Sept. 2-3, 2019: Strongest Swarm In Over 2 Decades Hits Mount Shasta

Start on Sept. 1, 2019, seismicity started to increase under the ESE base of Mt. Shasta. Only a few small quakes struck not surpassing M2.1. However, on Sept. 2, three large bursts of seismicity were detected in the same location at Mt. Shasta. The entire swarm, including the 3 bursts, was not large compared to other locations around the United States. But it was a major swarm for Mount Shasta especially when Shasta rarely sees much seismicity.

Please click the title of this post or "read more" to continue.

Sept. 2-3, 2019 Shasta Earthquake Swarm

USGS EQ Map for this Swarm

Mt. Shasta Station Map

Starting primarily around 17:34UTC on September 2, 2019, a major swarm broke out at Mt. Shasta in Northern California. It wasn't major compared to other swarms in other locations around the world, but it sure was major for Mt. Shasta. Shasta is usually extremely quiet so swarms like these are somewhat rare.

The swarm saw mainly 3 bursts of seismicity with the 2nd burst striking like a "rapid-fire" swarm that we see at Yellowstone or Long Valley from time to time. This swarm is also one of the largest for Mt. Shasta since at least 1993, around 26 years ago. According to geologists, the last major swarm for Mount Shasta was in 1993 with likely over 50 events recorded in 1 month (LINK).

On Dec. 19-20, 2008 there was a similar swarm but it was much weaker and didn't last as long as the most recent swarm. I will detail the Dec. 19-20, 2008 swarm in just a minute.

Also, for your information, Shasta is an andesitic to dacitic stratovolcano and is the largest stratovolcano in the continental USA in terms of volume. It supposedly erupts every 600-800 years on average based on the past 10,000 years of volcanic activity. The last eruption was supposedly 200-300 years ago, around 1786 or so, and that means we won't see an eruption for a while, right? Well, as of August 6, 2019, the 1786 eruption has been erased from the record. That means we could be in the warning time frame of the next Shasta eruption. I am not saying this swarm will lead to that, but we should monitor Shasta closely over the next few decades.

CLICK HERE to visit the link about how Shasta's 1786 eruption actually didn't happen.

Below I will show some information and data pertaining to this earthquake swarm. Usually I include seismic audio for earthquake swarms, however NCEDC does not have an audio converter like the IRIS Time Series builder does.

Sept. 2-3, 2019 Mt. Shasta swarm:
 
Start of first burst: Sept. 2, 2019 at 17:34UTC
End of first burst: Sept. 2, 2019 at 18:16UTC (about 42 minutes)
Total earthquake count for first burst: ~24 events

Start of second burst (largest burst): Sept. 3, 2019 at 00:14UTC
End of second burst (largest burst):  Sept. 3, 2019 around 00:32UTC (about 18 minutes; though some tiny quakes occurred after this time)
Total earthquake count for second burst (largest burst in terms of time vs quantity): ~39 events

Start of third burst: Sept. 3, 2019 at about 03:59UTC
End of third burst: Sept. 3, 2019 at about 06:56UTC (2 hours and 57 minutes)
Total earthquake count for third burst: ~47 events

Start of entire swarm: Sept. 2, 2019 at 17:34UTC
End of entire swarm: Sept. 3, 2019 at about 06:56UTC
Total earthquake count for entire swarm: ~125 earthquakes of all sizes if you count the some events in between the 3 bursts.

Reported count: For the time period stated above, for all 3 bursts, USGS reported 28 earthquake events. If you count all earthquakes within the total 3 days of seismicity, from Sept. 1 through Sept. 3, they reported 32 earthquakes. Please refer to the USGS EQ map button in the beginning of this post to see all reported earthquakes.
Largest reported earthquake of this whole swarm (all 3 bursts): M2.7 at 7.1km in depth at 17:34UTC on Sept. 2, 2019.
Severity: Major (This is a major swarm for Mt. Shasta, seeing historical seismicity is usually very low.)
Closest seismic station to swarm: LBR in the NC network
Depth of swarm: Between 7.1km and -0.8km in depth.

Below is a quick video showing the depth and spacings of each event via the Interactive 3D Earthquake Viewer which you can find HERE:

Below are the helicorder plots from 4 stations around Mount Shasta. Remember, LBR is the closest station.

Below are seismic plots of the Sept. 2-3, 2019 Mt. Shasta earthquake swarm (39 images within the slideshow below):

Also, about 8 1/2 hours prior to the earthquake swarm, a very strange emergent tremor-like event was spotted on seismic stations around Mt. Shasta. It was not of local, regional, or teleseismic origin and appears solely around Mt. Shasta. Was this event possibly a precursor to the earthquake swarm? Or was this an avalanche? Below is the event from 3 select stations around Shasta.

Sept. 2-3, 2019 swarm in relation to the Dec. 19-20, 2008 swarm.

Again it seems like the Sept. 2-3, 2019 swarm was the largest since 1993 for this volcano. However, I tried to find other swarms since that have come close. The only one I was able to find was the Dec. 19-20, 2008 earthquake swarm. You can see both swarms labeled via the image above.

If you wish to see the USGS EQ map for the 2008 swarm, please CLICK HERE.

Below is some information about the 2008 swarm. As you can see, this swarm was much smaller than the Sept. 2019 one.

Dec. 19-20, 2008 Mt. Shasta swarm:
 
Start of first burst: Dec. 19, 2008 at 17:03UTC
End of first burst: Dec. 19, 2008 at 17:05UTC (about 2 minutes)
Total earthquake count for first burst: No more than 6 events.

Start of second burst (largest burst): Dec. 19, 2008 at about 17:46UTC
End of second burst (largest burst):  Dec. 19, 2008 around 18:16UTC (about 30 minutes)
Total earthquake count for second burst (largest burst in terms of time vs quantity): ~23 events

Start of third burst: Dec. 20, 2008 at about 03:52UTC
End of third burst: Dec. 20, 2008 at about 04:52UTC (1 hour)
Total earthquake count for third burst: No more than 10 events.

Start of entire swarm: Dec. 19, 2008 at 17:03UTC
End of entire swarm: Dec. 20, 2008 at about 04:52UTC
Total earthquake count for entire swarm: ~39 earthquakes of all sizes.

Reported count: For the time period stated above, for the entire swarm, USGS reported 12 earthquake events. Please refer to the USGS EQ map button in the beginning of this post to see all reported earthquakes.
Largest reported earthquake of this whole swarm (all 3 bursts): 3 M2.1 events between 7.4km and 4.7km in depth.
Severity: Minor
Closest seismic station to swarm: LGB in the NC network
Depth of swarm: Between 8.3km and -1.7km in depth.

Below are helicorder plots from 4 of the closest stations to the 2008 swarm:

Below are seismic plots of the 2008 swarm from the closest seismic station: