A new study published by Arizona State University researchers found nearly 1,000 earthquakes occurring in Arizona over a three-year period from April 2006 to March 2009, using the EarthScope network of 58 seismometers temporarily deployed for the USArray program. AZGS bought 8 of the stations before they were redeployed eastward, and they now form the Arizona Broadband Seismic Network. [Right, Figure 3a from the study. "Analysis comparing epicenter elevation and hypocenter depth for
earthquakes located in this study. (a) Geographical distribution of
earthquakes in relation to statewide elevation." ]
Jeffrey Lockridge, a graduate student in ASU's School of Earth and Space Exploration used new seismic data collected as part of the EarthScope project to develop methods to detect and locate small-magnitude earthquakes across the entire state of Arizona. Many parts of the state had never been sampled by seismometers prior to the deployment of the EarthScope USArray, says Lockridge.
One-thousand earthquakes over three years may sound alarmingly high, but the large number of earthquakes detected in the study is a direct result of the improved volume and quality of seismic data provided by EarthScope.
Ninety-one percent of the earthquakes Lockridge detected in Arizona were "microquakes" with a magnitude of 2.0 or smaller, which are not usually felt by humans. Detecting small-magnitude earthquakes is not only important because some regions experiencing small earthquakes may produce larger earthquakes, but also because geologists use small magnitude earthquakes to map otherwise hidden faults beneath the surface.
The results appear in a paper titled, "Seismicity within Arizona during the Deployment of the EarthScope USArray Transportable Array," published in the August 2012 issue of the Bulletin of the Seismological Society of America. Ramon Arrowsmith and Matt Fouch, professors in ASU‚s School of Earth and Space Exploration, are Lockridge's dissertation advisors and coauthors on the paper.
"From April 2007 through November 2008, other networks detected only 80 earthquakes within the state, yet over that same time we found 884 earthquakes, or 11 times as many, which is really quite staggering," says Lockridge.