As Lake Mead filled this summer with runoff from record snowpack in the Rockies, the ground beneath it began to shake.
A string of small earthquakes, more than 20 measuring around 2.0 on the seismic scale, led geologists to infer that the rapid refilling of the lake was triggering the movement of the earth beneath it.
It's not the first time.
Geologists documented a correlation between lake filling and earthquakes in the early decades of the lake's existence.
A 1972 report on reservoir-caused earthquakes concluded that the area, streaked by a series of minor faults, was not geologically active until after completion of Hoover Dam on the Arizona-Nevada border.
When the lake filled, the shaking began, said Arizona State Geologist Lee Allison, who has been documenting this year's quakes on his "Arizona Geology" blog.
"The crust there is really bouncing," Allison said. "This all just kind of armchair geology at this point, but there seems to be a correlation between rapid filling and low-level seismic activity," he said.
A 1972 report by the National Research Council said the first Lake Mead shocks were recorded when it was filling in 1936. The largest shock in the area, a 5.0 earthquake, was recorded in 1939 when the lake was at 80 percent capacity.
People seem surprised when they learn that human activity can cause quakes, said Leonardo Seeber, a research professor at Lamont-Doherty Earth Observatory at Columbia University, but the cause-and-effect is only common sense.
"It's not surprising that if you change the stress regime, even by a little, you can trigger earthquakes."
Reservoirs are not a small change. Dams make it possible to move vast weight from one place to another. Lake Mead, at its current level, contains more than 16 million acre-feet of water. That's more than 5 trillion gallons of water, at 8 pounds per gallon, or more than 20 billion tons.
Quakes can also be triggered when you take weight off the ground in mining and quarrying. Injecting and extracting vast amounts of water, as is done in hydraulic fracturing operations, can be triggers, Seeber said.
Dams have caused major earthquakes in China and India, he said. "This belief that if you trigger earthquakes, they are going to be small, is simply not true," he said.
Contact reporter Tom Beal at email@example.com or 573-4158.