Fewer big earthquakes in '17 forecast

A map showing earthquake hazard risk
A map showing earthquake hazard risk

The U.S. Geological Survey predicts that fewer damaging earthquakes will occur in the central and eastern United States this year, in part because of the reduction of wastewater injection processes used during natural gas drilling in the areas that scientists have linked to tremors.

The survey released its yearly forecast Wednesday, saying 3.5 million people live and work in areas with "significant potential for damaging shaking from induced seismicity." The majority of the population is in Oklahoma and southern Kansas, areas that have had scores of earthquakes in the past few years after gas drilling companies used hydraulic injections of wastewater deep into the earth's crust.

An additional 500,000 people live in areas with a high risk of natural earthquakes along the New Madrid Seismic Zone, the survey said. The zone is a system of faults that runs from Marked Tree to southern Illinois. A second system crosses from southern Missouri to western Tennessee.

Last year, the survey estimated that 7 million people were at risk for man-induced or natural earthquakes in the areas.

According to the survey, there is a 2 percent to 5 percent chance of a damaging earthquake occurring in the New Madrid seismic zone this year, which was the same probability the survey forecast for last year.

A small area of central Oklahoma north of Oklahoma City and west of Tulsa has a 10 percent to 12 percent chance of a damaging quake this year, the survey said. Last year, the area had a 5 percent to 10 percent chance of damage, the report indicated.

From 1980 to 2000, Oklahoma averaged only two earthquakes a year of magnitude 2.7 or stronger. In 2014, after drilling companies used injection well techniques, the number of similar-sized quakes rose to 2,500. A year later, it jumped to 4,000.

In 2016, Oklahoma quakes dropped to 2,500 after the state restricted the volume of wastewater injections, according to the survey.

"The good news is that the overall seismic hazard for this year is lower than in the 2016 forecast," Mark Peterson, chief of the survey's National Seismic Hazard Mapping Project in Denver, said in a news release. "But despite this decrease, there is still a significant likelihood for damaging ground-shaking in the year ahead."

The survey began including man-induced quakes in its yearly reports last year because there were hundreds of tremors near injection well sites in Oklahoma and Texas. This year's survey also noted a reduced risk in north-central Arkansas, where quakes have virtually ceased after the Arkansas Oil and Gas Commission issued a permanent disposal-well moratorium in 2011.

The survey's addition of the man-induced earthquakes shows the immediate dangers in areas, but some geologists say it also may overshadow the longer-range concerns for quakes in the New Madrid zone.

"This [report] is a snapshot," said Scott Ausbrooks, the Arkansas Geological Survey's geohazards supervisor. "The USGS struggled with how to handle the man-made quakes. There were induced-seismicity areas popping up all over.

"When you include that, you'll see significant changes in the [hazard] maps every year. The New Madrid has stayed the same."

Ausbrooks said because the southern edge of the New Madrid Seismic Zone did not have any quakes registering 2.7 or above in magnitude last year, the Arkansas section of the fault system saw a slight reduction in damage risk this year in the survey's forecast.

"Overall, the New Madrid has had more seismicity during the past three years," he said. "If we get one 3.0 or 4.0 in the southern end near Marked Tree, next year's map will show that."

The most active area of the seismic zone has been around the Reelfoot Lake area in Tennessee, said Steve Horton, a geologist with the survey in Memphis.

"The report is comparing two slightly different things -- induced and natural earthquakes," Horton said. "We're also looking at two different time periods. We can see that last year there were more earthquakes on the New Madrid than the average over the last 50 years."

Horton said that while the survey has not definitively claimed that the injection wells cause quakes, there is "very good evidence" that it does.

"Injections change the physics of the earth," he said.

The injections add pressure to existing fault systems, causing them to slip easier, he said.

"Earthquakes may eventually happen on those faults naturally," Ausbrooks added. "But when you inject the wastewater, you tip the balance. You speed up the clock. The energy is there on the fault. They just needed a little nudge."

Ausbrooks said he will continue to raise awareness about the dangers of the New Madrid fault system.

"The induced earthquakes can change," he said. "The New Madrid hazard is not changing."

State Desk on 03/02/2017