A series of atmospheric rivers slammed California in early 2023, dumping as much as 300% of the historical annual average rainfall on some parts of the state. The rains replenished surface water storage above its historical average, but scientists are still unraveling how the influx of precipitation affected the state’s groundwater, which suffered from extreme drought conditions from 2020 to 2022.
“They’re able to see how, over the course of several years…subsurface water storage changes.”
In a new study published in Science, researchers used an existing network of seismic instruments to probe that question. They found that surface water, such as lakes, streams, and reservoirs, had recovered above its historical average after the winter storms. But the volume of groundwater—defined in the study as water stored more than 50 meters (164 feet) below the surface—lagged far behind.
The seismic data allowed for “unprecedented spatial resolution, especially with depth,” said Roland Bürgmann, a geophysicist at the University of California, Berkeley, who was not involved in the study. “They’re able to see how, over the course of several years…subsurface water storage changes.”
Seismic Solutions
It’s “very important” to monitor groundwater because it makes up 40%–60% of California’s total freshwater supply, said Shujuan Mao, a geophysicist at the University of Texas at Austin and coauthor of the new paper.
Scientists traditionally measure groundwater depth in monitoring wells. In recent years, they have developed geophysical methods such as sensing ground uplift and subsidence due to water using airborne or satellite data. But these methods have shortcomings. Monitoring wells show water height at a single point, and because they are expensive to drill, they can’t be placed at a high spatial resolution. And many airborne and satellite methods don’t give the depth profile of water, indicating only how the volume of groundwater has increased or decreased.
“It’s a buy-one-get-one-free technology.”
Seismic data get around both problems.
Geologic and human activity send constant vibrations called seismic waves through the ground. The speed of the waves varies depending on the amount of water present in the rocks through which they move.
The Los Angeles basin, which is prone to earthquakes, is home to a dense network of seismometers that record these waves traveling throughout the region. “The seismic array is already in place in Los Angeles for seismic hazard monitoring,” Mao said. “It’s a buy-one-get-one-free technology.”
That dense network allows seismometers to record volumes of water present not at just one location, like a monitoring well, but between seismometers, giving researchers a more detailed look at where groundwater is within the basin.
Seismic waves of different frequencies travel at different depths and can therefore record underground properties such as pore pressure—the pressure that groundwater exerts on surrounding soil or rock—at those depths. In rocks saturated with water, seismic waves travel more slowly where pore pressure is lower. Inferring pore pressure using seismic wave speed changes at various frequencies can reveal the amount of groundwater at different depths.
In the new study, researchers used the seismic wave speed in different frequencies present in 2 decades of seismic data from 68 seismographic stations to calculate a “seismic drought index,” which showed how the water content of aquifers in the Greater Los Angeles area changed over time.
The researchers found, as others have, that surface and near-surface water storage in Greater Los Angeles had nearly fully recovered from the drought because of the 2023 rains. But despite an increase after 2023, groundwater had regained only about 25% of water lost since 2006.
The transport of water from the surface into deep aquifers can take years, Bürgmann said.
A New Monitoring Method
Bürgmann said the most benefits of the seismic monitoring method are achieved when a dense network of seismometers exists, making the Los Angeles area particularly convenient for this type of monitoring.
But the method could work virtually anywhere, he added. “It may actually make sense in some areas to deploy seismometers for the purpose of imaging aquifers—making that part of the toolbox to study the aquifer system.”
Scientists have begun to use fiber-optic cables to measure seismic waves for other purposes, and Bürgmann said those data could also indicate water storage changes.
Mao said she hopes that water managers can use their seismic drought index to guide decisions about sustainable water use. The California Department of Water Resources said it “will continue to evaluate the applicability and development” of the seismic monitoring methodology for broader use across California.
“The use of a seismic drought index may be helpful in areas with adequate monitoring densities and can help provide information to understand drought recovery of vertically stacked aquifers,” according to a statement from the California Department of Water Resources.
—Grace van Deelen (@gvd.bsky.social), Staff Writer