Parts of southern Kansas and northern Oklahoma have as high a probability of damaging earthquakes this year as parts of California, according to a new study released Monday by scientists with the U.S. Geological Survey.
It’s the first time the USGS has created a map to identify potential ground-shaking hazards from both human-induced and natural earthquakes — and it places a target for highest probability on a region spreading out from central Oklahoma and reaching into several counties in Kansas.
“The chance of having (a damaging earthquake) is 5 to 12 percent per year in north-central Oklahoma and southern Kansas,” the study states, “similar to the chance of damage caused by natural earthquakes at sites in parts of California.”
“People living in areas of higher earthquake hazard should learn how to be prepared for earthquakes, and guidance can be found through FEMA’s Ready Campaign,” a news release from the USGS stated.
For the forecast, the scientists use the Modified Mercalli Intensity scale, which measures shaking felt by people, rather than seismic equipment.
The projections are for MMI VI (6) or greater earthquakes. An MMI intensity of 6 indicates minor damage, such as fallen plaster or cracks in the walls, while intensities of 7 to 10 relate to heavier damage.
A damaging quake, said Mark Petersen, chief of the USGS National Seismic Hazard Mapping Project, is generally about a magnitude 2.8 or higher on the Richter scale, though the depth of the quake and the type of ground influence the level of ground shaking.
“We’re hearing calls from people who feel shaking and wonder if their house is going to collapse, or if they should move,” Petersen said. “This is to tell them, there is a potential for damage, but mostly for cracking damage, not collapsing buildings. There’s a chance that could happen, but mostly what will happen is what we’ve seen.”
MMI intensities of 4 and 5, incidentally, are felt by many, but typically cause only limited damage to windows or dishes.
The new map is not broken down to county level, but it generally indicates possibilities — based on a 1 percent probability of occurrence — of MMI 8 or greater damage in an area including portions of Harper and Sumner counties in Kansas, and MMI 6 and 7 in an arc spreading several counties out from that.
The map is only a one-year projection based on earthquake activity since 2000, and it does not take into account changes in oil and gas productions or new regulation on wastewater disposal.
Scientific studies have linked the majority of increased earthquake activity in the central and eastern U.S. to the injection of wastewater from oil and gas production into deep disposal wells. Lower oil prices in the past year have significantly reduced oil exploration, and regulators in Kansas and Oklahoma have imposed some restrictions on wastewater injections levels.
“In the past five years, the USGS has documented high shaking and damage in areas of these six states, mostly from induced earthquakes,” Petersen said. “Furthermore, the USGS ‘Did You Feel It?’ website has archived tens of thousands of reports from the public who experienced shaking in those states, including about 1,500 reports of strong shaking or damage.”
Those included some 1,300 reports of strongly felt quakes in Oklahoma and 102 in Kansas.
“We don’t think we’re taking a big step in saying this,” Petersen said. “Many of the places where damage is likely in our 2016 model have experienced damage in the past decade. We do want people to understand how much concern we have and what we’re concerned about, so we can continue to dialogue about what to do, to mitigate effects in the future.”
The most significant hazards from induced seismicity, the study states, are in six states. Listed in order from highest to lowest potential hazards, they include Oklahoma, Kansas, Texas, Colorado, New Mexico and Arkansas.
In developing the map, USGS scientists identified 21 areas around the country with increased rates of induced seismicity. In all, some 7 million people live in areas exposed to potentially damaging quakes, with Oklahoma and Texas having the largest populations exposed to induced earthquakes.
The map includes close to half the state of Oklahoma, while in Texas, the danger zone centers on Dallas.
The central U.S. has undergone the most dramatic increase in seismicity over the past six years, the study states. From 1973 to 2008, there was an average of 24 earthquakes of magnitude 3.0 and larger per year in the central U.S. Since then, the rate steadily increased, averaging 318 per year and peaking in 2015 with 1,010 earthquakes.
The study, drafted by a nearly a dozen scientists, is intended for use by government officials “to make more informed decisions, as well as emergency response personnel to assess vulnerability and provide safety information to those who are in potential danger,” the authors stated.
Engineers can also use the map to evaluate earthquake safety of buildings, bridges, pipelines and other important structures, though the scientists, speaking during a media conference call, declined to “get in the middle” of how the projections should be used, leaving that up to engineers and public officials to determine.
Asked about the potential magnitude of induced quakes, the scientists said there is not an industry consensus. Current research indicates the maximum magnitude of induced earthquakes may be lower than for natural earthquakes, but many scientists suggest that induced earthquakes can trigger larger earthquakes on known or unknown faults.
Induced earthquakes also tend to exhibit swarm-like behavior, with more numerous, though smaller, earthquakes at shallower depths.
The largest natural earthquake would probably be the limit for induced quakes, said Justin Rubinstein, deputy chief of the Induced Seismicity Project for the USGS. The largest believed induced quake in Oklahoma was a magnitude 5.6 near Prague in 2011, but scientists estimate the largest natural quake in the state was a prehistoric magnitude 7.1, near Meers, in the southwest part of the state.
“To me, I’d not be surprised by a magnitude 6, and it’s possible it could be higher,” Rubinstein said. “In many cases, whether it’s a 6 or 7 doesn’t much matter. It would cause a lot of damage.”