SALT LAKE CITY (ABC4) — An earthquake shook the area near Zion National Park late last week, and more rounds of wet weather are expected in the coming days, as the remnants of a powerful atmospheric river wash over Utah.
This had the ABC4 newsroom wondering: Does this set the stage for rockfall in the iconic national park?
“The two together is probably not an ideal combination,” said Sam Hudson, a geology professor at Brigham Young University. “Luckily, so far, we haven’t seen any mass wasting or destruction from this earthquake, but I think it’s a valid concern.”
The magnitude 3.5 earthquake happened just before 7:30 p.m. Friday night, about two miles east of Springdale, the town nearest to the park’s southwest entrance.
Katherine Whidden, a research scientist with University of Utah seismograph stations, said an earthquake that strong could cause a home to slightly shake, as if a large truck were passing by or kids were roughhousing inside.
“Those are usually the first things people think of before realizing that, ‘Oh, it’s an earthquake,’” she said.
An earthquake of that magnitude is not uncommon near Zion National Park, which sits near the Hurricane Fault, the most active and earthquake-prone fault in southern Utah.
“It’s a place that’s geologically unstable,” Hudson said. “It’s a place that we love because of its ruggedness and huge cliffs … but it is prone to earthquakes and landslides and failure.”
He said that Friday’s earthquake could be a catalyst for failure in the future, noting that the heavy recent rainfall and the wet weather expected this week could lead to instability in the area.
Tyler Knudsen, senior geologist with the Utah Geological Survey, said prolonged precipitation can result in water collecting on the surface of the rocks, which can undercut and erode the steep slopes.
“So if you’ve got already steep terrain and add moisture to that, it can weaken and help destabilize the slope, and then you add onto that seismic energy and that can be a trigger for slope movement,” he said.
He described Zion National Park and the area near Springdale as essentially a deep canyon, with enormous, steep cliffs faces and fairly weak rock units.
“It is kind of landslide and rockfall country,” he said.
However, Knudsen noted that rockfalls and landslides generally happen immediately following an earthquake.
Such was the case in Sept. 2, 1992, when a magnitude 5.8 earthquake happened in St. George, triggering a large landslide in Springdale.
That landslide destroyed three homes, disrupted utility lines, threatened other buildings, and led to a temporary closure of Zion National Park’s southwest entrance, according to the U.S. Geological Survey.
“That was pretty eye-opening to a lot of people, because the epicenter of that earthquake was about 30 miles away, and so it just kind of showed just how unstable that slope was already,” Knudsen said.
ABC4 reached out to National Park Service in Zion, but they were not available for an interview on Monday. In a note, a spokesperson said there wasn’t great cause for concern related to last week’s earthquake.
The only current closure in the park related to rockfall stems from an incident in November, when there was a rockfall near Weeping Rock. The shuttle stop, parking lot and trail have been closed since then.
