Rare Earthquake Trio Shakes Phoenix: What Happened?

A rare trio of earthquakes shook central Arizona Sunday (Nov. 1), startling residents in Phoenix event surrounding areas.

The largest quake was a magnitude-4.1 temblor, which hit at 11:29 p.m. local time. It was preceded by a magnitude-3.2 foreshock at 8:59 p.m. and was followed by a magnitude-4.0 aftershock at 11:49 p.m. Smaller aftershocks may follow, said Ryan Porter, a seismologist at Northern Arizona University.

Earthquakes are "pretty uncommon for Arizona," Porter told Live Science. "So it's not unexpected, but it's not a very common event.

This does not mean Arizona is seismically inactive. In fact, geophysicists have recorded hundreds of quakes each year in the state, though most are too small to be felt by humans. Particularly active is the Northern Arizona Seismic Belt in the north central part of the state, which is part of the wider Intermountain Seismic Belt of Nevada, Utah, New Mexico and southeastern California. Central Arizona is seismically quieter, however.

Researchers haven't yet fully analyzed Sunday's quakes, which occurred between the depths of 3.1 miles and 5.9 miles (5 and 9.5 kilometers), near Black Canyon City.

http://www.livescience.com/52673-phoenix-earthquakes-explained.html