Tucson- Earthquake aftershocks of about magnitude (M) 3 continue to lightly rattle Duncan, Arizona and environs. The latest event, a M3.6 temblor, was felt 30 miles north in Morenci, Arizona. Aftershocks stem from the M5.2 earthquake that struck near Duncan, Arizona, (approximately 40 miles east of Safford) on the evening of June 28.
Since the M5.2 event, 14 felt aftershocks (from M2.8 to M3.9) have rattled the area around Duncan in southeastern Arizona and southwestern New Mexico. Like the main shock, these were shallow events occurring about 3 miles deep. The largest aftershock, at M3.9, occurred at 3:56 p.m. on July 3. Since yesterday, July 10, five felt earthquakes have been reported. The largest were M3.6 temblors at 2:59 p.m. on July 10, and 10:33 a.m. on July 11. Hundreds of smaller magnitude aftershocks – less than ~ M3.0 – have occurred since the onset of activity, and most went largely unfelt.
People in southeastern Arizona should be prepared for ongoing M3.0 to M4.0 earthquakes over the next weeks or even months. In the event of strong ground shaking, follow the “Drop, Cover and Hold On” convention used in earthquake-prone areas. For more information on what to do in the event of an earthquake, please visit the Great Arizona ShakeOut website athttp://www.shakeout.org/arizona.
Over the past week, “the Arizona Geological Survey (AZGS) deployed a temporary array of five portable seismometers around the location of the M5.2 main shock in hopes of learning more about the behavior of the earthquakes and faulting in the region,” said Jeri Young, AZGS geophysicist. These five devices complement existing seismometers of the Arizona Integrated Seismic Network and form a seismic net for monitoring earthquake activity.
According to Jon Spencer, Senior Geologist at the Arizona Geological Survey, “The recent Duncan earthquake occurred because Earth’s crust in southern Arizona and northern Sonora is gradually extending in an east-west direction.”
The largest historical earthquake in the region was the M~ 7.5M event in May 1887 on thePitaycachi fault of northern Sonora, Mexico, about 25 miles south of Douglas, Arizona. This is considered the largest earthquake likely to occur in this region. A M5.5 earthquake occurred on August 17, 1938, near Buckhorn, New Mexico, and M4.5 events occurred soon after in the Duncan and Clifton areas. In May 2010 and October 2012, small earthquake swarms, with earthquake events ranging from M2.0 to M4.1, occurred about 45 miles north-northeast of Duncan, in the Morenci-Clifton area of northern Greenlee County.
Historical earthquake activity for all of Arizona is available online at the interactive Natural Hazards in Arizona Viewer. Attached image shows the M5.2 main shock (9:59 p.m., 6/28/14) and 14 aftershocks that occurred through 10:33 a.m. on July 11, 2014.