Seismicity within Arizona during the deployment of the EarthScope USArray transportable array

A detailed record of earthquake frequency and distribution is essential to understanding regional tectonic strain and seismic hazard, particularly in regions of low, but significant, seismicity levels. Comprehensive analyses of seismicity within Arizona have not been previously possible due to a lack of seismic stations in many regions, contributing to the perception that earthquakes within Arizona are rare and generally limited to the north-central and northwestern portions of the state. The EarthScope USArray Transportable Array (TA) was deployed within Arizona from April 2006 to March 2009 and provided the opportunity to examine seismicity on a statewide scale. In this study, we developed a streamlined workflow for producing a comprehensive earthquake catalog using TA data. We combined our new catalog with historical earthquake catalogs from several sources to produce the first comprehensive historical earthquake catalog for the state of Arizona. The TA-derived catalog is complete to local magnitude (M _L ~ 1.2, contains crustal events as small as ML 0.0, and includes events located within several previously unidentified areas of seismic activity in Arizona. We also identified 16 earthquake clusters, many of which have swarmlike behavior. These earthquake clusters account for 42% of the events identified during the study period, and they occur in all physiographic, geophysical, and tectonic settings. We suggest that swarms and clusters, such as those documented in this study, may represent an important mechanism for small-scale tectonic strain release within intraplate regions with otherwise apparently low seismicity levels.