by David Brumbaugh
Professor and director of Arizona Earthquake Information Center

On October 16, 2014, Arizonans will participate in the national Earthquake preparation exercise known in Arizona as “Arizona Shakes.” The function of this exercise is to show a person what to do in the event the earth begins to move so that the chances of surviving such an event are increased. Most folks in Arizona are aware that California is earthquake country due to the continual earthquake activity in that state. But Arizona? What are the chances here to experience an earthquake large enough to cause damage or injury? That is a difficult question to answer with numbers, so perhaps the best way is to look at the earthquake history of the state.

Significant damage from earthquake ground shaking begins at about a 5.0 magnitude. Such was the case in 1993 when a magnitude 5.3 tremor shook the northern part of the state causing minor damage at the Grand Canyon National Park. More recently a magnitude 5.2 earthquake shook southeastern Arizona on June 29, 2014, causing minor damage in Duncan, Arizona. The real hazard comes from the knowledge that there have been quakes as large as magnitude 6.2 in Arizona that fortunately caused little damage because of their location in what at the time was a sparsely populated state (1906, 1910, 1912).

The potential that such events or ones even a bit larger can occur in Arizona’s future does exist, especially in the northern part of the state. The Lake Mary fault is about 25 miles long and at its northern end lies inside the city limits of Flagstaff. If this fault were to slip along its length it could generate a magnitude 6.9 tremor. Other seismically active faults exist just outside of the Arizona state boundary in southern California and Mexico that are relatively close to Yuma and Tucson, Arizona. A magnitude 7.4 earthquake occurred in 1887 in northern Mexico that was felt in Phoenix and Mexico city. A recurrence of such an event in the future has serious implications for the Tucson area.

The Arizona Earthquake Information Center at Northern Arizona University and the Arizona Geological Survey in Tucson are operators of a shared seismic network known as the Arizona Integrated Seismic Network. The data generated by its seismographs is being catalogued and studied so that we can track and locate areas of elevated seismicity that could generate future hazardous earthquakes.