Shujuan Mao et al., Depth-dependent seismic sensing of groundwater recovery from the atmospheric-river storms of 2023.Science387,758-763(2025).DOI:10.1126/science.adr6139
The article below is predominately excerpted from the articles cited above.
Mao et al. in their recent research study showed
that something called seismic ambient field interferometry, which is simply the
measurement of seismic vibrations , can be used to measure groundwater. Utilizing
the existing seismic monitors and decades of data in the Los Angeles
groundwater basin to measure the recharge of the groundwater from the 2023
atmospheric river that hit the region. They found that only about 25% of the
groundwater lost since 2006 was replenished by a very wet 2023. These
observations highlight the need for more continuous monitoring and provide a
new way to estimate groundwater resources.
The Los Angeles area is densely populated and has long faced
challenges of water supply in what is naturally a semi-arid region prone to droughts.
Most of the drinking water for the region is piped in from other parts of
California.
The region is also subject to earthquakes-seismic activity. In
response to the risk of earthquake, the regions has a network of instruments designed
and developed to study seismic hazards, and continuously records the ambient
seismic field, which is the ever-present ground vibrations resulting from
natural and manmade sources (trucks, construction etc.). Analysis of these
passive seismic records enables the calculation of spatiotemporal changes in
seismic velocity (20).
Seismic velocity changes as water saturation (moisture) of
the medium changes. Thus seismic velocity changes can serve as a measure of the
total water content in the subsurface. Passive sensing of seismic velocity
changes (Δv/v) has recently emerged as a noninvasive, cost-effective
approach for the continuous monitoring of aquifers.
Mao et al used the seismic data recorded by 68 stations in
the Los Angels area to calculate the seismic velocity changes, Δv/v over
the past two decades (32).
And then using all the other data available for the region demonstrated that
analysis of seismic noise can capture changes in the state of groundwater
storage with better depth resolution than the traditional method of soil
moisture content and simplified water balance. They verified the regional
seismic hydrograph against the water equivalent thickness (WET) changes derived
from gravity observations from the GRACE and GRACE-FO missions. NASA’s Gravity
Recovery and Climate Experiment (GRACE) and GRACE Follow-on (FO) have provided
a global picture of water storage trends for over two decades. The GRACE
missions are able to monitor monthly water storage changes within river basins
and very large aquifers using variations in gravity.
Most drought indices are derived using measurements of
above-surface and near-surface water, including precipitation, streamflow,
surface reservoir gauge, snow cover and melt, and soil moisture. However,
quantifying groundwater drought conditions remains challenging owing to the
limited resolution of groundwater monitoring data despite the essential role of
groundwater in the total water supply.
Over the two-decadal study period, surface-water
availability (indicated by Palmer-DI) varies rapidly in response to
precipitation, exhibiting severe to moderate drought during dry years, but
quickly returning to wet conditions in storm years. By contrast, the Seismic
drought index and GRACE drought index consistently suggest much more severe
conditions in groundwater drought that have continued to accumulate over the
study period. The researchers found that the storm replenishment over the past
20 years did not compensate for the substantial depletion during dry years. In
the years from 2014 to 2022, the surface water recovered and reached “wet”
conditions amid storms, but the deep aquifers remained persistently in moderate
to extreme drought conditions.
Groundwater aquifers are extremely important sources of/ and
storage for fresh water, especially in regions experiencing flash droughts that
can rapidly diminish surface-water supplies. Groundwater is essential not only
in California, but in much of the nation and even here in Prince William
County. Unsustainable groundwater use
can have detrimental effects such as aquifer depletion, loss of storage
capacity, chemical and waste contamination, saltwater intrusion, and land
subsidence. Of all the things mankind knows we seem to know the least
about what’s happening under the ground. This important work may help us change
that.
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