After an earthquake, regional stream flows will sometimes increase because of an influx of groundwater being released from aquifers. This phenomenon is well documented, but the details of the underlying mechanisms remain somewhat mysterious.
A new study looking at the effect of the 2011 Tohoku earthquake in Japan on groundwater systems in China is shedding some light on how Earth’s subsurface can be affected by large earthquakes.
Groundwater systems often comprise layers of permeable rock called aquifers separated by low-permeability layers called aquitards. Previous studies have analyzed the effects of earthquakes on either aquifers or aquitards, but to date, nobody has quantified the effects of earthquakes on both aquifers and aquitards in the same groundwater system, said Zheming Shi, a hydrogeologist at the China University of Geosciences in Beijing and an author of a new study published in May in Water Resources Research.
“The commonly used tidal response models can only identify either aquifer or aquitard permeability, not both at the same time,” Shi said.
Shi and colleagues took a different approach, combining a traditional tidal response model with an analysis of barometric pressure changes detected in a 2,600-meter-deep well near the Shunyi-Qianmen-Liangxiang fault zone, one of the largest fault systems in Beijing.
Scientists have been monitoring subsurface changes in this well—drilled into an aquifer in porous limestone that is capped by layers of impermeable mudstone, sandstone, and andesite—for decades as part of the country’s extensive earthquake monitoring program. By comparing barometric pressures recorded 4 months before the 11 March 2011 Tohoku quake with data collected up to a year after the event, they found that the earthquake boosted the permeability of the aquifer by a factor of 6, whereas the permeability of the aquitard doubled.
“As far as I know, this is the first time that changes in both an aquifer and an accompanying aquitard have been quantified after an earthquake,” said Michael Manga, a planetary scientist at the University of California, Berkeley, who was not involved in the new study but who coauthored a commentary about the study’s findings in June in Water Resources Research along with Steve Ingebritsen, a hydrologist with the U.S. Geological Survey.
“Permeability is not usually thought of as a quantity that can change over time,” Manga said. “When you drill a well, you measure the rate of flow and then that’s the number you use” to describe the productivity of that well.
But during an earthquake, subsurface pressure changes, and new fracture networks and shifting gases and fluids can open new pathways for groundwater movement. The new study also demonstrates that these subsurface shifts are not permanent: As the area settled after the Tohoku event, detectable changes in the well returned to preseismic levels in about 4 months, Shi and his colleagues wrote.
“This is a very clever study that’s adding a lot to this discussion of how permeability can change in space and time,” Ingebritsen said. “I think there’s growing interest in this idea that permeability is a mutable property. I’d like to see more of these kinds of studies done in other wells in other geologic settings.”
The work may also have implications for groundwater quality after an earthquake, Shi said. Increases in the aquitard’s permeability may allow pollutants to find their way into groundwater supplies.
“The aquitard is a good indicator of the aquifer’s vulnerability to pollution,” Shi said. “If an earthquake causes changes in permeability in the aquitard, groundwater may move upward or downward, thus increasing the risk of groundwater contamination.”
—Mary Caperton Morton (@theblondecoyote), Science Writer