A new study of the strength and frequency of earthquakes nationwide ranks Hawai'i fourth out of the 50 states in seismic activity, behind Alaska, California and Nevada.

Previous rankings put Hawai'i slightly ahead of Nevada, and it may seem odd in the wake of the magnitude 6.7 and 6.0 earthquakes off the Big Island on Oct. 15 that the research by the Nevada Seismological Laboratory would drop Hawai'i down a notch.

John Anderson, director of the Nevada Seismological Laboratory and one of the authors of the study, said the point of the research is not so much to make comparisons between states as it is "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," Anderson said.

Paul Okubo, seismologist with the USGS Hawaiian Volcano Observatory, said the most important issue is the risk from seismic activity, which is also a function of where people live. In Hawai'i, most of the population centers are away from the Big Island, where volcanic activity makes the island the hub of the state's earthquake activity.

"The concern truly is what are the seismic risks that we are going to be exposed to, to somehow generate an awareness of what the respective exposure might be," Okubo said.

The Oct. 15 earthquakes were "a reminder to a lot of folks. It was probably news to many folks who have moved here since we had a big earthquake, so this was a real eye-opener in a lot of ways."

The study grew out of Anderson's curiosity about a ranking earlier this year from the United States Geological Survey that actually showed Nevada fourth, behind Hawai'i.

Anderson had long believed Nevada was the third-most seismically active state.

He then began a study with Yuichiro Miyata of the Nevada Seismological Laboratory to review the historical data, which led to a national study that produced the list of the top-10 most seismically active states. The research was published in the November/December issue of Seismological Research Letters, a bimonthly publication of the Seismological Society of America.

Alaska and California are clearly the most seismically active states, but the third- and fourth-place ranking depends on the method used. Anderson placed Nevada third and Hawai'i fourth based on a comparison of greatest-magnitude earthquake that is reached once a year on average.

By that measure, Alaska comes in at 6.7, California at 6.02, Nevada at 5.1 and Hawai'i at 5.0.

Hawai'i has large numbers of small earthquakes each year, along with fewer larger earthquakes and occasional quakes that can be quite destructive. For example, some estimates put the damage from the Oct. 15 earthquakes at more than $200 million.

A USGS map of recent earthquakes around the Big Island showed that as of yesterday afternoon, there had been 41 temblors detected in the last two weeks, with the largest a magnitude 3.7 near Waikoloa on Nov. 12.

"In terms of magnitude-7 earthquakes, Nevada and Hawai'i are essentially tied, but in terms of magnitude-5 and greater earthquakes, Nevada is ahead," Anderson said. "Then, counting magnitude-3.5 and greater, Hawai'i again leads. Considering the uncertainties, Nevada and Hawai'i are essentially tied."

Other seismologists with the U.S. Geological Survey said there are a number of ways to look at the likelihood that an earthquake will affect an area.

Peter Cervelli, research seismologist with the USGS Alaska Science Center, joked that "it isn't quite fair" to compare the two states because Nevada is about 10 times larger than Hawai'i. Since Hawai'i is tiny and Nevada is enormous, more earthquakes would be expected in Nevada given how much more area is within the state's boundaries.

Cervelli suggested a better way to gauge earthquake activity in an area would be the total seismic moment release over the course of a year on average, which would take into account both the size of the earthquakes and the number in an average year.

Reach Kevin Dayton at kdayton@honoluluadvertiser.com.