A small swarm of earthquakes occurred south of Colorado City between December 12th and 13th. The largest event in the swarm was magnitude 3.1, the smallest was 1.7.

Year Month Day Lat Long Depth Hour Min Sec Mag (local) Location Catalog
2011 12 13 36.765 -113.018 13.9 0 43 0 2.8 Colorado City, AZ UU
2011 12 13 36.764 -113.017 8.2 23 36 22 3.1 Colorado City, AZ UU
2011 12 12 36.782 -113.001 2.8 9 44 37 1.7 Colorado City, AZ UU
2011 12 12 36.757 -113.02 7.9 8 3 0 2.2 Colorado City, AZ UU

This small swarm occurred between the terminus of the Southern Intermontain Seismic Belt (ISB) and the start of the Northern Arizona Seismic Belt. This region regularly experiences small to moderate sized earthquakes.  Earlier this summer there were several quakes of similar magnitude in the same area between the Hurricane and Sevier/Toroweap faults.

The National Earthquake Information Center Reports of the southern ISB:
The ISB in southern Utah coincides with a transition between east-west-directed stretching in the Basin and Range to the west and more stable crust of the Colorado Plateau to the east. Tectonic movement on generally north-trending, east- and west-dipping range- and plateau-bounding normal faults, which results in horizontal extension, characterizes this part of Utah. The Sevier Valley is an area of variable and complex deformation involving significant components of folding and both normal and strike-slip faulting. The most prominent geologically young faults in southwestern Utah are the Hurricane and Sevier faults. The Hurricane fault forms the west-facing Hurricane Cliffs, which define the eastern edge of the Basin and Range within the ISB. Faults in the ISB in southern Utah locally show evidence of displacement younger than 10,000 years, but average recurrence intervals are generally longer than those on faults in the ISB in northern Utah. Recurrence intervals for surface faulting on the most active segments of ISB faults in southern Utah are generally many thousand to tens of thousands of years.


The largest event in the swarm (3.1) as seen on the Arizona Broadband Seismic Network



Lisa LinvilleDecember 17, 2011