Arizona Earthquakes Not As Rare As Once Thought
August 15, 2012

Earthquakes In Arizona Are Not As Rare As Previously Believed

April Flowers for - Your Universe Online

Earthquakes are a scary phenomenon. They are enormously destructive and much more common than you might think.

Several million earthquakes are estimated to occur worldwide each year. Most are too small to feel, but they can be measured by arrays of seismometers. Some are so large they claim hundreds of thousands of lives (Shaanxi China, 1556, claimed 830,000 lives) while some create massive economic hardship (San Francisco, USA, 1906, over $400 million in damages). One of the scariest overall was the New Madrid earthquake of 1811. The Mississippi River actually ran backwards from the force of this earthquake.

Historically, though, Arizona has not been a hot bed of seismic activity. According to previous data and scientific thought, Arizona has only experienced low levels of seismicity, with infrequent moderate and large earthquakes. Researchers have not been able to make comprehensive seismic analysis of the state previously due to a lack of seismic stations in most regions.  This lack of data has led to the idea that widespread earthquakes in Arizona are rare.

Debunking that myth, a new study from Arizona State University, found nearly 1,000 earthquakes rattling the state over a three-year period.

Led by graduate student Jeffrey Lockridge, from ASU's School of Earth and Space Exploration, the research team 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.  EarthScope's USArray Transportable Array was deployed within Arizona from April 2006 to March 2009. This array, with its increased sensitivity, provided the first opportunity to examine seismicity on a statewide scale and allowed Lockridge and team to find almost 1,000 earthquakes in areas that were previously thought to be seismically inactive.

EarthScope's USArray is a continental-scale seismic observatory designed to provide a foundation for integrated lithosphere and deep Earth structure.  When finished, it will consist of over 400 portable seismometers that will be deployed across the United States over a 10-year period.  In addition to this, the Magnetotelluric Transportable (MT) array is a flexible component, which will comprise shorter-period investigations at hundreds of sites.

"It is significant that we found events in areas where none had been detected before, but not necessarily surprising given the fact that many parts of the state had never been sampled by seismometers prior to the deployment of the EarthScope USArray," says Lockridge. "I expected to find some earthquakes outside of north-central Arizona, where the most and largest events had previously been recorded, just not quite so many in other areas of the state."

One thousand earthquakes in three years sounds like an alarmingly high number.  However, the large number of quakes detected in the study is a direct result of the improve volume and quality of seismic data provided by EarthScope.  Ninety-one percent of the earthquakes detected were "microquakes" with a magnitude of 2.0 or smaller.  Earthquakes this small are generally not felt by humans. Detecting such small-magnitude quakes is not only important because some regions experiencing small quakes might also produce larger ones, but also because geologist use small magnitude earthquakes to map previously hidden faults beneath the surface.

The largest earthquakes and the majority of seismicity recording within Arizona have historically been located in an area of north-central Arizona.  More recently, a pair of magnitude 4.9 and 5.3 earthquakes occurred in the Cataract Creek area outside of Flagstaff.  Large earthquakes, 4.0 or higher on the Richter scale, have also occurred in other areas, including a magnitude 4.2 earthquake in December 2003 in eastern Arizona and a 4.9 near Chino Valley in 1976.

The Richter magnitude scale is how scientists compare the size and power of earthquakes.  The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs and expressed in whole and fractional numbers from 1.0 to the largest earthquake recorded so far at 9.5.

"The wealth of data provided by the EarthScope project is an unprecedented opportunity to detect and locate small-magnitude earthquakes in regions where seismic monitoring (i.e. seismic stations) has historically been sparse," explains Lockridge. "Our study is the first to use EarthScope data to build a regional catalog that detects all earthquakes magnitude 1.2 or larger."

Lockridge's results appear 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, "Seismicity within Arizona during the Deployment of the EarthScope USArray Transportable Array."

"The most surprising result was the degree to which the EarthScope data were able to improve upon existing catalogs generated by regional and national networks. 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. "It's one of countless examples of how powerful the EarthScope project is and how much it is improving our ability to study Earth."