SALT LAKE CITY — More rocks similar to ones that killed a couple when they smashed into their home near Zion National Park could come loose from a cliff, Utah geologists warn.
A 375-foot high sandstone slab near the former rock slide now threatens to flatten homes below, according to the Utah Geological Society report released Thursday.
When it comes to hazards of falling rock, ‘‘some are annoyances, some are deadly,’’ said Bill Lund, a senior scientist at the Utah Geological Survey. ‘‘And we’re dealing with one of the deadly ones here.’’
There is ‘‘no question,’’ said the report, that future rock falls will hit Rockville, population 250. The report comes from a team of state geologists who surveyed the December wreckage and nearby cliffs on foot and from a helicopter.
Rockville Mayor Tracy Dutson declined to be interviewed but said in an e-mail that the town would use the report to help current residents and potential homeowners understand the risks.
The warning comes less than two weeks after at least 30 people were killed by a mudslide in Washington state.
In mid-December, officials recounted, an explosive boom sent up a cloud of dust just before dinnertime on a Thursday evening, killing the couple and crushing their small chalet, garage, and the front half of a sport utility vehicle.
Other looming rock masses and crags could unhinge at any moment, Lund said, tumbling down onto a string of homes in and around Rockville, about 250 miles south of Salt Lake City.
Residents should pack and move, they caution, or understand they could die if they choose to stay.
Rockville faces higher risk than neighboring Springdale and La Verkin because the canyon walls taper in there, forcing homes closer to cliffs.
‘‘They just don’t have a lot of room to maneuver in that town,’’ Lund said.
In December, the massive slab gave way then shattered when it hit an underlying slope a bit steeper than a typical mall escalator, the report found. Boulders raced down onto the Main Street home.
The largest boulder to slam the home weighed 520 tons, about as much as 30 gravel-filled dump trucks.
It marked first time anyone in Rockville has died, but rocks have fallen six times in the last 13 years, twice striking a building, the report finds. Since 1850, Utah has counted at least 20 deaths from falling rock.
An earlier report from the same team of geologists in 2011 pointed out hazardous areas, which included the home destroyed in December.
Efforts to brace against the falling blocks, including fences, or bolts into cliffs, likely won’t prevent future slides, it found.
The report recommends the town try to buy up homes and lots in dangerous areas but says it is unknown whether federal, state, or other sources could help relocate residents.