At 12:32 am Alaska time on January 23, 2018, a magnitude 7.9 earthquake shook Alaska residents out of their beds and set off fears of a tsunami all down the West Coast. Fortunately, the tsunami was only a few inches in height, but within an hour of the earthquake in Alaska, waves of a different sort were hitting far away in Florida.
Alaska Earthquake Rattles Florida’s Groundwater Plumbing
More than 3,500 miles from the Kodiak Earthquake’s epicenter, water levels at the USGS groundwater well near Madison, Florida, spiked by about two inches, while levels at the USGS groundwater well near Fort Lauderdale, Florida, dropped by an inch and a half. Both recovered to their previous levels within an hour.
Hydrogeologic responses to earthquakes have been known for decades, and have occurred both close to, and thousands of miles from earthquake epicenters. Water wells have become turbid, dry or begun flowing, discharge of springs and ground water to streams has increased and new springs have formed, and well and surface-water quality have become degraded as a result of earthquakes.
This is not even the first time a major Alaska earthquake caused groundwater effects far from its original epicenter. Water-level fluctuations caused by the 1964 magnitude 8.5 Alaska earthquake were recorded in 716 wells in the United States; the earthquake also was registered on water-level recorders in many other countries.
One common type of observed ground-water response is an instantaneous water-level offset, or step, which may be either an increase or a decrease and may occur near or far from the epicenter. Recovery to the pre-earthquake water level can be so rapid that no change will be detected if the water level is measured infrequently, or it may take as long as days or months.
The other common type of ground-water response is a water-level oscillation, which may occur during many earthquakes, but is rarely recorded because they do not last long enough for many groundwater monitoring systems to record them. In the few cases where oscillations have been recorded, they resemble long-period seismograms.
The change in water level can be small, measured in inches, or dramatic, measured in feet. However, the changes rarely exceed a couple of feet change.
So Why Does This Happen?
For the changes seen near Madison and Ft. Lauderdale, they are likely oscillations caused by the seismic waves. Think of it as the ripples in a glass of water on a table when a truck drives by outside.
Other causes for groundwater effects from earthquakes include the compaction of the overlying rock layer like what happens during liquefaction. That compaction can lead to temporary spikes in groundwater levels. Meanwhile drops in groundwater levels can be caused by the escape of gas from the rock layers. In a fractured rock environment, changes in groundwater levels can be caused by the unclogging, widening, or narrowing of a fracture, or the creation of new fractures.
USGS monitors groundwater levels all over the country through its Groundwater Watch network. These wells are located in rural, suburban, and urban areas, and are monitored regularly. Some are even provide real-time data.
In addition, USGS continues to study the effects of seismic activity throughout the United States.