are therefore treated differently and reported separately (hence the lack of a traditional magnitude and their exclusion from other regional catalogs).
  3. Because the tremors accompany slow slip, we use them as a marker for when and where the fault is slipping. As you see the tremors migrate over the course of days, you’re really observing the rupture propagation of a very slow earthquake occurring on the interface between the subducting Juan de Fuca Plate and North America. Ultimately it is the slip from this slow earthquake that we care about; the tremors are just a seismic symptom of this underlying process.
  4. It can be alarming to see 427 red dots appear beneath the Washington, but it is important to remember that this kind of activity happens all the time. Big tremor episodes, like the current one, occur repeatedly in Puget Sound every 14 months or so, and elsewhere along the fault with different intervals. 

To round out that perspective: the above image shows the entire tremor catalog for the past 10 years plotting time (x axis) vs. distance along the coast (y axis) from northern California to Vancouver Island. When viewed this way we see that this activity is business as usual. These things, large and small, happen all the time. There are over 300,000 tremor dots plotted there. The big vertical stripes are major tremor episodes that have migrated hundreds of km, like what we’ve observed for the last 3 weeks from Vancouver Island to Washington (seen as a tiny sliver on the right hand side). There have been 9 such events in Puget Sound since 2006, but that’s nothing compared to southern Cascadia. Northern California has tremor activity constantly, and hardly a day goes by when there isn’t tremor somewhere in the subduction zone.

The relationship between slow slip and megathrust earthquakes is unresolved and very much an area of ongoing research. However, a large body of work from Cascadia as well as other subduction zones worldwide (e.g. Japan, Mexico, Alaska, Costa Rica, New Zealand) suggests that these events are an integral part of the greater earthquake cycle. That doesn’t mean they have no effect. We honestly don’t know and need more than 10 years of data to answer that. But it does mean they are neither anomalous nor acutely ominous. The activity is a good reminder that we are sitting on an active fault and need to be prepared, but it’s important to keep a broad spatial and temporal perspective. The current episode isn’t distinctly more anomalous or sinister than the previous one. Or the 8 before that. Or the dozens we’ve observed in northern California. Or the likely hundreds that have occurred all along the subduction zone since that last great earthquake. It isn’t necessarily good. It isn’t necessarily bad. It just happens.