Arizona is no stranger to small magnitude earthquakes. But detecting and monitoring swarms of small earthquakes had to wait until 2006 – 2009, when the Earthscope USArray Transportable Array rolled into Arizona.   Once Earthscope picked up its seismometers and moved eastward, the Arizona Integrated Seismic Network (AISN) – a consortium of the Arizona Geological Survey, Arizona State University, Northern Arizona University and NAU’s Arizona Earthquake Information Center (AEIC), and University of Arizona – cobbled together 21 seismometers, 13 broadband and 8 analog seismometers, to continue State-wide monitoring.

Since 2006, several swarms of small-magnitude events were captured by Earthscope or AISN and characterized by geophysicists at Arizona State University or AEIC.  Swarm locations include  Sunset Crater, Theodore Roosevelt Lake, Big Chino Valley, Uinkaret volcanic field, Shonto on the Navajo Nation, and most recently near Clifton, in eastern Arizona (Fig. 1) . 

An earlier swarm occurred on 21-22 December 2003, near Hannegan Meadow, 50 miles southwest of the Springerville/Eagar area.   The fall 2007 issue of Arizona Geology contained a detailed analysis by Eager and Fouch . 

Sunset Crater swarm

More than 120 discrete earthquakes, most M 2.0 or greater,  occurred over a six-hour period on 31 October 2009, on a northeast trend with a distinct shallowing to the southwest adjacent to Sunset Crater, 15-miles northeast of Flagstaff.  The largest event, magnitude 2.9, occurred late in the swarm.   According to Young (2010), the combined factors of a lack of a mainshock and aftershock sequence, the depths  and periodicity of the events points to a volcanogenic source mechanism. 

Background:  Sunset Crater is the youngest of the 600 scoria cones of the San Francisco volcanic field (SFVF), and lies 15 miles northeast of Flagstaff, Arizona.   Repose between successive cinder cone eruptions in the SFVR is probably on the order of 1,000 to 3,000 years.  

Roosevelt Lake swarm

Over 8 days in June 2007, 62 earthquakes occurred near Theodore Roosevelt Lake, 80 mile east-northeast of Phoenix, Arizona (Lockridge et. al., 2010).  Average depth was 3.8 km, with a maximum depth of 9.7 km.  Possible sources of the swarm are: 1) reservoir filling, 2) deformation in the Arizona Transition Zone. (See above feature banner image - Epicenters for events of Theodore Roosevelt Lake earthquake swarm)

Background:  Theodore Roosevelt Lake filled in 1911 behind a newly constructed masonry dam on the Salt River.  Nearly 10-miles of the former Salt River channel are submerged as is about 8-miles of the tributary Tonto Creek. 

Uinkaret Volcanic field

From 4 December to 17 December 2007, 345 seismic events (average magnitude 2.0, maximum magnitude 3.2) were registered in the Uinkaret volcanic field (Lockridge et. al., 2010), sandwiched between the Hurricane Fault and Toroweap Fault along the north rim of Grand Canyon.  The focal depth of the swarm was markedly deeper than other swarms; average depth of the events was 13.5 km, with maximum depth of 15.4 km.    

Shonto, Navajo Nation

Located about 20-miles north of the intersection of US 160 and Arizona SR 98, Shonto lies in northwestern-most Navajo Nation.  From late August 2008 to late September 2008, two discrete small-magnitude seismic swarms, totaling 172 events, occurred here.  Average magnitude was 2.0, with a maximum of 2.8 M.  Average depth was 5.8 km, with a maximum depth of 8.5 km.

Big Chino Valley

A microswarm of seven events over a three-day time-window,  in March 2008,  occurred adjacent to the Big Chino Fault, 40-miles north-northeast of Prescott, Arizona.  The Big Chino fault is an active Quaternary fault with surface displacement and geomorphology suggesting an oblique fault, predominantly normal with a component of right-lateral slip. 

Clifton, Arizona

Twelve earthquakes, ranging from about M 2 to  M 3.5, occurred north and north-northeast of Clifton Arizona from 24 to 26 May 2010.  The largest quake, an M 3.6 event, was felt locally and reported widely.  All events were shallow with focal depths from 1.5 miles to 5.0 miles. 

Summary

Earthscope’s USArray program and the new Arizona Integrated Seismic Network permit  systematic, State-wide monitoring of small magnitude earthquakes and earthquake swarms in Arizona.  Analysis of these data should inform seismic risk analysis studies and regional tectonic stress models in theSouthwestern U.S.

Other Recent Seismic Events in Arizona (from Arizona Earthquake Information Center - Go to AEIC)

                                               Depth
D/M/Year    Lat. (N)   Long. (W)    Km      Hour    Min.   Sec.         Mw        Location
3.21.2010       36.6098      113.318           10            17        40          6.08         2.7*        Hurricane Cliffs, AZ
4.1.2010         38.8996      113.0616         20            17        26        42.42        2.6          Colorado City, Arizona
5.12.2010       34.748        112.161          12.56        16        19         51.3         2.05        Woodchute Mtn
5.24.2010       33.315        109.209           5             12         27          8.41        3.5          Apache National Forest
5.24.2010       33.2496      109.233           5             14         43          4             2.4          Apache National Forest
5.24.2010       33.273        109.193           5             23         53       43.04        3.0           Apache National Forest
5.25.2010       33.2772      109.2717         2.07         15         15       36             2.3           NNE of Clifton
5.24.2010       33.2494      109.2327         5.0           23         53       43             3.6           N of Clifton
5.25.2010       33.2914      109.2853         5               8         22        87             2.5           NNE of Clifton
5.25.2010       33.2914      109.2605         5              16        59        49             2.7           NNE of Clifton
5.25.2010       33.0327      109.805           5.00         19          2        32             2.2           N of Clifton
5.26.2010       33.367        108.987           1.79          1         29         3             2.1           N of Clifton
5.26.2010       33.3017      109.2883         4.06          6           3        48             2.2           N of Clifton
5.26.2010       33.2717      109.2697         1.31          7          17        25            2.4           N of Clifton
5.26.2010       33.2153      109.403           5.00          7          15        17            1.8           N of Clifton, Az
5.27.2010       36.862        113.516           2.3          16          39        53            1.7           Quail Hill
5.29.2010       36.905        112.834           5              7            2        24            2.5           SE of Colorado City
5.29.2010       36.463        113.259           6.72         15         31        54            2.7           SE of Colorado City
6.21.2010       36.940        113.509           2.1           20         27        23            1.7           11-mi S St. George, Ut
6.25.2010       33.607        111.231         13.58        10          30         34            3.58         Roosevelt Lake
7.14.2010       36.943        113.517           4.7          13          25         36            1.4       18 km SSE of St George7.21.2010       36.993        113.509           6.3          12          29         15            1.5           13 km SSE St. George, UT

References:

Eagar, K.C.  and Fouch, M.J., 2007, Detection of a Unique Earthquake Swarm in Eastern Arizona, Arizona Geology, V. 37, no# 3, p. 1-5

Lockridge, J.S., Arrowsmith, J.R., Fouch, M.J., 2010,  Analysis of Spatial and Temporal Seismicity Patterns within Arizona During the Deployment of the EarthScope USArray Transportable Array (March 2006 - April 2009).  2010 IRIS Annual Workshop.