July 8-10, 2019: Mt. Hood Sees Very Energetic Swarm

During July 8-10, 2019, an extremely energetic earthquake swarm broke out at Mt. Hood. Swarms at Mt. Hood, especially in this location, are quite common. However, they usually do not carry this much energy in such a short time frame. Most swarms in the past are part of local tectonics, but the characteristics of this swarm may indicate something more. Please click the title of this post or "read more" to continue...

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USGS EQ Map

Mt. Hood Station Map

First off, if you wish to see all reported earthquakes for the entire swarm period at Mt. Hood, please use the USGS EQ map button above. There is also a 2nd button which shows all available stations in the Mt. Hood area.

July 8-10, 2019 was a very spectacular day for seismicity at Mt. Hood. Magnitudes were not large, never exceeding M2.1 or so. Mt. Hood has seen swarming in the past with much higher magnitudes. However, Mt. Hood rarely, if ever, sees earthquake swarms occurring in such rapid succession you can barely count them all.

Although the earthquake swarm started on July 8, 2019 and lasted only two days, the vast majority of seismicity was concentrated within two narrow time periods on July 9th and July 10th. For this post, and the plots below, I will only deal with those two specific episodes of seismicity.

Station TIMB was the closest station to this earthquake swarm. However, it did not detect these events as well as TDH did. TDH will be the station I use for the seismic plots and audio below. On the helicorder to the right of this text, you can clearly see two intense periods of seismicity for July 9th and July 10th. Again, magnitudes were not as large as seen before at Mt. Hood. But the energy and quick succession of these events suggest possible fluid migration and triggering of the local fault system in this area. This is for sure one for the record books!

July 9-10, 2019 Mt. Hood swarm
 
Start of first burst: July 9, 2019 at 15:47UTC
End of first burst: July 9, 2019 around 22:39UTC (about 6 hours and 52 minutes; though some quakes occurred between bursts)

Start of second burst: July 10, 2019 at 02:56UTC
End of second burst:  July 10, 2019 around 05:58UTC (about 3 hours and 2 minutes; though some tiny quakes occurred after this time)

Total earthquake count (includes even the tiniest, unreported events): This is determined by analyzing the P/S wave arrivals from 2 of the closest stations to this swarm. Some were so tiny that they only appeared on TDH. However, you can easily tell if something is an earthquake by experience and judging P/S wave arrivals. Remember, there is a difference between an obvious earthquake and a locatable earthquake. For the first burst of swarming, there were approximately 207 earthquakes in total, counting negative magnitude earthquakes as well. For the second burst of swarming, there were approximately 91 earthquakes of all sizes, including negative magnitude quakes. That means there were a total of approximately 298 earthquakes of all sizes during the two bursts of seismicity. This number may be slightly larger, seeing it was hard to separate some of the events because many were occurring very quickly. Most of the earthquakes that were unreported likely were not able to be located. However, they were earthquakes nonetheless.

Reported count: For the time period stated above, for both bursts, USGS/PNSN reported 83 earthquake events, including a few tiny aftershocks after the second burst. If you count all earthquakes within the 2 days of seismicity, from July 8 through July 10, they reported 111 earthquakes. Please refer to the USGS EQ map button in the beginning of this post to see all reported earthquakes. Again, I am only dealing with the two main bursts of seismicity for July 9-10, 2019 for the plots below.
Largest reported earthquake of this whole swarm (both bursts): A total of 4 M2.1 events between 3.6km and 4km in depth.
Severity: Major (major for the Mt. Hood area; magnitudes were not large, but the energetic nature of this swarm makes this one of the most energetic swarms at Mt. Hood; possible magmatic fluid migration, along with tectonic activity)


As of right now, July 27, 2019 at 03:00UTC, random aftershocks from this swarm still continue under Mt. Hood.

Helicorder Plots

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First Burst

The first main burst in seismicity occurred at 15:47UTC on July 9, 2019 and lasted approximately 6 hours and 52 minutes. There were a total of approximately 207 earthquakes of all sizes in this first burst alone. The slideshow below contains 61 images.

The following is the seismic audio of the first burst, taken from the seismic station used for the plots above. All seismic audio is best heard when it is sped up. Please use your headphones but be wary of the volume! Every pop or crack you hear is an earthquake. Enjoy!

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Second Burst

The second burst of seismicity started at 02:56UTC on July 10, 2019 and lasted approximately 3 hours and 2 minutes. There were a total of approximately 91 earthquakes of all sizes for the second burst. The following slideshow contains 23 images.

The following is the seismic audio of the second burst, taken from the seismic station used for the plots above. All seismic audio is best heard when it is sped up. Please use your headphones but be wary of the volume! Every pop or crack you hear is an earthquake. Enjoy!