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Just in time for the Fourth of July, CalVO scientists have been seeing some interesting - but unusual - acoustic signals on our seismometers. 

A spectrogram made up of five bars dark blue background, each overlain by a black-and-white helicorder trace. About halfway across each bar, a series of small, regularly-spaced arrowheads appear on the helicorder, with bright aqua stripes appearing below each in the spectrograms. The stripes are brightest between 5 and 10 Hz.
Spectrogram and helicorder signals of "Noon Booms" recorded at California's Clear Lake Volcanic Field on 26 June 2023.

Endearingly called "the noon booms" by our duty scientists, these signals are short, repetitive bursts that decay quickly. Observed on our seismic network for over a decade, they tend to occur around midday - hence their nickname - and can show up on multiple seismometers across the entire state. They most often appear on the Long Valley area network, but are sometimes visible as far away as western California and southern Oregon. They aren't necessarily audible at that distance, though. The 'booms' lose higher frequencies quickly and what's left rarely exceed 15Hz, which places the remaining signal in the infrasonic range.

How can we be sure they're not earthquakes? First, the waves move slowly from station to station because they are traveling in the air rather than through the ground; we can tell this from their arrival times at stations in different locations. The signals also occur around the same time of day with a very characteristic pattern, which means they are unlikely to be coming from nature.

Do you think you know what's making California's Noon Booms? Put your answers in the comments or tweet us @USGSVolcanoes! And be sure to tune in next week for the answer...

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