Tonasket Earthquake Swarm or Aftershock Sequence?

At 6:47pm PDT, on April 12, 2025 a magnitude 3.1 earthquake located about 6 km (4 miles) northeast of Tonasket, WA at a depth of ~14 km (8 mile) was reported felt by only about 60 people.  Most of them reported very light shaking.  OK, so no big deal.  But this event was followed within 10 hours by almost 50 detectable, but smaller earthquakes.  See Figure 1.  Fourteen of these events were large enough to be located by the PNSN, meaning they recorded on at least three seismograph stations other than the local station station near Tonasket.  To us this seemed like a normal aftershock sequence that dies out over a few days or a week or so with events decreasing in number and size.  The largest aftershock typically following a large “Main” shock is significantly smaller than the main shock, often a full magnitude unit.  In this case there was a magnitude 2.9 event within 2 hours.  OK, smaller, but not a lot smaller.  When a magnitude 3.2 earthquake took place just a day and half later, we said, OK, looks to be more like an earthquake swarm.  So what is the difference?

Like in a lot of science, geologists and seismologists like to try to summarize observations in easy to understand words that classify sets of observations (data) with the hope of understanding something about a common process. Here are some “definitions” I use for some classifications. (I think most seismologists, though not all, would say these are reasonable definitions):
1. Earthquake sequence - A time series of earthquakes in a general geographic region.
2. Mainshock-aftershock sequence - The Largest earthquake in the sequence is very near the beginning and the rate and size of events decreases over time.
3. Earthquake swarm - A sequence that builds up in number and size over time and then dies out and in which the largest event is not at the beginning and there may be several events of a similar large size.

The recent (on-going?) Tonasket sequence has characteristics of both cases.  Figure 2 is a time plot showing in part A the magnitude versus time and part B showing cumulative number versus time.  It is clear from part A that the largest event was not at the beginning and there have been 6 events just below or above magnitude 3.  So, it looks swarm-like. However, from both part A and  part B it looks like a very typical aftershock sequence where the event rate is dying out over time.

This begs the question,  what difference does it make? And, can we say anything about the likelihood of the sequence continuing or perhaps having a significantly larger earthquake.  Many seismologists would agree that mainshock-aftershock sequences often take place on well established large fault systems that may be capable of large damaging earthquakes.  On the other hand, earthquake swarms take place on smaller faults where stresses are concentrated or influenced by fluids such as at volcanoes, or geothermal areas or where the the rock is highly fractured with many different orientations.

A Washington State Department of Natural Resources fault map does show several faults in the general area but no significant ones where this sequence is located.  Also, unfortunately the distribution of seismic stations is such that getting what is called a “focal-mechanism” that shows the orientation of possible faults causing the earthquake are not adequate to obtain a reliable mechanism.  We feel it is unlikely that this sequence is occurring on a major fault capable of a large earthquake.  However, we don’t know enough to place high confidence in this interpretation.   

As always, it is wise to be prepared at all times for natural hazards such as earthquakes.