Update on northwest Arizona earthquake swarm - 19 events so far

 On March 29th, a magnitude (ML) 2.3 earthquake occurred about 23 miles south-southwest of Littlefield, Arizona. This event marked the onset of a swarm of 18 small magnitude earthquakes that continued through April 3rd. The largest event, ML 2.6, occurred at 8:36 am on April 3rd. There were no reports of damage or injuries.  [Top right. The location of the 19 seismic events in northwestern Arizona; red circles with magnitudes of each event; red lines show location of known, active faults; vertical white line is the NV-AZ border. Background image by Google Earth.]

The magnitude, date, and location of events are tabled below. The Google Earth map shows the locations of individual events which form a distinctly northwest trend. Also included for the ML 2.6 event are seismograms from the Arizona Geological Survey’s Arizona Broadband Seismic Network.

Active faults in the vicinity of the earthquake swarm include the Mesquite/Overton Arm, about 10 miles north along the western front of the Virgin Mountains, and the Grand Wash Fault system 11 miles to the east. The latter represents the boundary between the Colorado Plateau and the Basin and Range Provinces, an area with extensive historical earthquake activity.

Both fault systems are normal faults with down-dropping western blocks, characteristic of many active faults in the Basin and Range Province of northwestern Arizona and Nevada.

The northwest trend of this earthquake swarm (see above) is similar to other earthquake sequences observed in central and northern Arizona. In addition, strong lineaments in the nearby landscape suggests that there are potentially active faults immediately north of the largest event. More investigative mapping is needed to determine if the lineaments in the general area are indeed active faults.  

The 2.0+ events were resolved from data of the Arizona Integrated Seismic Network, while the small magnitude events were approximately located by the Nevada Seismological Laboratory. The small magnitude and dearth of seismometers in this remote area precludes more precise locations. Calculating depths of the individual events in northwestern Arizona’s rugged terrain was similarly difficult, resulting in a broad range of depths from 0.0 miles to 8.6 miles.  [Bottom right, seismograms from AZ stations for the ML 2.6 April 3rd event.]

Over the past 22 months, moderate-size earthquakes have been reported in the Phoenix area - the Black Canyon City ML 4.1 and 4.0 events on the night of Nov. 2, 2015, which were felt widely in the Valley of the Sun; the Kachina Village ML 4.7 event of Nov. 30, 2015, felt in both Sedona and Flagstaff; and the ML 5.3 earthquake that rattled Duncan in eastern Arizona and resulted in aftershocks that were felt more than one year after the main shock. 


Table of earthquake events associated with the Mesquite swarm in northwestern Arizona,
29 March – 3 April 2016

Most Recent to Oldest	Earthquake Date	Magnitude (ML)	Latitude (N)	Longitude (W)
1	04/04/2016	1	36.489	113.989
2	04/03/2016	1.1	36.514	114.024
3	04/03/2016	2.6	36.456	113.979
4	04/03/2016	0.7	36.521	114.031
5	04/03/2016	1.1	36.481	113.971
6	04/03/2016	1.1	36.489	113.971
7	04/03/2016	1.0	36.489	113.989
8	04/02/2016	1.2	36.488	113.987
9	04/02/2016	1.0	36.473	113.979
10	04/02/2016	1.7	36.492	113.998
11	04/01/2016	1.5	36.493	114.002
12	04/01/2016	2.0	36.498	113.992
13	03/31/2016	1.6	36.486	113.922
14	03/30/2016	1.0	36.524	114.014
15	03/30/2016	1.7	36.439	113.969
17	3/29/2016	1.6	36.506	114.004
18	3/29/2016	0.8	36.483	114.004
19	3/29/2016	2.3	36.54	114.013

This material taken from a report prepared by Dr. Jeri, Young, AZGS, and posted at http://www.azgs.az.gov/news_releases2016.shtml#apr5