Ancient earthquakes: Scientists investigate fault line that helped shape Colorado’s Rocky Mountains
share
FRISCO, Colo. — While it is often said that mountains don’t move, in reality, the Colorado Rocky Mountains are the result of a slow dance that has been occurring beneath the earth’s surface for hundreds of millions of years.
The state’s mountains contain thousands of faults, where two blocks of rocks are fractured and move relative to each other, usually slowly, but sometimes suddenly, causing earthquakes.
On Friday, Sept. 6, a group of U.S. Geological Survey scientists scraped and studied the inside of a freshly dug trench just off the Rainbow Lake Trail near Frisco, in search of evidence of mountains moving.
“The big question is — when was the last big earthquake on the Gore Range fault?” Research geologist Alex Hatem said. “When was the last big earthquake that broke the ground surface? That’s what we’re after.”
The trench, which was marked with yellow caution tape and signs declaring “open hole,’ was dug perpendicular to the Gore Range fault, which runs along the east side of the Gore Range and Tenmile Range.
“There’s a few different kinds of fault, this particular kind of fault is called a normal fault,” Hatem said. “It’s a dipping fault, so one side moves down relative to the other. So basically, it raises mountains and deepens valleys.”
A version of the Gore Range fault has been active on and off for hundreds of millions of years, helping first to build the Ancestral Rocky Mountains and later the modern Rocky Mountains, according to the U.S. Geological Survey.
The fault pushed blocks on top of each other as the rocks were squeezed together to form the Rocky Mountains. Then, about 30 million years ago, scientists say compression between blocks of rocks transitioned into extension and, ever since, the Gore Range fault has been pulling two blocks of rock away from each other, opening up the Blue River Valley.
“This fault in particular isn’t contributing a whole lot to the height of the Gore Range and the Tenmile Range, there’s probably pretty minimal fault slip that is contributing to that height and the depth of the valley,” Hatem said. “But before this was a normal fault with the valley deepening, these were all compressional faults. That’s what’s building the beautiful Rockies as we know them and those peaks. This particular fault isn’t as active as the mountain-building faults once were.”
The U.S. Geological Survey maintains the U.S. National Seismic Hazard Model, which forecasts potential future earthquake shaking across the country and is always collecting more information on active faults to make the model more accurate.
While damaging earthquakes are rare in Colorado, smaller earthquakes occur every year. There have been more than 700 recorded earthquakes of magnitude 2.5 or higher in the state since 1867, according to the Colorado Geological Survey. The largest known earthquake in Colorado occurred Nov. 7, 1882, and had an estimated magnitude of 6.6.
Being the closest active fault to the Front Range and the Denver metro area, scientists are especially interested in the Gore Range fault “because of the people aspect,” and “just to learn what’s in our backyard, really,” Hatem said.
The rough location of the Gore Range fault has been noted for decades but it was hard to map it under thick vegetation, Hatem said. But new observations from light-detection-and-ranging technology, which is often called lidar and is similar to radar, has allowed for better mapping of the fault.
Lidar helped scientists locate scarps, or raised areas of land indicative of where the ground slipped during an earthquake, that line the surface, providing evidence of a fault below.
“Beneath us the fault extends for miles, it goes about 15 kilometers deep, so what we see on the surface is a representation of what we see below,” Hatem said. “Earthquakes happen quite deep 5 to 10 to 15 kilometers deep, and then (to form a scarp) they have to be big enough to then slip all the way to the surface.”
Inside the trench, Hatem and a group of three more paleoseismologists — or scientists who study ancient earthquakes — used a variety of tools to scrape at the dirt to inspect the different layers. The lidar technology provided scientists a better idea of the exact location of the Gore Range fault. But it is the data that can be collected beneath the surface that will tell scientists when the last earthquake occurred on the Gore Range fault.
“There’s a lot of different layers in this trench, but we can group some of them together based off of how they were deposited and their relative importance for understanding the faulting story here,” Hatem said.
The scientists plan to sample layers that are offset by the fault, as well as layers that are not offset by the fault, so that they can estimate when the earthquake occurred, Hatem said. The earthquake will be younger than the oldest offset layer and older than the youngest non-offset layer.
Along the walls of the trench, the scientists have mapped out the layers in the subsurface with different colored flags. Midway through the trench, a row of white flags suddenly dips down, the layers evidencing a possible earthquake.
When did the earthquake occur? The scientists don’t know yet. They’ll continue to work in the trench, making observations, taking samples and collecting data, for a few more weeks before heading back to the lab. It could be a year or so until they have results.
“We’re digging this trench to find out more about the activity, when this earthquake happened and what these features really are,” Hatem said. “We think these are representative of fault scarps, but we have to keep digging to understand what these features really are.”