According to the Arizona Geological Survey, 131 earthquakes were detected in 2011 compared with 53 in 2010. That was twice as many as in 2009 and about a third more than in 2008. Most of the earthquakes were in the northwestern part of the state. The Yuma area was also shaken by earthquakes associated with the Gulf of California Rift Zone.
Many of these earthquakes (magnitude ca. 1.6) occurred near Lake Mead. These are attributed to mining and quarrying, and also to crustal adjustments to water going into and out of the lake. The strongest earthquakes (magnitude ca. 3.6) occurred near Clarkdale in the central part of the state. The Survey says that these events are consistent with past behavior: "a propensity for deeper seismicity to occur in two pockets, the northwestern Utah-Arizona border and well within the Colorado Plateau in the northeast corner of the state" and "the highest concentration of energy release correlates well with the pattern of established Quaternary faulting, indicating that this portion of the crust continues to be an active area of strain release and of particular interest for hazard studies in Arizona." The strain is due to on-going crustal extension.
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. The Arizona Geological Survey provides several videos dealing with earthquakes and geothermal energy on its Youtube Channel. Give it a look. Also take a look at the new issue of Arizona Geology Magazine