Earth shaking history

The news of an earthquake near Saratoga is nothing new. According to the U.S. Geological Survey, there was a 3.0 magnitude quake February 4, 2020 at approximately 5:36 p.m. The epicenter was at 41.3810 N, 107.1370 W which is near the Jack Creek area at a depth of 5 kilometers.

Five years ago there were two earthquakes of similar magnitudes.

Earthquakes are the result of movement of the earth along faults, movements of magma chambers like in Yellowstone National Park and from man-made events such as blasting at mines.

Earthquakes can occur at various depths depending on the orientation of the fault. The initial rupture on a fault that results in seismic waves is called the earthquake hypocenter or focus. The area directly above, on the ground surface is called the epicenter.

Earthquakes are characterized by magnitude and intensity.

Magnitude is a number based on the measurement of the maximum motion of the earth recorded by a seismograph. The most common scale used to describe the magnitude of an earthquake is the Richter scale which was invented in 1935.

Intensity is a measurement of the degree of shaking an earthquake imparts on people, structures and the ground. Intensities can vary depending on the distance from the center of the earthquake. There is a twelve level intensity scale called the Modified Mercalli Intensity Scale that is used to describe a quake.

Wyoming is classified by the Federal Emergency Agency and US Geological Survey as having a very high seismic hazard. The greatest percentage of earthquakes, however, happen in the vicinity of Yellowstone National Park. Approximately 700 to 3,000 earthquakes occur there every year but most are not felt.

Wyoming is not the most earthquake prone state though. Alaska is. It experiences a magnitude 7 earthquake almost every year, and a magnitude 8 or greater earthquake on average every 14 years.

One of Wyoming’s top ten earthquakes was felt in Carbon County. It happened approximately 4 miles west-northwest of Toltec in northern Albany County. On October 18, 1984, a magnitude 5.5, intensity VI event occurred. The earthquake was felt in Wyoming, South Dakota, Nebraska, Colorado, Utah, Montana and Kansas. It cracked buildings in Douglas and Medicine Bow and cracked chimneys in Casper, Douglas, Guernsey, Lusk and Rock River.

According to a report in 2002 titled ‘Basic Seismological Characterization for Carbon County, Wyoming’ by the Wyoming State Geological Survey, the first recorded earthquake that was reported in Carbon County occurred just southwest of Rawlins on March 28, 1896. It was an intensity four earthquake that shook for about two seconds. Residents of Rawlins reported that dishes rattled and lamps swayed.

Most of the earthquakes that were recorded in Carbon County happened in the 1970’s. Of the 13 recorded earthquakes, three were attributed to explosions in the areas of the quakes. The last earthquake in the 1970’s occurred on March 3, 1977 approximately 18.5 miles west-northwest of Encampment. It was a magnitude 4.2 with an intensity of 5 in the Sierra Madre Mountains.

Carbon County contains an active fault system, the South Granite Mountain fault system. It is composed of several segments; the Seminoe Mountains, the Ferris Mountains, Muddy Gap, the Green Mountain and the Crooks Mountain segments. The WSGS also indicates that there may be unmapped faults in Carbon County.

According to the WSGS the Ferris Mountain segment of the South Granite Mountain fault system is capable of generating an earthquake of magnitude 6.5-6.75 every 5,000 to 13,000 years. The Green Mountain segment is capable of generating a magnitude 6.75 earthquake every 3,00 to 6,00 years. The WSGS did not find evidence of movement in the other segments but still consider them to be potentially active.

Because the South Granite Mountain fault system has been given a potential maximum magnitude of 6.75, the chart below shows the intensity the towns in Carbon County would feel if an earthquake of that magnitude were to occur.

The WSGS suggests taking the following precautions if a large earthquake should strike Carbon County:

• If you are outside, stay outside and away from buildings, fences, trees and power lines.

• If you are in your car, pull over and stop away from high structures, power lines, overpasses and trees. Stay in your car.

• If you are inside, stay inside unless conditions warrant otherwise. Duck under a sturdy table or desk, sit with your back against a strong inside wall or stand under a doorway. Stay away from windows and glass doors. Stay away from heavy standing objects such as bookcases.