New maps from the U.S. Geological Survey identify potential ground-shaking hazards from both human-induced and natural earthquakes.
In the past, USGS maps only identified natural earthquake hazards.
The latest report from the federal agency shows that about 7 million people live and work in areas of the central and eastern U.S. — CEUS — with potential for damaging shaking from induced seismicity.
Within a few portions of the CEUS, the chance of damage from all types of earthquakes is similar to that of natural earthquakes in high-hazard areas of California, according to the USGS.
“By including human-induced events, our assessment of earthquake hazards has significantly increased in parts of the U.S.,” stated Mark Petersen, chief of the USGS National Seismic Hazard Mapping Project. “This research also shows that much more of the nation faces a significant chance of having damaging earthquakes over the next year, whether natural or human-induced.”
Induced earthquakes are triggered by human activities, with wastewater disposal being the primary cause for recent events in many areas of the CEUS. Wastewater from oil and gas production operations can be disposed of by injecting it into deep underground wells, below aquifers that provide drinking water.
The agency, in its release, emphasized that USGS scientists only distinguished between human-induced and natural seismicity in the CEUS. In the western part of the United States, scientists categorized all earthquakes as natural. Scientists also used a different methodology in looking at the CEUS compared to the West.
Six States Face Highest Hazards
The most significant hazards from "induced seismicity" are in six states, listed in order from highest to lowest potential hazard: Oklahoma, Kansas, Texas, Colorado, New Mexico and Arkansas. Oklahoma and Texas have the largest populations exposed to induced earthquakes.
“In the past five years, the USGS has documented high shaking and damage in areas of these six states, mostly from induced earthquakes,” said Petersen. “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.”
USGS scientists identified 21 areas with increased rates of induced seismicity.
Induced earthquakes have occurred within small areas of Alabama and Ohio, but a recent decrease in induced earthquake activity resulted in a lower hazard forecast in these states for the next year. In other areas of Alabama and small parts of Mississippi, there has been an increase in activity and scientists are still investigating whether those events were induced or natural.
The central U.S. has undergone the most dramatic increase in seismicity over the past six years, according to the agency. From 1973 to 2008, there was an average of 24 earthquakes of magnitude 3.0 and larger per year. From 2009 to 2015, the rate steadily increased, averaging 318 per year and peaking in 2015 with 1,010 earthquakes. Through mid-March in 2016, there have been 226 earthquakes of magnitude 3.0 and larger in the central U.S. region.
To date, the largest earthquake located near several active injection wells was a magnitude 5.6 in 2011 near Prague, Oklahoma.
The USGS published a study in 2014 that only considered natural earthquakes. The largest changes in this new report are primarily due to hazards from induced earthquakes, but the calculations also consider updated forecasts for natural earthquakes.
For example, the New Madrid Seismic Zone near Memphis has experienced a higher rate of natural earthquakes in the past two years, leading to a slightly higher hazard potential in small portions of Arkansas, Missouri, Illinois, Kentucky and Tennessee.
Wastewater disposal is thought to be the primary reason for the recent increase in earthquakes in the CEUS. While most injection wells are not associated with earthquakes, some other wells have been implicated in published scientific studies, and many states are now regulating wastewater injection in order to limit earthquake hazards.
Many questions have been raised about hydraulic fracturing — commonly referred to as “fracking” — and the USGS said its studies suggest that this process is only rarely the cause of felt earthquakes.
To determine whether clusters of earthquakes were natural or induced, the USGS relied on published literature and discussions with state officials and the scientific and earthquake engineering community.
Scientists looked at whether an earthquake occurred near a wastewater disposal well and whether the well was active at the time these earthquakes occurred. If so, it was classified as an induced event.
Current research indicates the maximum magnitudes 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.
In the CEUS, there may be thousands of faults that could rupture in a large earthquake. Induced earthquakes also tend to exhibit swarm-like behavior with more numerous and smaller earthquakes at shallower depths. These factors were taken into account in the analysis.
“We are using the best available data and principles to determine when, where and how strong the ground could shake from induced earthquakes,” said Petersen. “Of course there is a level of uncertainty associated with this and all hazard maps, as we are still learning about their behavior and can only forecast with probability — instead of predict with certainty — where earthquakes are likely to occur in the future. Testing these maps after a year will be important in validating and improving the models.”
The federal report came shortly after the Sierra Club and Public Justice filed a federal lawsuit against three energy companies engaged in hydraulic fracturing in Oklahoma. The suit alleges that wastewater from fracking and oil production has contributed significantly to the alarming increase in earthquake activity.
Oklahoma has seen a rapid increase in earthquakes registering at or above a 3.0 magnitude per year, with 109 in 2013, 585 in 2014, and more than 900 in 2015.
Sierra Club released the following statement: "Today’s report once again highlights the dangers the fracking cycle poses to our communities. The world is already experiencing deadly storms, droughts, and erratic climate and weather extremes due to climate change, and the rapid increase in earthquakes caused by wastewater injections from the oil and gas industry only raises the threat to communities across the country.”