Evidence Casts Doubt on Quake Risk of Some Continental Faults

October 28, 2006

This sloping feature in the Mormon Mountains of Nevada indicates a detachment fault, but is it a rooted fault that extends to the mid-crust and could generate large earthquakes? Or is it a small landslide that slipped down the hillside?

A close-up of the detachment shown above reveals that the detachment surface is overlain by a gravel typically found at the base of landslides.

For several decades, geologists have thought the western North American tectonic plate was riddled with a type of fault that permitted the continent to expand over the past several million years. However, a new study published in the November issues of The Journal of Geology challenges that assumption and suggests that these faults are actually the remains of massive, gravity-driven rock slides and not tectonically active features of the Earth's crust.

Mark Anders, Nicholas Christie-Blick and Christopher Walker, all from the Lamont-Doherty Earth Observatory, looked at select faults in the Basin and Range region of the southwestern U.S. that have been described as "rooted low-angle faults" that extend deep into the Earth's crust. During this study, they found a widespread layer of material similar in texture to concrete and several other signs indicative of massive block slides along shallow, rootless faults.

As a result, Anders and his colleagues conclude that the faults are not rooted, and that the idea of extreme continental extension in the Basin and Range region needs to be re-thought. Their work also suggests the risk of large mid-continental earthquakes in the western U.S. and elsewhere may be less than currently believed.

The notion that these were rooted faults has led scientists to estimate that the North American Plate has expanded east-to-west by as much as 35 miles along the eastern margin of the Basin and Range over the past 15 million years. This interpretation has held sway in geology despite the fact that theories of rock mechanics say faults near the surface should lock up if they lay at angles of less than 20 degrees to the surface.

“In the early 1980s several key papers argued for a new class of fault, the upper crustal low-angle normal fault, based principally on faults within our study area," said Anders. "We suggest these early examples are not mega-extension faults but actually just simple rock slides which accommodated no crustal extension at all. Our work does not mean there has not been significant extension in the western third of the U.S., rather that the extension has been greatly overestimated."