The Israeli city Tiberias looks over the Sea of Galilee, under which a series of mysterious earthquakes rumbled in 2013 and 2018.
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By Michael Price
One evening in September 2013, windows rattled and ceiling fans swayed in northeastern Israel as a small earthquake rumbled beneath the Sea of Galilee—the water Jesus is said to have walked on—also known as Lake Kinneret. Four more tremors struck over the next 4 days. Then, in July 2018, a dozen small earthquakes shook the same spot beneath the lake on a fault—the slip surface along which an earthquake ruptures.
Although far more powerful earthquakes have struck the Levant, swarms such as these are rare and mysterious. A new study suggests human activity is to blame. Pumping too much water from the region too quickly may have “unclamped the fault,” the authors argue. And, they add, Californians and others living above major faults better pay attention, lest a similar appetite for water trigger a far larger, more dangerous quake.
Researchers have long known that humans can cause earthquakes by injecting fluid into the ground—a technique used in hydraulic fracturing, or fracking, to break open rocks and release natural gas or petroleum. Pumping water into geothermal vents to generate steam for heating and electricity has also triggered temblors. But earthquakes set off by pumping fresh water out of underground aquifers haven’t received much attention, says study co-author Emily Brodsky, a seismologist at the University of California (UC), Santa Cruz. “This is not the first example of groundwater extraction triggering earthquakes, but it’s uncommon,” she says.
For decades, Lake Kinneret provided up to a third of Israel’s domestic water supply, but a booming population and declining rainfall caused lake levels to fall precipitously. In the 1990s, Israeli officials encouraged people to pump groundwater from the wells surrounding the lake instead. After the 2013 and 2018 earthquake swarms, Nadav Wetzler, a geologist with the Geological Survey of Israel in Jerusalem, wondered whether the events were connected.
To find out, he, Brodsky, and colleagues compared the dates, locations, depths, and magnitudes of the earthquakes to regular measurements of groundwater levels in the region’s aquifer. They found that the quakes, most of which registered relatively harmless magnitudes between 3 and 4, occurred after steep groundwater declines from 2007 to 2013, and again from 2016 to 2018.
Injecting water into aquifers is thought to trigger earthquakes by increasing the water pressure in pores within the rocks, lubricating faults and allowing them to slip more easily. But how might removing groundwater trigger an earthquake swarm? In a paper published last week in Geophysical Research Letters, the authors suggest a model in which extracting groundwater reduces the gravitational load on a fault—lessening the forces that push rocks on either side of the fault together to keep it locked. “Pulling the water out allows the rocks to kind of relax away from one another, and therefore unclamps the fault,” Brodsky explains.
It’s a reasonable explanation, says earth scientist Manoochehr Shirzaei at Arizona State University in Tempe. He says scientists have traditionally assumed that freshwater fluid removal can’t easily trigger earthquakes. “This is one of the first studies to show that it can.”
Wetzler says the 2018 earthquake swarm, although relatively small, was a “wakeup call” for Israeli seismologists and government officials to improve earthquake readiness and bring buildings up to code. The Dead Sea Transform fault, which runs underneath the Sea of Galilee and the Dead Sea and accommodates the grinding motion between the Arabian and African tectonic plates, has been responsible for far larger earthquakes. Historical records reveal that in 1138, for example, a quake with an estimated magnitude between 7 and 8 hit the region, killing some 230,000. More recently, a 6.25-magnitude earthquake in 1927 killed close to 300.
Though the 2013 and 2018 swarms were comparatively mild, Brodsky says officials should be wary of them, no matter their initial cause, as small quakes can trigger larger ones. “Earthquakes beget earthquakes,” she says. “Once the rock breaks, it doesn’t know how it started.”
That means Californians should also appreciate the link between groundwater and seismicity, Wetzler says. In recent years, aquifers underneath California’s Central Valley have been overpumped, even amidst severe drought conditions. Worryingly, Wetzler adds, the nearby San Andreas fault, responsible for some of California’s largest earthquakes, is similar to the Dead Sea Transform.
The findings may even help explain why California sees slightly more earthquakes during the state’s dry summers, when groundwater pumping is at its peak, than in comparatively wetter months, says Roland Burgmann, a geophysicist at UC Berkeley. “We used to say there’s no such thing as ‘earthquake weather,’ but now it’s looking like there might be a subtle connection.”
Source: Science Mag