Earthquake Swarms in Northern Arizona
During the last couple weeks northern Arizona has experienced an unusually high level of seismicity. In addition to the occurrence of a number of events around the state during the month of June, there were two small swarm events over the last ten days.
The first cluster of events occurred between Tusayan, Arizona and Red Butte. That swarm included nine events. The largest magnitude was near 2.8 and occurred over a period of 5 days. The nearest fault system is the northeast-southwest trending Bright Angel fault zone which extends through the Grand Canyon. These events were between 3.5-10 km deep.
The second swarm began on June 21 and continued through this weekend for a total of 13 events over a 5 day period. The largest events were magnitude 2.7 and the smallest around 2.0. Average depth was around 6, though the depth range was between 1.8 and 17.3 km. These event occurred near the Anderson Mesa fault, just south of Flagstaff, Arizona.
Though the Lake Mary/Anderson Mesa fault is known to be an active zone which local seismicity has been linked to in the past, the discrete location of the recent swarm has not experienced any comparable clusters since a group of very small, relatively deep events in October of 1979. The Tusayan/Red Butte group lies just south of the majority of seismicity in the Grand Canyon area. More specific investigation of these swarms will be ongoing. For specific event data please visit the bulletin page of the Arizona Earthquake Information Center Website (http://www.cefns.nau.edu/Orgs/aeic/bulletins.html)
Arizona is not new to swarm events. In 2009 the Halloween swarm produced 120 events over a 1 day period. Additionally, Jeff Lockridge of ASU is researching several recent swarms in Arizona using high density network coverage. We hope to learn more about the character of earthquake swarms in the state through his research.
June 28, 2011
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