Miniscule changes in the Earth’s surface could indicate an impending earthquake—and now they can be sensed by a satellite in space.
Regions in eastern Texas have experienced an exponential increase in earthquake activity in recent years following the injection of wastewater from energy operations, such as fracking, into wells in the ground. When and how this makes a fault line fail and cause an earthquake is still poorly understood. By combining an improved mathematical model and disposal records with satellite tracking, scientists are developing a tool that might help predict future earthquakes.
Using remotely sensed radar data from the ALOS satellite, scientists at the University of Arizona reported being able to track changes in the Earth’s surface as small as 3 mm per year—one tenth of an inch—around wastewater injection sites. ALOS works by beaming down microwaves, which hit the ground and are scattered back to the satellite. By comparing these data over three years, it was possible to detect vertical movement of the Earth’s surface towards the satellite—surface uplift—which can be a sign of changing pressure underground. The study, published recently in Science , is the first to demonstrate surface uplift as a result of wastewater disposal.
“We knew that injection could cause surface deformation or uplift, but we were never able to prove it [before now],” said Manoochehr Shirzaei, the lead author of the study. “The satellite is about 800 km above the surface, and we are measuring 3 mm—that’s crazy.”
The study focused on a set of injection sites linked to a 4.8-magnitude earthquake that shook Timpson, Texas, in 2012. The earthquake hazard of a region is linked to the region’s background seismicity—the more earthquakes, the greater the chance of a bigger earthquake following them.
“We had less than one earthquake per year [in this region], and now we’ve had more than 1,500 since 2009. If you wanted to translate this to seismic hazard, we would give this a red flag,” Shirzaei said. “It’s a cascade of events. Over time, the size of earthquakes and the number of earthquakes will be upgraded. This is alarming.”
The majority of wastewater in this region is injected into limestone carbonate rocks above the basement layer, and hydraulic pressure propagates down into the basement. “Most of the earthquakes we see in east Texas and Oklahoma are occurring in this basement layer,” said Jake Walter, an earthquake seismologist at the University of Texas at Austin who was not involved in the study. “That’s the big jump right now. If we are injecting everything in these units above the basement, how are all these faults getting triggered? This study shows how it might be [that it’s the cause].”
While it’s possible to detect tiny surface changes, Walter said it will take time before its clear how useful these data will be for understanding the influence of wastewater injection on earthquakes.
Timpson is located between four injection wells—the two west wells are deep and allow water to percolate down to a fault, while the two east wells are shallower and are sealed from both the top and the bottom by an impermeable rock layer. Art McGarr, a research geophysicist at the U.S. Geological Survey, said that these differences in depth and permeability are likely why surface uplift was almost exclusively detected near the east wells, though the west wells were actually responsible for the Timpson earthquake, a conclusion that several previous studies have reached.
But because the east wells were not associated with the earthquake, McGarr said that “it’s premature to make any guesses as to whether [satellite monitoring] is going to become a useful procedure, at least in the foreseeable future.”
Though fracking has slowed in recent years, it shows no sign of stopping. And active disposal doesn’t immediately produce earthquakes—the wells in question were closed and sealed five years before the Timpson earthquake, suggesting that monitoring might be useful in other closed sites.
Shirzaei said that current practices can be modified, for example, by injecting at a slower rate to reduce earthquakes. Satellite sensing may also illuminate where pressure is moving to avoid further injection in those areas. “The best example is that you have a glass, and you can only fill it with a certain amount of water. If you keep filling it, it will spill,” Shirzaei said.