So many earthquakes have struck the west Texas county of Scurry in the past week – more than 100 at last count – local officials have declared a state of emergency.

Scurry County Judge Dan Hicks wrote in his Friday declaration of disaster that since the first earthquake, registering magnitude 4.9, was felt the night of July 22, "damage has been found throughout Scurry County in businesses and residences."

The county’s buildings can handle a few quakes here and there, but the cumulative effects of so many small ones, punctuated by larger shaking, has become cause for concern.

From his second floor office in the Scurry County Courthouse, Hicks told the Abilene Reporter News, part of the USA TODAY Network, workers in the building became alarmed during a 5.0 quake Friday, which hit at 9:28 a.m. local time. Trophies rattled inside display cabinets, and pictures bounced against the walls.

“The building was shaking pretty good,” Hicks said.

103 earthquakes in eight days

From the first large quake of magnitude 4.9, the county has experienced a total of 103 earthquakes, including 12 of magnitude three or more, said Justin Rubinstein, a geophysicist with the U.S. Geological Survey in Menlo Park, California.

The rash of earthquakes is not naturally occurring, as Texas in general is not a very seismically active part of the country. Instead, it is"almost 99% likely" to be linked to local oil fields, Rubinstein said.

“We can say with confidence that these are related to oil and gas extractions," he said.

The temblors are very likely linked to new forms of oil and natural gas drilling technology that allow companies to drill not just down into the earth but horizontally along an oil formation.

They are reaching deeply buried oil and natural gas deposits that are the decomposed remnants of plants and animals in ancient oceans. When the oil comes up, the salt water, which can be millions of years old, also comes up.

This is called "produced water" and there's a lot of it.

"The ratio of oil to saltwater is low. It can be five or 10 or even 20 barrels of salt water for every barrel of oil," said Rubinstein.

This prehistoric water is much saltier than ocean water and can't be disposed of in rivers or even the ocean, in part because it can contain contaminants such as hydrocarbons.

Instead, it must be pumped back deep underground where it cannot leech into groundwater, a process called saltwater disposal.

The large amounts of water being pumped underground in turn can cause earthquakes. "We've found evidence that saltwater disposal is the most likely cause of the earthquakes in Scurry County. This specific area has seen seismic activity going back to 2020," Rubinstein said.

How to interpret:Earthquakes happen all the time, you just can't feel them. A guide to how they're measured

Magnitude is a measurement of the strength of an earthquake. Officially it's called the moment magnitude scale. It's a logarithmic scale, meaning each number is 10 times as strong as the one before it. So a 5.2 earthquake is moderate, while a 6.2 is strong.

Texas is investigating the quakes

On Friday, the Railroad Commission of Texas – which regulates the state's oil and natural gas industry – announced it was looking into any connections between the tremors and the injection of fluids into the ground for the extraction of petroleum products.

"In efforts to reduce seismicity possibly caused by underground injection of produced water, several operators in the area have converted deep saltwater disposal wells to shallow saltwater disposal wells within the last year," the Commission said in a statement.

Commission inspectors are now inspecting saltwater disposal wells within two and a half miles of the cluster of earthquakes.

"The RRC has shut-in two deep disposal wells in the area following inspections; and staff will continue to monitor wells and seismicity data to mitigate earthquakes and protect the environment and residents in the region," the statement said.