During October, six earthquakes, ranging from 2.5- to 4.1-magnitude, occurred about 25 miles north-northeast of Morenci in Greenlee County, east-central Arizona. These earthquakes were recorded by several seismographs around Arizona, including one in Tucson. In addition, there have been swarms of lesser magnitude earthquakes in the area according to the Arizona Geological Survey (AZGS). AZGS has a short article on these recent earthquakes here. The article includes maps and seismograph records.
According to Wallace (1989) “The background seismicity level for southern Arizona is quite low, especially compared to California. The two most seismically active regions in southern Arizona are the southeastern corner of the State, extending north from Douglas along the New Mexico border to the Clifton-Morenci area, and the southwestern corner south of Yuma along the Mexico-Arizona border.”
Just why earthquakes occur in the Morenci area is subject to speculation. No faults have been positively identified, but the topography suggests that a fault exists in the area. The Morenci area has long been seismically active. In May of 2010, 12 earthquakes ranging from Md 2.0 to 3.5, and 5 events below Md 2.0 occurred near the area of recent earthquakes.
Arizona is divided into two main physiographic provinces. In the northeast is the high-elevation Colorado Plateau characterized by mainly flat-lying sediments. The southwest part of the state is the Basin & Range province, lower in elevation, and characterized by long, thin mountains ranges separated by fault-bounded valleys. The Basin & Range topography is the result of crustal stretching during the past 20 million years.
Separating the Colorado Plateau from the Basin & Range, is the so-called Transition Zone which runs diagonally through Arizona from the northwest corner to the southeast corner. This area is characterized by faults and marks the boundary between the zone of crustal extension and the more stable plateau. The Morenci area earthquakes occur in this transition zone and may reflect continued crustal adjustment to the on-going extension.
UPDATE: A magnitude 3.4 earthquake hit just before 1a.m. this morning, November 1, about 22 miles NNE of Morenci, in eastern Arizona.