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Earthquake Zone of Basin & Range

Had you been on top of Mt. Borah on October 28, 1983, around 8 a.m., you would have suddenly found yourself a good eight inches higher. The earthquake, a ground-rupturing magnitude 7, also lowered the nearby valley floor by several feet, creating a 21 mile long gash, or fault scarp, along the edge of the mountain range.

That it happened near the Lost River Range is no surprise to geologists. This is part of what's called the Basin and Range Province, all the proof you need that we are living atop an unpredictable landscape shaped by forces outside our control.

Mt. Borah [Credit: Bruce Reichert]

The Basin and Range Province covers a good chunk of the West. One geologist described the unique topography of narrow parallel mountain ranges as an "army of caterpillars marching toward Mexico." Idaho has its own set of caterpillars.

The Lost River Range runs basically north and south. Parallel to that range is the Lemhi Range; and parallel to that range is the Beaverhead Range. While most mountains are eroding away, these mountains are still growing. That's because tectonic forces are breaking this part of the world up into blocks, stretching the continent east and west.

This is Idaho's earthquake zone, one of the most active earthquake zones in North America.

Geologist Marty Godchaux explained the geologic process of Basin and Range this way, using the Lost River Range as an example. "Every block that sinks due to stretching has a small range on one side where that rotated block is tilted up a bit; that would be the White Knob Mountains. Then on the other side, we have a great fault scarp and imposing high mountains such as Borah Peak; and a flat, absolutely flat valley floor in between, where the erosional debris pours in and makes a flat elongated valley floor."

Earthquake damage [Credit: Idaho Geological Survey]

And why does this lead to earthquakes? "The major earthquakes that we might expect now would be the type that the Borah earthquake was, what we call dip-slip. In other words, they result from tension in the earth's crust pulling the crust apart slightly. Rock is going to accumulate some elastic strain. They are like elastic or a sling shot, so they can be stretched. Every rock has its breaking point, and when it breaks, that's when you have an earthquake."

Godchaux believes there will be more earthquakes like the 1983 earthquake that will raise the Lost River Range slightly higher and drop the valley floor slightly lower.

The breaking and bending of rocks continues southeast of Mt. Borah, along the Idaho-Wyoming border. The Teton Range is the youngest of the Rocky Mountains; in fact, it's one of the youngest mountain ranges in the world. But at its core are some of the oldest rocks in North America. Here you can see all the different rock types piled on top of each other: the sedimentary, adjacent to the fire-created igneous, next to the pressure-treated metamorphic rocks.

They all look solid enough, but these mountains are under enormous stress deep within the earth. In fact, they are still growing, in some places a foot every hundred years. And as fast as they are rising, glacial ice and snow and rain are eroding them away, creating surfaces that perhaps only mountaineers can truly appreciate.

Graphic showing earthquake frequency [Credit: Idaho Geological Survey]

Small earthquakes are a regular occurrence here. Almost every day seismic instruments record earthquakes up to magnitude 5 in the Teton-Yellowstone region. And the last big one with fatalities was in 1959, when twenty eight people perished.

You might ask yourself, why does eastern Idaho have more earthquakes than, say, northern Idaho? And part of the answer may be that the Snake River plain seems to be some great crustal dividing line between the extended basin ranges to the south and the less extended ranges to the north.

"Much of the action in the present day is the stretching of the Basin and Range Province," explains Godchaux. "And so if you're along the eastern edge of the Basin and Range Province, or in the seismic bow wave around Yellowstone, that's where the earthquakes are most likely to happen. The historic earthquakes have happened because Basin and Range extension tends to move eastward through time. In other words, in the early tertiary Basin and Range extension, the focus of it was further west. It's as if every time you try to stretch the North American continent, you bite off another piece, and the zone of maximum activity now is along the Wasatch front. That's kind of the eastern limit of Basin and Range extension, but only at the present time. So the zone of southern and eastern Idaho is more likely to get earthquakes because it's in the intermountain seismic zone."

People are always surprised to hear that climbers find marine life on the top of Idaho's highest mountain, Mt. Borah. So we asked geologist Bill Bonnichsen how that could be.

Bill Bonnichsen and Marty Godchaux at scarp [Credit: Bruce Reichert]

"I think that's a great question. You have to look at the succession of geologic events that brought that about. In the first place, you had to have an ocean wherever those sediments were deposited; and it was somewhere in the Idaho vicinity and perhaps a little bit west of where Mount Borah is today. So there was an ocean, and there were animals in the ocean, and their remains got trapped in the sediments.

"And then later in time, nature assembled the state of Idaho. There was a push from the west as a large micro-continent crashed into the side of North America, bringing an end to the ocean basin at that time; and this also brought about raising up of the landscape. This would have been during the period of time we call the Mesozoic.

"And accompanying that, but probably happening just a little bit later, was the development of the Idaho batholith. As material is brought down underneath the edge of the continent by subduction, part of that material melted, came up into the earth's crust, melted more material and developed this large, massive but very viscous, body of granite we call the Idaho batholith. The effect of that was to raise the landscape up even higher, including the Paleozoic sediments that were already sitting there.

"That situation of an area being raised very high permitted these layered rocks at the top to slide off. A lot of them slid toward the east to make what is called the thrust belt. You've got a series of sheets that slid off one after the other from a position a little bit further west, and they took the fossils with them of course.

"And then even later in time, we had block faulting — the very large, fairly steep faults that gave rise to the Lost River Range and the Lemhi Range and the Beaverhead Range. And it's those last faults that really pumped the fossil bearing beds up to their present elevation, where we're 12,000 feet above sea level.

"So, there's a succession of events. You had to have the ocean there at one time; then you had to bring in the micro-continent; then you had to have the development of the batholith; then you had to have the thrusting and gravity slides; and then you had to have the block faulting. What's that, five successive events to bring that about."

The mountains of the Basin and Range remind us of the words of historian Will Durant: "Civilization exists by geological consent, subject to change without notice."

Text by Bruce Reichert