Fracking-related quakes make central U.S. as vulnerable as California to tremor damage
In developing this new product, USGS scientists identified twenty-one 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 has 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.
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.
One-year outlook: The nation’s shortest forecast yet
The new hazard model estimates where, how often and how strongly earthquake ground shaking could occur in the United States during calendar year 2016. The USGS chose this short timeframe of one year because induced earthquake activity can increase or decrease with time and is subject to commercial and policy decisions that could change rapidly.
The USGS National Seismic Hazard Map uses a 50-year forecast because that is the average lifetime of a building, and such information is essential to engineering design and the development of building codes. Building code committees are still determining whether it is appropriate to treat induced earthquakes in building code revisions, in part because induced seismicity changes on short time scales compared to the years it takes for building codes to be updated, reviewed, and adopted.
How will this help protect communities?
USGS say that the new report can be used by both government officials to make more informed decisions and by emergency response personnel to assess vulnerability and provide safety information to those who are in potential danger. Engineers can use this product to evaluate earthquake safety of buildings, bridges, pipelines and other important structures.
Dramatic change in the Central U.S.
The central United States has undergone the most dramatic increase in seismicity over the past six years. From 1973 to 2008, there was an average of twenty-four 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.
Research in the western U.S.
The CEUS has experienced the most significant U.S. increase in seismic activity due to induced earthquakes in recent years. Therefore, in the 2016 forecast, scientists distinguish between human-induced and natural seismicity only for the CEUS. Scientists also used a historical catalog of seismic events leading back to the 1700s, putting a strong emphasis on earthquakes in 2015.
While there are some areas of induced earthquakes in the western United States, they do not significantly change the regional hazard level compared to the much more abundant natural earthquakes. Therefore scientists just considered the historical catalog in the western U.S. and did not separate natural from induced earthquakes. Future research could take a more detailed look at induced seismicity in the west, including in California at The Geysers, Brawley, or the Los Angeles Basin.
Explanation of updates — mostly, but not all, are induced
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 since the previous hazard map was released. 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 USGS studies suggest that this process is only rarely the cause of felt earthquakes.
Learn more by reading six facts and common questions about human-caused earthquakes.
Distinguishing between induced and natural earthquakes
To determine whether particular 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 factors such as 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 that 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.
Testing and future research
“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.”