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The Mystery of the Noon Booms Part 2

By California Volcano Observatory July 5, 2023

In last week's post (https://www.usgs.gov/observatories/calvo/news/mystery-noon-booms-part-1), we challenged you to give us your best hypotheses as to what was causing the 'Noon Booms' CalVO seismometers were picking up.

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Screenshot of announcement which reads: "New Bomb Operations Monday through Thursday June 26 - June 29, 2023, between the hours of 7am-5pm, weather permitting. You may hear loud noises, see smoke, or feel vibrations."
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Screenshot of munitions testing announcement from Hawthorne Army Depot's Facebook page.

There were some great ideas (heat effects, sonic booms, mining operations) and some less plausible ones (aliens), but in the end a few of you came up with the same conclusion as our seismologists. (Drum roll...)

If you guessed munitions detonation at Hawthorne Army Depot, you're bang on!

According to our seismologists, the signals picked up by our seismometers are consistent with an explosive source located near the Hawthorne Army Depot. These seismograms show Noon Booms on instruments in the Clear Lake Volcanic Field from June 26, 2023 as well as an older example from Long Valley on March 11, 2020. The instruments are ordered by distance from the depot, with dashed lines that follow the speed of sound in air (343 m/s or 767 mph). Atmospheric effects (such as wind direction) affect where we see these signals, which is likely why we didn't see last week's boom on the much closer Long Valley seismometers.

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A series of seismograms with lines showing the recorded motion of the ground at these sites in light gray. The measurements are ordered by distance from the source of the noise, with the closest at the bottom and furthest at the top. Darker, slanted lines show the speed that sound would travel in the air from that source to each of the seismometers, one for each burst, that line up with when the ground shakes. This example shows data from the Clear Lake Volcanic Field on 26 June 2023.
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A series of seismograms shows the geographical progression of acoustic signals caused by munitions detonations at Nevada's Hawthorne Army Depot (26 June 2023).
Media
A series of seismograms with lines showing the recorded motion of the ground at these sites in light gray. The measurements are ordered by distance from the source of the noise, with the closest at the bottom and furthest at the top. Darker, slanted lines show the speed that sound would travel in the air from that source to each of the seismometers, one for each burst, that line up with when the ground shakes. This example shows data from the Long Valley Caldera on 11 March 2020.
Sources/Usage: Public Domain. View Media Details
A series of seismograms showing acoustic signals from munitions detonations at the Hawthorne Army Depot. Signals were observed on 11 March 2020 in the Long Valley Caldera.

The Hawthorne Army Depot is actually located in Nevada about 50 miles north of the Long Valley Caldera. According to their website, they "store conventional munitions, demilitarizes and disposes of unserviceable, obsolete, and surplus munitions". An example of one of the controlled detonations can be seen in this video at 1:38: https://youtu.be/6Zhjkh_KA7c?t=98

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