When you add it up, it's actually a major geological hazard. As more people move into more mountainous environments, the opportunities for interactions between human infrastructure and people, and landslides, increase. —David Montgomery, geology professor
JACKSON, Wyo. — What's happening in this Wyoming resort town might be better described as a land creep than a landslide, but the lack of speed has not hindered the sheer power of the moving earth.
Over the past two weeks, a piece of East Gros Ventre Butte has slowly collapsed toward the west side of Jackson — shearing one hillside home in half, threatening to devour several others and looming ever more ominously over a cluster of businesses below.
No one can say precisely when the mountainside will cease its slow droop into Jackson or finally give way. But it appears increasingly likely that it's going to take a piece of Jackson with it.
Emergency workers have tried in vain to shore up slow-moving slope, attracting a steady parade of the curious and camera-wielding gawkers.
"We don't know what Mother Nature wants to do here. She's shown us quite a bit," Jackson Fire Chief Willy Watsabaugh said as he stood at the edge of the slide zone, its rocky slope rising sharply behind him.
The rate of movement slowed Saturday, giving crews a chance to get back in and reassess the damage, Watsabaugh said. The chief said he's seen many slides in the mountains around Jackson but never one in town.
Town officials first noticed significant hill movement on April 4. They evacuated 42 homes and apartment units on April 9.
Workers and residents had watched helplessly on Thursday and Friday as a sudden acceleration of movement prompted authorities to suspend their efforts to shore up the slope as falling rocks created a hazard.
Work resumed over the weekend with a new focus: repairing some of the damage the slide already has caused, including a break in a sewer line on Friday. On Saturday evening, officials postponed the sewer line work until Monday because of access issues.
Bart Moudy, a construction manager from Etna, a town south of Jackson, said he has been keeping a close eye on the slide as the small cracks initially seen at the top of the slope widened.
"It's a little reminder of where we live — in a dynamic region," he said. "It's amazing. We were looking at on Friday, and it's moved a bunch since then."
Authorities are looking into whether recent construction at the foot of East Gros Ventre Butte made the slope unstable. They say there could be a variety of other causes, including prior construction at the site, warmer weather and a wet winter that put more water into the ground, where it acts as a lubricant for unstable rocks and soil.
By Saturday morning, the shifting earth had caused bulges in a road and a parking lot at the foot of the hill that were as big as 10 feet. The groundswell pushed a small town water pump building 15 feet toward West Broadway, the town's main drag.
Because of its more stable geology, the slope is unlikely to suddenly collapse like the March 22 landslide in Oso, Wash., that killed 39 people, experts said. More likely, large blocks of earth would tumble down piece by piece.
The ground had been moving initially at a rate of an inch a day. That's is expected to speed up as time goes on, said George Machan, a landslide specialist consulting for the town.
Rockslides are common in the surrounding Rocky Mountains in the spring, when melting snow and warmer weather unleash the region's dynamic geology. In the early 1920s, a massive slide caused by heavy rains north of Jackson formed a natural dam across a small river. The dam gave way two years later, unleashing a flood that killed six people.
But other factors appear to be in play on East Gros Ventre Butte, a small mountain that looms over the west side of town, its base dotted with homes and businesses.
The area of the landslide has been graded for roads and businesses in recent years, including a new Walgreens. That could have weakened the hillside and set the stage for its collapse.
Landslides in scenic, mountainous areas like Jackson are a lot like the wildfires that occur in the same areas. Both hazards are natural events that present more of a problem when people move in and build subdivisions or shopping areas.
"When you add it up, it's actually a major geological hazard," said David Montgomery, a geology professor at the University of Washington in Seattle. "As more people move into more mountainous environments, the opportunities for interactions between human infrastructure and people, and landslides, increase."