Northwest Arkansas sits almost exactly between two of the most seismically active regions of the country, and seismologists say the risk of a significant earthquake is growing.

About 200 miles west is central Oklahoma, where geologists said the rate of noticeable earthquakes jumped 240 times what it was in 2008. The New Madrid fault zone in Arkansas' northeast corner, site of several of North America's strongest earthquakes, sits the same distance to the east.

Large earthquakes in either area, though relatively rare, could reach and potentially cause damage in Washington and Benton counties.

"Arkansas is a seismically active state," said Scott Ausbrooks, a geologist with the Arkansas Geological Survey. The survey monitors seismograph stations across the state, including in Hobbs State Park in Benton County. "In this business, you don't take anything for granted."

Unruly Earth

Patterns new and old are driving up seismic risk in both regions, scientists said.

Oklahoma's noticeable earthquakes -- generally with a magnitude of at least 3 or so -- have surged to unprecedented numbers. The state is on track to experience 500 this year, up from one or two per year before 2009.

As a result, roughly one in six quakes of that size worldwide originated within Oklahoma's borders, according to the U.S. Geological Survey. Most fall along a north-south line through Oklahoma City.

"We've never seen anything like this before," said Austin Holland, a research seismologist with the Oklahoma survey. "It's remarkable."

It's also potentially a growing problem, Holland said, because earthquakes tend to build off each other at fairly predictable rates. About 100 magnitude 1 quakes occur for every magnitude 3, for example.

Likewise, the more 3s there are, the more likely another 5 or 6 is to hit, and so on. A quake of that size carries enough energy to spread hundreds of miles outward, including across the Arkansas border.

A magnitude 5.6 quake, the state's largest, hit in November 2011. It destroyed 14 houses near Oklahoma City and was felt in Fayetteville and as far away as Milwaukee, according to the USGS.

Meanwhile, a different process could be slowly bumping up the risk in the New Madrid Fault Zone to the east: Time.

Two centuries ago, an earthquake there famously clanged church bells in South Carolina and rattled people in Boston. It was one of at least three major earthquakes in late 1811 and early 1812, each with magnitudes between 7 and 8, among the continent's largest on record.

For comparison, an Alaskan earthquake of similar strength in 1946 caused 60-foot waves in Hawaii, 2,400 miles away.

Earthquakes of this size have struck New Madrid every few centuries for millennia, and while the pattern isn't consistent enough to set a watch by, Ausbrooks said, the clock is ticking.

"There's no real reason to think that area's died out," said Jeff Connelly, earth sciences chairman at the University of Arkansas at Little Rock.

The fault has generated a handful of moderate quakes since 1811, including a pair at magnitude 4.1 in 2005, according to the Arkansas survey.

In a 2009 report, the USGS predicted roughly one-in-three odds for an earthquake of at least magnitude 6 in the next 50 years from the area. A repeat of 1811-strength quakes, meanwhile, has up to a 10 percent chance of happening, the report found.

This time around, the effects would be far more severe.

Hundreds of thousands of buildings would be damaged and tens of thousands of people could be injured or killed in Memphis, St. Louis, Northwest Arkansas and elsewhere, according to a 2009 Federal Emergency Management Agency study. Economic losses could total $300 billion, more than four times the cost of Hurricane Katrina, it found.

"We've thought about it and planned for it," said Robert McGowen, Benton County emergency director, noting disaster response largely depends on the specific damage. In Northwest Arkansas, McGowen said, the biggest concerns would be damage to society's connective tissue: gas and power lines and bridges.

The more likely scenario is assisting parts of the state closer to New Madrid, such as sending search-and-rescue teams or the mobile command unit, he said.

Old And New

Different processes seem to be driving up seismic risk in each area, scientists said.

A study published this month in the journal Science blamed Oklahoma's activity on water that's used to pump for oil and natural gas throughout the state. The process is called hydraulic fracturing, or "fracking."

After the fracking process, companies re-inject billions of gallons of the wastewater deep underground, building pressure along miles-long faults, according to the study. The pressure can be enough to push the sides of these faults past each other, causing a tremor.

"We're looking at that as a real and likely contributor" to the recent activity, Holland said, stressing the issue is the re-injection of the wastewater, not the fracking itself.

The oil and gas industry says more research is needed for a definitive link.

"We've been doing injection in the state for a long time," Chad Warmington, president of the Mid-Continent Oil and Gas Association of Oklahoma, told Bloomberg this month. "It deserves a lot more investigation before making a determination."

The same process was blamed for more than a thousand earthquakes in north-central about three years ago, Ausbrooks said.

The quakes clustered between Conway and Greers Ferry Lake and mostly faded after wastewater re-injection was banned in the area, Ausbrooks said.

Minor earthquakes continue to be recorded in the area, including a 3.8 quake last month.

"I heard it as much as felt it," said Linda Temple, who lives in Fairfield Bay on the lake. "I thought it was an explosion as opposed to an actual earthquake. It's not particularly common."

In contrast to Oklahoma, New Madrid's seismic activity stretches back eons.

The seismic zone formed roughly 600 million years ago, Ausbrooks said. A rift began to form in the North American tectonic plate, one of several enormous slabs of rock that sit on the Earth's surface like puzzle pieces. The rift failed, but the rock was left scarred and weakened.

The push and pull from other plates surrounding North America creates tension, which can be released through this weakness in the form of earthquakes.

Scientists still have much to learn about faults in the central U.S. like New Madrid, Ausbrooks and Connelly said. Such faults are more complicated than the better-known cracks at plate boundaries, such as the much more active San Andreas in California.

"We don't know or understand at this point how much energy is actually being transferred into that fault by the plate movement overall," Ausbrooks said. "It may be another 300 years before we get the next (quake). Then again, it could be tomorrow, or today."