Faults are breaks in the earth’s crust where adjacent sections, or plates, have moved relative to each other. When a lot of slippage happens all at once, the result is a major earthquake, like the one that struck Haiti in January along the Enriquillo-Plantain Garden Fault, which runs in an east/west direction across the Caribbean. North of that fault a piece of crust called the Gonave Platelet is advancing westward, while the Caribbean Plate south of the fault is grinding to the east at a rate of about one-third inch per year. This side-by-side motion defines what is known as a strike-slip fault. (The Yushu earthquake in Tibet on April 13 was also the result of strike-slip movement, but the huge Chilean earthquake in February was instead a subduction event, where one plate slips under another.)

California’s San Andreas Fault, shown here, is probably the most-studied fault in the world. Like the Haitian fault, the 800-mile-long San Andreas is a strike-slip system. Geologists and seismologists are probing these massive cracks to learn more about the earth’s internal dynamics and to get better at predicting dangerous quakes.

STRESSED OUT
A constant shifting of the earth’s tectonic plates causes a buildup of stress in the crust, which eventually leads to earthquakes. To study these deep stresses, scientists installed the San Andreas Fault Observatory at Depth: a borehole drilled two miles deep into the fault, along with an underground seismic monitoring station. A ton of rock samples were extracted in 2007, the first time researchers gained access to a major fault at the depths where quakes start.

EARLY WARNINGS
Long-term forecasts remain a distant goal, but close monitoring of faults is yielding clues that could signal an impending earthquake. In 2008, researchers examining data from the San Andreas Fault Observatory at Depth reported detectable changes in the way seismic waves traveled through fault rock in the hours before two quakes. The Southern California Earthquake Center says there is a 46 percent chance that the state will see a magnitude 7.5 or greater quake in the next 30 years.

SLOW AND STEADY
Not all fault movements are violent tremors like the four monster quakes that have shaken San Andreas since 1690. Portions of the fault slide slowly and continuously as the North American Plate shifts northward an inch per year relative to the Pacific Plate. Scientists working for the U.S. Geological Survey examining samples from the San Andreas Fault Observatory at Depth have reported that these slow-moving sections contain talc, a soft mineral, which may account for their slipperiness.

AFTERSHOCKS
Powerful quakes have lasting impacts on a fault zone, creating new fractures and stresses that leave some areas prone to future damage. A six-year study of seismic activity along the San Andreas Fault found that the 2004 Parkfield earthquake caused breakage and microcracking in shallow rock layers that changed the speed at which seismic waves moved through the area, making it more vulnerable to tremors for more than three years afterward.