Seismic Study: 2023 Storms Hit LA Aquifers Hard

Despite record rainfall in the region in early 2023, only a fraction of Southern California's groundwater reserves has been replenished, researchers report. Their study, which leverages seismic noise data from across Greater Los Angeles, highlights the urgent need for improved monitoring and management of the state's critical groundwater reserves. After enduring two decades of severe drought, California experienced an abrupt meteorological shift in water in 2023. A succession of 16 atmospheric rivers from late 2022 through early 2023, followed by the torrential rains of Hurricane Hilary in August, resulted in one of the wettest years on record. In the Greater Los Angeles region, rainfall reached 300% of historical norms. Although this period of intense precipitation rapidly replenished California's surface reservoirs to 128% of their historical average, the extent of groundwater recovery has remained uncertain. Accurately quantifying changes in groundwater storage is essential for assessing drought impacts and ensuring sustainable water management. Yet, it remains a complex challenge, particularly at the basin-to-watershed scale essential for resource planning. Focusing on the Los Angeles area, Shujuan Mao and colleagues demonstrate that seismic noise analysis can effectively track groundwater storage changes with high resolution. Using ambient-field interferometry and passive data from 2003 to 2023 from a network of existing seismometers, Mao et al. measured variations in seismic wave speed; these variations reflect shifts in subsurface water distribution due to groundwater recharge. The regional seismic hydrograph revealed that the 2023 storms replenished only about 25% of the groundwater lost over the past two decades, highlighting substantial long-term depletion and a slow recovery of deep aquifers on a decadal scale. "Integrating the seismic imaging techniques of Mao et al. with existing methods, such as in situ well monitoring and satellite-based or aerial remote-sensing observations, would allow for comprehensive tracking of groundwater," write Taka'aki Taira and Roland Bürgmann in a related Perspective. "This will help to ensure the preservation of sustainable water supplies for future generations."