For many years, John Anderson, director of the Nevada Seismological Laboratory at the University of Nevada, Reno, has been telling citizens, reporters and other scientists from throughout the world that in terms of seismic activity in the 50 states, Nevada ranked as the third most active.
Then, during a meeting of the Nevada Earthquake Safety Council earlier this year, he learned that the United States Geological Survey (USGS) website actually ranked Nevada fourth, behind third-place Hawaii.
"One of the Safety Council members said, 'John, what in the world is going on here?'" Anderson remembered.
His curiosity piqued, Anderson began a study along with Yuichiro Miyata of the Nevada Seismological Laboratory to take another look at the data. The project turned into a national study, and their lists of the top 10 most seismically active states have just been published in the November/December issue of Seismological Research Letters, a bimonthly publication of the Seismological Society of America.
The undisputed leader for numbers of earthquakes? Alaska, of course, with California solidly in second place. But beyond that point, depending on how you measure activity, the rankings change. On the list that Anderson likes the best, which gives the greatest magnitude that is reached once per year on average, Nevada inches into third, at 5.1. For fourth-place Hawaii, it's 5.0.
The top 10 rankings, based on the magnitude of earthquake that occurs once per year on average: 1, Alaska, 6.70; 2, California, 6.02; 3, Nevada, 5.11; 4, Hawaii, 5.00; 5, Washington, 4.97; 6, Wyoming, 4.67; 7, Idaho, 4.57; 8, Montana, 4.47; 9, Utah, 4.29; 10, Oregon, 4.24.
"Everybody is dedicated to accurately portraying how and when earthquakes occur," said Anderson, who, as director of one of the nation's premier seismological laboratories, monitors such events on a daily basis. "The reason for talking about this is not to change the rankings or to have one state move ahead of another – it's to motivate people to build structures that resist earthquakes. If you're on this top-10 list, hopefully it will motivate you to be better prepared in the event of a large earthquake."
The lab, part of the University's College of Science and Mackay School of Earth Sciences and Engineering, records earthquakes in Nevada and parts of eastern California, as part of the U.S. Geological Survey's Advanced National Seismic System (ANSS). It also operates a seismic network in southern Nevada for the U.S. Department of Energy.
To determine the rankings, Anderson and Miyata consulted the ANSS catalog of earthquakes, which contains earthquake data from 1898 to 2005. They then supplemented this catalog with data from the USGS catalog of significant U.S. earthquakes from 1568-1989.
Miyata's participation as a GIS expert was also key, Anderson said.
"Yui is a GIS expert, and a graduate of our Department of Geography," Anderson said. "With GIS, it's very easy to sort out how many earthquakes have occurred with each state's borders."
"It only took a couple of days," Miyata said. "First, we took the whole earthquake catalog and divided it by state. And, we made sure that with states with coast line, we extended the borders to go about 100 kilometers (about 62 miles) beyond the coast."
In this way, the study was able to take into account seismic "hot spots" that exist undersea, and underneath, a state such as Hawaii.
Many earthquakes, including the magnitude-6.7 event that occurred in Hawaii recently, causing upwards of $50 million in damage, are driven by stresses set up on the flanks of active volcanoes by accumulated magma intrusions. Due to this concentrated volcanic source, Hawaii has a large number of smaller earthquakes. Seismicity in top-ranking states of Alaska, California and Nevada, however, is driven by plate tectonics, Anderson said.
For those keeping score, the difference in seismic activity between Nevada and Hawaii is not great, Anderson added.
"In terms of magnitude-7 earthquakes, Nevada and Hawaii are essentially tied, but in terms of magnitude-5 and greater earthquakes, Nevada is ahead," Anderson said. "Then, counting magnitude-3.5 and greater, Hawaii again leads. Considering the uncertainties, Nevada and Hawaii are essentially tied."
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