Sources/Usage: Public Domain. View Media Details

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...