Since March, 2016, northwestern Arizona has experienced on-going earthquake activity. To date, there have been 57 earthquakes, the largest of which registered a magnitude of 3.8.
These earthquakes are not unexpected according to the Arizona Geological Survey. They are just now being detected because of enhanced instrumentation deployment.
Northwestern Arizona is on the southern end of a seismic belt that stretches from Montana and Idaho, through Utah and into Arizona. For the past 15 million years, the crust in this area has been stretching and stretching causes faulting and earthquakes.
The Arizona Geological Survey has a 4-minute video which explains what is happening: