Today’s episode is a follow-up to the Basin and Range discussion the other day, and a follow-up to the Sierra Nevada Batholith which we talked about back in the Jurassic, in October. The Sierra Nevada Mountain Range in California coincides pretty well with the Jurassic-Cretaceous Sierra Nevada Batholith, the deep roots of the subduction and magmatic arc system that was established out there by at least 150 million years ago.
But the modern range is vastly younger than that. The Sierra Nevada is basically one huge fault block, tilted to the west, with the fault marking its eastern front. It’s a big normal fault, with the mountains up to the west and the region to the east dropped down. That fault is essentially the western margin of the Basin and Range Province, the extended, faulted suite of uplifts and basins that continues all the way east to the Wasatch Mountains in central Utah.
The action on the Sierra Nevada Fault started about the same time as the Basin and Range became active, in the Miocene Epoch, 15 to 20 million years ago. And most of the uplift is in the past 5 million years. There’s easily 10,000 feet of displacement on the fault and probably quite a bit more.
|Intermountain Seismic Belt |
marked by dashed black line.
Today, there is some seismic activity within the Basin and Range, but it’s pretty spotty. In California, the main tectonic focus is along the San Andreas Fault Zone. But the Sierra Nevada Fault is almost certainly still active. The uplift of the Sierra Nevada isn’t simply on the one fault, and in 1872, a magnitude-7.8 quake hit the Lone Pine area, in the Owens Valley east of the Sierra. The movement wasn’t on the Sierra Nevada Fault, but on the nearby and related Owens Valley Fault, which is part of the system of faults continuing to uplift the Sierra Nevada. Some segments had vertical offset of as much as 20 feet.
On the other side, the Wasatch Mountain Front, which forms the backdrop of Salt Lake City and all of central Utah, is part of an earthquake belt called the Intermountain Seismic Zone. Its heritage is also about the same timing as the Basin and Range, the Miocene, but there has been some level of discontinuity there, at the western edge of the Colorado Plateau, for many tens of millions of years.
The activity along this belt is usually nothing like that on, say, the San Andreas Fault, but great earthquakes do occur, including the Hebgen Lake Quake in 1959, which had a magnitude of about 7.4. The Intermountain Seismic Belt goes from southern California, through southern Nevada and central Utah, eastern Idaho and western Wyoming, through western Montana and into Canada. If you drive Interstate 15, you’re pretty much following this zone. There hasn’t been a big earthquake on the Salt Lake City segment of the Wasatch Fault for about 1,300 years. That doesn’t mean one is due; typically, the historical record of earthquakes is far too short to infer any kind of regular periodicity, and it can be challenging to date prehistoric quakes with accuracy. Nonetheless, seismologists always try to figure this out for general hazard preparedness, and there do appear to have been sizeable earthquakes on some part of the long Wasatch Fault zone about every 350 years. A big one will happen there, sometime.
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Our anniversary today is an earthquake, on December 22, 856 A.D., at Damghan, Iran. It had an estimated magnitude of 7.9 and killed about 200,000 people, one of the most deadly earthquakes ever. It occurred in the Alborz Mountains, the range in northern Iran along the shores of the Caspian Sea. The South Caspian Basin is a small bit of oceanic crust that is trapped in the ongoing Cimmeride and Alpine-Himalayan collisions that we’ve talked about previously. Here, it seems likely that the dense bit of oceanic crust is subducting, at least somewhat, beneath the Iranian continental block. Volcanoes in the Alborz Range, including Damavand, support that idea. And so do the earthquakes.
—Richard I. Gibson
Sierra Nevada Fault Scarp (1898)
Intermountain Seismic Zone