The paper analyzes fault displacements on high-resolution seismic reflection profiles in two regions of the central US, including the Fort Worth Basin (FWB) of Texas and the northern Mississippi embayment (NME). They used these areas to assess whether seismicity is induced by human activity (drilling), or if it happens naturally.
Since 2008, Texas, Oklahoma, Kansas and three other states have experienced many more earthquakes than they had previously. Strikingly, Oklahoma's earthquake rate increased from one or two each year to more than 800. In the same time period, Texas has seen a sixfold surge in quakes.
Even though most of the tremors in Oklahoma have been small, the state has seen many damaging earthquakes that exceed a magnitude 5 level.
A majority of scientists agree that the uptick in earthquakes has been directly triggered by injections of wastewater from oil and gas production into deep wells. But some scientists have suggested the quakes are natural, and arise from faults in the earth's crust that sometimes move on their own, according to Scientific American.
However, researchers working on the paper looked back at 450 million years of fault history in the Dallas-Fort Worth area and found that these specific faults almost never move. Magnani and her colleagues concluded that these faults would not have created the uptick in earthquakes were it not for wastewater injection. The paper states that pressure from these injections spreads underground and can disrupt already-weak faults.
Although the work Magnani accomplished with her colleagues provides considerable evidence implicating drilling as the cause of the quakes, the Texas government has not officially acknowledged the link quakes have to drilling.
Magnani and her fellow researchers studied the the faults in Texas using images of the subsurface, which are similar to ultrasound scans. The images the researchers generated are called seismic reflection data, created by equipment that generates sound waves and records how fast waves bounce off faults and different layers of rock deep within the ground.
The study says faults that create earthquakes look similar to vertical cracks in a brick wall, in which one side of the wall has sunk down a few inches so the rows no longer appear to line up.
The researchers compared images of faults in the Dallas Forth Worth area, or north Texas, with images of other faults that have been active throughout geologic history. The other faults they looked at are in the New Madrid Seismic Zone, which encompasses parts of Arkansas, Kentucky, Missouri, and Tennessee along the Mississippi River. Faults in the New Madrid area produced earthquakes with a magnitude of seven or eight in the early 1800s, and have set off smaller quakes since then.
When Magnani and her colleagues examined the faults in New Madrid, they came across evidence showing the horizontal rows of bricks were offset at the fault line-- from the distant past up until the present time. However, the north Texas faults showed no disturbances in the last 300 million years.
The researchers proceeded to take another step to refine the results of their findings, but their seismic reflection data is not capable of picking up vertical offsets smaller than 15 meters, which is roughly 49 feet. However, the Texas quakes were so small that they caused offsets of only a fraction of a centimeter.
So the seismologists wanted to be positive the north Texas faults had not just been producing this type of tiny quake the entire time.
They were ultimately able to come to a conclusion because the offsets representing rock that are moved by earthquakes, are cumulative. This means that each new quake adds more distance.
The researchers took the 300 million-year time span, and calculated the maximum amount of small to medium sized earthquakes it would take to produce a cumulative offset just short of 15 meters. The answer the researchers got fell into the range of 3,800 to 6,000 earthquakes, or roughly, an earthquake every 50,000 to 79,000 years.
The paper states that even if earthquakes occurred that frequently, the likelihood of a natural earthquake sequence occurring in north Texas in the previous 10 years was only one in 60 million. North Texas has experienced five earthquake sequences during that same 10-year period, so the researchers wrote that it is “exceedingly unlikely” that the recent temblors were natural.