A new report by the U.S. Geological Survey (USGS) begins to address how to incorporate induced earthquakes into the seismic hazard maps.
Induced earthquakes are defined as those caused by human activities, such as fluid injection or extraction. The report is the result of “dramatic increases in seismicity rates” in the central U.S. over the last five to seven years. The increase is suspected to be due to injection of wastewater or other fluids in deep disposal wells.
The USGS identified 17 areas in eight states (Alabama, Arkansas, Colorado, Kansas, New Mexico, Ohio, Oklahoma and Texas) that have experienced substantial increases in earthquakes since 2009.
“This new report describes for the first time how injection-induced earthquakes can be incorporated into U.S. seismic hazard maps,” said Mark Petersen, chief of the USGS National Seismic Hazard Modeling Project, in a press release. “These earthquakes are occurring at a higher rate than ever before and pose a much greater risk to people living nearby. The USGS is developing methods that overcome the challenges in assessing seismic hazards in these regions in order to support decisions that help keep communities safe from ground shaking.”
Hazard maps are used to determine building codes, insurance rates and emergency plans and have typically forecast the likelihood of ground shaking over a 50-year period. However, the new models for induced earthquakes will attempt to predict the likelihood of ground shaking on a year-to-year basis because induced activity can vary rapidly with time and is subject to commercial and policy decisions.
According to the report, wastewater, such as that generated from oil production, increases underground pore pressure and can potentially lubricate nearby faults. The largest earthquake thought to be related to the deep injection of wastewater was a 5.6 magnitude temblor at Prague, Okla., in 2011.
A broad range of experts initially identified 14 areas that contained increased seismicity suspected to be induced by fluid injection or extraction and added three additional areas that are believed to have increased earthquake activity due to coal production.
“We acknowledge that natural earthquakes could occur within the defined zones during the stated time windows; however, we treat the seismicity as induced for this study because the earthquakes are all located near deep fluid injection wells or other industrial activities capable of inducing earthquakes,” the report says.
One example cited in the report is that of the Guy-Greenbrier zone in Arkansas, which experienced increased earthquake activity, attributed as induced quakes, until 2011 when deep well injection ceased, when the number declined. Studies in Oklahoma indicate that induced earthquakes end more quickly but aftershocks occur across a larger area than typically expected.
“Relatively small earthquakes can cause damage and earthquakes as small as M1.8 can be felt,” the report says. “It was reported at the workshop that an M3.4 earthquake in Oklahoma caused damage to residences. Damage from earthquakes, as low as mid-M3, could also affect the well systems, such as casing and pipes. Communities experiencing elevated levels of induced seismicity are looking to the USGS to understand the associated ground shaking hazard and earthquake risk to society.”
The USGS concluded that it is necessary to be able to detect induced earthquakes in new regions early on, requiring the reduction of the detection threshold for the national network from M3 to at least M2 or lower. While most induced earthquakes that can be felt are suspected to be caused by wastewater disposal, there are also a number of reports of earthquakes from the fracking process itself. The lower magnitude detection will allow the USGS to gain more insight into the factors that contribute to induced seismic activity.
Participants at the workshop at which the data for the report was reviewed sought advice for reasonable guidelines on when to stop and start injection.
“This is a difficult problem because it is unknown why induced seismicity occurs near some fluid injection wells and not at others,” the report says. “A helpful first step would be to compile a common format (and publicly accessible) database for wastewater injection operations that can be used to compare observed seismicity with exploration activities. This would also encourage the information exchange between industry and scientists.”
The full report is available from the USGS.