On 3 January 1998, an 8-meter-deep sinkhole suddenly opened up at a campsite bordering the Dead Sea, swallowing up a member of the camp’s staff. Since then, thousands of other sinkholes have developed in the area—a matter of great concern in this region, which draws tourists seeking to stand at Earth’s lowest point on land and float in water 10 times saltier than the oceans.
These maws, some of which are several tens of meters in diameter, have destroyed numerous buildings and roads and have forced local farmers to abandon their fruit orchards. Researchers now have used seismic waves to study an area near the southeastern tip of the Dead Sea that is riddled with sinkholes. They showed that the layering of buried sediments, rather than a thick band of salt, as was previously thought, likely predisposes the region to sinkhole formation.
An Artificial Earthquake
Sinkholes around the Dead Sea are not just destructive, explained Hussam Alrshdan, a geophysicist at the Ministry of Energy and Mineral Resources in Amman, Jordan. They’re also unpredictable. “Suddenly sinkholes appear,” he said. “It’s a big problem.”
In this new study, Alrshdan and his colleagues relied on a technique called shear wave reflection seismic imaging to trace how materials like clay, silt, salt, and sediments were layered in an alluvial fan near Ghor Al-Haditha, Jordan. Using a wheelbarrow-mounted vibrating source, the scientists launched seismic waves into Earth. “It’s an artificial earthquake,” Alrshdan said of the method.
These waves penetrated to a depth of roughly 200 meters. Seismic waves travel through different materials with different telltale velocities. By studying how quickly the waves propagated underground before being picked up again by seismic receivers positioned some distance away, the researchers could, in essence, take an “ultrasound” of the buried material.
With this ultrasound, Alrshdan and his colleagues inferred the composition and layering of the material in the alluvial fan with meter-scale resolution. Although the researchers studied only this one alluvial fan, these features are found around the Dead Sea in other areas characterized by sinkholes, the team noted.
One finding immediately stood out in the data. “We were surprised that we didn’t find any salt layer,” said Alrshdan. Previously, scientists studying the Dead Sea had suggested that a 2- to 10-meter layer of compacted salt lay roughly 40 meters below the surface. This salt, the reasoning went, played a key role in sinkhole formation: As freshwater runoff down nearby valleys slowly eroded this layer, it would weaken and produce cavities that eventually would turn into sinkholes.
But Alrshdan and his colleagues didn’t find any evidence of salt: The seismic wave speeds they recorded were several times slower than what would be expected if salt were present. Furthermore, two boreholes drilled in Ghor Al-Haditha down to 45 and 51 meters, respectively, showed no evidence of a salt layer. What then was responsible for the numerous sinkholes pockmarking the area, the team wondered?
The researchers found their answer in their seismic imaging, which showed regions that reflected seismic waves poorly. Instead of forming compact, ordered layers, the material in these areas was “loosening and cracking,” the team reported late last month in Solid Earth. The team hypothesized that water washed away fine-grained sediments like sand from the upper layers of the ground, weakening those layers’ structure. Over time, as these sediments were transported to the Dead Sea, they would leave behind an increasingly porous matrix of coarser materials—gravels and boulders—that would eventually give way and create a sinkhole, Alrshdan and his colleagues proposed.
The results indicate that shear wave reflection seismic imaging can be applied to help determine the factors that help form sinkholes in other settings and at other locations, noted Pauline Kruiver, a geophysicist at Deltares, an independent research institute in the Netherlands, who was not involved in this work.
On a more immediate level, this research is important for ensuring that future construction projects in the Dead Sea region aren’t built on sinkhole-prone ground, said Alrshdan. He knows that he and his team can’t stop nature; all of this work, Alrshdan says, is to help people “live in peace with these sinkholes.”