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Paul Myrow: Cambrian Rocks in Colorado Show Signs of Catastrophic Earthquakes

Colorado College Geology Professor Paul Myrow has determined there were earthquakes in the central Rocky Mountains on the scale of 7.0, a greater magnitude than the earthquake that rocked Northern California on Aug 24 and similar to the devastating 1989 San Francisco earthquake and the 2010 Haitian earthquake.

Myrow and a colleague, Jitao Chen of the Nanjing Institute of Geology and Paleontology, worked on outcrops of Cambrian rocks, which are about 490 million years old, in central and western Colorado. Their research, which centered around Glenwood Springs, has been published in the leading geology journal Sedimentology, in an article titled “Estimates of Large Magnitude Late Cambrian Earthquakes from Seismogenic Soft-Sediment Deformation Structures: Central Rocky Mountains.”

Although Myrow and Chen’s estimates of the earthquake’s magnitude indicate these may have been some of the largest in the geologic past for the central Rocky Mountains, they note there is no evidence to suggest earthquakes of this magnitude will occur in the immediate future.

The Cambrian rocks are part of the Dotsero and Manitou Formations, which are composed of shale, limestone, and dolostone. These sedimentary rocks were deposited in a shallow sea that covered the western United States at the time. “The sedimentary rocks show evidence that the sea floor was disrupted by large shock waves produced by earthquakes. The region was jostled, dislodging thick pieces of the sea floor and turning some layers into quicksand, i.e., liquefaction,” Myrow said.

Based on the regional distribution of the structures and the size of dislodged pieces of the sea floor, Myrow and Chen estimated the size of earthquakes required to produce the features. They determined ground motion velocities of 1.6 meters per second, which would correspond to earthquake magnitudes of 7.0 or more, thus classifying them as “major” quakes. An earthquake of this magnitude would correspond to a Mercalli Index of X or greater. The index is a measure of the effects of an earthquake, and in this case the earthquakes would have ranged from “intense” to “catastrophic.” Myrow notes that for comparison, the 1906 San Francisco earthquake was rated an index of X.

The geologists’ research indicates the earthquakes are linked to a fault zone in Glenwood Canyon called the Grizzly Creek Shear Zone, in the vicinity of Grizzly Creek. It has been active several times in the earth’s history, but episodically with long periods of dormancy.