Earthquakes Shake Central Arizona

Late Sunday night, three earthquakes rattled central Arizona, including Phoenix and environs. The three temblors, magnitude (M) 3.2, M 4.1 and M 4.0, occurred approximately 3- to 11- miles north of Black Canyon City – see attached figure. The largest event, an M 4.1, occurred at 11:20 p.m. The earthquake(s) were felt from north of Flagstaff to south of Casa Grande, including most of central Arizona.

The U.S. Geological Survey initially located the epicenters and depth to focus. Using 15 Arizona-based seismometers from the Arizona Broadband Seismic Network and the Arizona Earthquake Information Center, Flagstaff, Arizona, Dr. Jeri Young (Research Geologist), refined the locations for the foreshock, mainshock, and the large aftershock from the November 1^st  Black Canyon City sequence of quakes (Table below).

The M3.2 foreshock was located at approximately 3miles north of Black Canyon City at a very shallow depth, between 0.6 and 3 miles. The mainshock was located approximately 11 miles NNE of Black Canyon City and occurred at approximately 7 mile depth.  The largest aftershock an Mw 4.0 occurred only 20 minutes following the mainshock at a depth of approximately 3 miles.

There have been multiple smaller aftershocks, but there are not enough seismometers close enough to the area to accurately locate them.  The closest station is located near Payson at about 40 miles, with the Wickenburg station being 55 miles away.

The closest known active fault is the Horseshoe Fault, located 22 miles SSE of the mainshock.  The Black Canyon City Sequence has not generated a quake large enough to break the surface.  Aftershocks will continue for several months or more. Most aftershocks will go unfelt, but residents of Black Canyon City and environs should potentially anticipate small felt aftershocks.

From a report by Dr. Jeri Young, AZGS Research Geologist.  Note that all times are reported in UTC.

Online Resources from the Arizona Geological Survey

Natural Hazards in Arizona – showing historic earthquake epicenters and young, active faults.

Geologic Map of Arizona – showing major geologic features and rock units.