On November 20, 2014, from 11 am to 1 pm, visitors in Zion National Park heard several large booms and felt a vibration. The University of Utah's Seismology Station inside Zion told the St. George News that it wasn't an earthquake. They're not sure what it was.
Zion does sit near a few of fault lines in southwestern Utah where very minor earthquakes are not uncommon. However, Zion is also not too far from Nevada's Nellis Air Force Base where combat training exercises have been conducted during that time period. The training exercises are the most likely cause of the rumble.
When the Zion area experiences earthquakes they are usually less than 1.0 on the Richter scale. Click to see a Seismic Station map of the Zion area with the most recent quakes. In the week following the reported vibration, four earthquakes occurred in the area ranging from 0.2 to 1.3 in scale. The closest one to Zion was near St. George, Utah at 1.0.