More recently, researchers have been using satellite measurements to detect small changes in Earth’s surface deformation or gravity field, which works well to infer changes in groundwater storage over time and area. “These surface measurements couldn’t tell us what’s happening at different depths,” said Mao, “but that’s where we as seismologists can help.”
With data from 65 broadband seismographs in the Southern California Seismic Network, the researchers looked at changes in the propagation speed of seismic waves. The data they used are the “background” seismic vibrations generated by the oceans, winds, and human activity—not the seismic waves associated with earthquakes.
“These background seismic vibrations are continuous, which allow us to measure and monitor the seismic velocity changes continuously,” Mao said.
Seismic wave speed varies with the mechanical state of materials that the waves are passing through. When the groundwater level increases, the pressure in the porous space among rocks increases, and the seismic waves propagate slower through this porous rock.
The researchers found that their estimates of groundwater storage calculated from seismic velocity change compared well with groundwater storage measurements from well and satellite data.
The researchers also found prominent aquifer replenishment in San Gabriel Valley and Raymond Basin, likely due to surface or subsurface flows from the San Gabriel mountains.
The combination of a dense seismic network and a pressing water shortage made Los Angeles “an ideal place to showcase how existing seismic data can contribute to the monitoring, understanding, and management of groundwater aquifers,” said Mao.
Seismic data would likely be integrated with many types of measurements to produce the comprehensive picture of groundwater dynamics needed to manage the valuable resource in a data-informed and sustainable manner, she added.
Mao, who will be an assistant professor at The University of Texas at Austin in August, said she will apply seismic techniques in Austin to monitor how aquifers in that region respond to artificial recharge operations.