A Colorado expert said today that halting the rescue of six trapped Utah miners because of dangerous ongoing "aftershocks" makes sense, even though quake sensors aren't detecting the shaking.

Settling of unstable earth after the Crandall Canyon mine collapse "could be too small for the instrumentation to detect," said John Bellini, a geophysicist at the National Earthquake Information Center in Golden. "That's entirely possible."

Dangerous "seismic" conditions forced the halt of rescue efforts until until this afternoon and could prevent crews from reaching the men for at least a week, Bob Murray, chairman of Murray Energy Corp., owner of the mine, said Tuesday night.

"Since the original collapse, there have been a number of small, you could call them aftershocks," Bellini said. "After you have a large (mine) collapse like that, the ground is going to be settling or there's some stress changes in the area."

"But the last one of those aftershocks that was able to be recorded and located was about 3:42 yesterday. It was 1.7 magnitude, that was very small," he added. "The largest of these aftershocks was 2.2 magnitude at 1:13 a.m. Tuesday."

Yet, Bellini said there could be hazardous post-cave- in shifting — just not big enough to register on quake sensors.

However, there's an ongoing debate over whether a quake caused the early Monday mine collapse as mine chairman Murray insisted.

He also lashed out at news media for suggesting his men were conducting "retreat mining," in which miners pull down the last standing pillars of coal after mining out an area and let the roof fall in.

"This was caused by an earthquake, not something that Murray Energy ... did or our employees did or our management did," he said. "It was a natural disaster. An earthquake. And I'm going to prove it to you."

Bellini, however, said preliminary analysis indicates the 3.9 magnitude temblor recorded early Monday morning at the time of the shaft collapse was the actual cave-in — not an earthquake.

"The wave forms between an earthquake and a mine collapse have different characteristics," he said. "This event did not have the characteristics of a typical, natural occurring earthquake."

Amy Louviere, a federal Mine Safety and Health Administration, said retreat mining was going on at the mine in a remote canyon 140 miles south of Salt Lake City. She said that exactly what the miners were doing, and whether that led to the collapse, can be answered only by a full investigation.

Retreat mining has been blamed for 13 deaths since 2000.

The government requires mining companies to submit a detailed plan before beginning the process. Murray Energy submitted such a plan and received approval in 2006, Louviere said.

gathrighta@RockyMountainNews.com or 303-954-5486

The Associated Press contributed to this report.