Analysis of the 2009 earthquake swarm near Sunset Crater volcano, Arizona

• The 2009 Sunset Crater, Arizona swarm is a potential candidate for a magmatic swarm
• Depths and trend of the swarm suggest control by basement fractures
• Swarm depth and characteristics match a mid-to-lower crust intrusion as a source

A swarm of microearthquakes occurred on October 31, 2009 within 5 km of the Sunset Crater, Arizona, volcano. A detailed study of the swarm was warranted because of its location near a young volcanic construct and its proximity to the population center of Flagstaff, Arizona. The question posed in this study was whether the swarm was the result of tectonic stress release during fault slip, or due to stresses driven by magmatic processes. This question was addressed by analyzing and comparing the physical and seismic characteristics of the swarm to the regional tectonic environment and to the characteristics of tectonic swarms in Arizona and magmatic/volcanic swarms elsewhere. This analysis included swarm duration, frequency of events, b-value, focal depths and epicentral pattern of the swarm. The comparison of the salient features of the 2009 Sunset Crater swarm to both magmatic and tectonic swarms indicates that the Sunset Crater swarm has features similar to magmatic swarms and is a potential magmatic swarm candidate.