When six men got trapped in a Utah coal mine on August 6, the mine’s chief executive declared that the cave-in was caused by a natural earthquake. The University of Utah Seismograph Stations did record a magnitude-3.9 earthquake, but the quake was probably caused by the mine collapse—rather than the other way around.

Seismograph stations recorded a smaller seismic event on Thursday, when a second implosion killed three men who were participating in the rescue effort, including an inspector from the U.S. Labor Department’s Mine Safety and Health Administration. Called a “bump” by mining officials, it is the seismic event recorded in blue on the lower right section of this chart.

Seismologists at the University of Utah, the U.S. Geological Survey and the University of California, Berkeley, say that the downward motion of seismic waves from both events is strong evidence of collapse. The second event happened less than a mile underground, which would be quite shallow for a natural earthquake.

The “bumps” are likely to continue. The roof of the Crandall Canyon mine is held up by pillars of coal. When some pillars fail, that can increase the stress on nearby pillars that are still standing.—Dawn Stover