The mine is built into a mountain in the Manti-La Sal national forest, 225km south of Salt Lake City.
 
US geologists have been investigating whether a 3.9 magnitude tremor recorded early on Monday morning was a result of the cave-in or an earthquake that could have caused the collapse.

Unstable conditions
 
Unstable conditions below ground have thwarted rescuers' efforts to break through to the miners, who have been trapped 1,500ft below the surface for nearly two days, Murray said.
 
The seismic activity and other factors "have totally shut down our rescue efforts underground", he said.
 
"They're digging as much as they can, even with their hands"

Julie Jones, councillor
"There is absolutely no way that through our underground rescue effort we can reach the vicinity of the trapped miners for at least one week."
 
Rescuers have been using heavy drilling equipment and teams inside the tunnels to attempt to dig through to the men.
 
Earlier on Tuesday, Murray said: "There is plenty of air in there for them to survive for weeks and there is water"
 
He said there were 30 pieces of mining equipment in place and 134 people dedicated to the rescue.
 
Cave-in's cause

The National Earthquake Information Centre in Colorado said 10 seismic shocks have been recorded since the collapse, but only one since 3am (0900 GMT) on Tuesday.
 
That one struck at 3:42pm (2142 GMT) with a magnitude of 1.7.
 
Murray has insisted the cave-in was caused by an earthquake.
 
But government seismologists have said the pattern of ground-shaking picked up by their instruments around the time of the accident on Monday appeared to have been caused not by an earthquake, but by the cave-in itself.
 
Natural event?
 
"Based on the information and preliminary analysis we've done so far, this event doesn't look like a natural event. It doesn't have the proper characteristics of a natural earthquake," Rafael Abreu, a geologist for the earthquake information centre, said.
 
"Even though it's not a natural earthquake, it could still generate aftershocks, which is exactly what we're seeing in this particular situation."

Concerns about mine safety in the US rose last year when 12 miners were killed in an explosion at International Coal Group's Sago mine in West Virginia.

Source: Agencies