From the Pacific Northwest Seismic Network's blog:
Earthquake swarm NE of Bremerton
The Seattle Fault Zone runs West to East right through downtown Seattle and has the potential of being the source of a large (M6+) earthquake. It was therefore quite exciting when a small earthquake (Ml=3.3) happened right below the Seattle Fault Zone near Bremerton last week and was followed by aftershocks, and then again by a Ml=3.4 earthquake and another Ml=3.6 earthquake last night (as I am writing this our prototype Early Warning App is telling me another earthquake just happened, it is 10:57PDT on the 11th, this one turns out to be near Whidbey Island instead).
Rather than call it a mainshock-aftershock pattern, we can now call it a swarm, which basically means a cluster of earthquakes close in space and time that doesn't have an obvious mainshock. Swarms of small earthquakes are not unusual in our region. In fact, I found at least 6 more PNSN blog posts about swarms in WA or OR (links at the bottom of this post). However, this swarm is interesting in that it might be related to the Seattle Fault Zone.
See the mapview below for locations of the earthquakes in this swarm. Important to know is that the smallest dots on the map are probably not in the correct location. These earthquakes are so small that they are not well-recorded on all the nearby seismometers, which makes it difficult to locate them well. We are quite convinced that these ocurred in the same area as the bigger ones (except for one, but more about that later) because their waveforms look similar to the bigger ones on those seismometers that did see all of them.
The joys of living in a very geologically active area.
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