Earthquake Swarm at the End of the Cascadia Subduction Zone: Petrolia, CA

In this post I will show some data and info regarding the earthquake swarm that recently has been occurring near Petrolia, California where the Cascadia Subduction Zone ends. This was a moderate swarm but did contain some good sized earthquakes. This post will deal with the activity between 00:00UTC February 2, 2019 to 04:47UTC February 4, 2019. Also, this swarm was associated with a decrease in seismic activity throughout the United States (except for Montana and Petrolia, CA). As always, please click the title of this post or "read more" to continue.

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Around the start of the new UTC day for February 2, 2019, an interesting earthquake swarm started to break out near Petrolia, California, where the Cascadia Subduction Zone ends and meets the California coastline. Swarms have occurred here before, but to the best of my knowledge this is the largest swarm this area has seen in a long time. Also something strange is that right when this swarm was starting, seismic activity throughout the entire continental United States started to calm. Well, except for Montana which did see a swarm as well. However it was eerily quiet except for those two locations.

The first image above shows the swarm area in respect to the surrounding states. The second image above is a zoomed in look at the earthquakes reported in this swarm in respect to the closest seismic station, which is the station I will use for all of the data below. During the time period stated in the beginning of this post, there were 15 reported earthquake events with many other earthquakes that have yet to be reported.

For example (in regards to the activity on February 3, 2019):
-Between the earthquake reported for 1:38UTC to the quake reported for 7:37UTC, there were approximately 16 events that have yet to be reported. They range in size from about M0.5 to M2.0.
-Between the earthquake reported for 7:37UTC to the quake reported for 22:18UTC, there were approximately 25 events that have yet to be reported. They range in size from about M0.5 to M2.0.

Below I will first show the helicorders in slide show format. Then I will show the event pages and plots for the 3 largest earthquakes during this swarm. After that I will show 7 extra 3-plot images in slide show format. Please pay attention to chart labels and any captions beneath any images. The event pages you see below, generated by USGS, might be slightly different as time moves on. So the events pages shown are as of 5:57UTC February 4, 2019. Also many of the earthquakes that occurred contained elongated codas. That simply means the "end tail" of some events were much longer than what you would expect.

Please CLICK HERE to visit the USGS EQ map for this time period and location.

This was the largest earthquake during this swarm and was felt by over 262 people! Remember the felt count on any USGS event page is ONLY the number of reports. Not everyone knows how or cares to report feeling an earthquake to USGS. Therefore, when seeing multiple felt reports, it is likely the real number of people that felt it is much higher. These were some interesting earthquakes! So why did a swarm strike Lincoln, Montana and Petrolia, California while the rest of the continental USA pretty much went silent?