Detecting and interpreting these changes would help forecast earthquakes, but that detection has proven difficult, partly because scientists don't yet fully understand the complex chain of events that precipitates a quake.
Now, a team of researchers has claimed to find hidden clues that can help them predict looming quakes.
Studying the San Andreas Fault, Fenglin Niu and colleagues from Rice University in Houston, Texas, recently found a way to help determine that an earthquake is coming.
The team found a way to detect stress changes along the San Andreas Fault in California - the world's most studied fault - by measuring how fast seismic waves traveled through the deep rock.
A few hours before two separate earthquakes rumbled along the fault, the researchers saw a decrease in seismic wave speed that they think was triggered by stress changes leading up to the quakes.
The team designed instruments capable of sending and receiving seismic waves through the ground at depths of about one kilometer.
Scientists have shown that the speed with which seismic waves travel through rock changes with the level of stress.
"That's because the increased pressure squeezes tiny cracks in the rock together," Niu said. "Therefore, measuring seismic wave speed could, in principle, lead to a stress meter," he added.
So, he and his colleagues dropped their seismic wave generator and the receiver into two adjacent boreholes at the San Andreas Fault Observatory at Depth (SAFOD), sent seismic waves from one well to the other and measured their travel time continuously for two months.
The researchers saw that seismic wave speed closely followed variations in the surrounding air pressure in the atmosphere, which puts stress on the rock.
"The higher the air pressure was, the shorter was the travel time," Niu said.
A few weeks into the experiment, seismic waves suddenly slowed down, and 10 hours later, a magnitude-3 earthquake shook the ground. The same happened a few days later, two hours before a magnitude-1 earthquake struck.
"These drops in seismic wave speed may be related to pre-rupture stress-induced changes in crack properties," the team said.
According to Niu, though more studies are needed, but the method could form the basis for an early warning system for impending quakes.