Geologists working near California's San Andreas Fault have found a way to monitor the strength of a geologic fault — a finding which could be used to predict when a fault would fail and cause an earthquake.

Faults are breaks in the Earth's crust where one side of the break moves relative to the other. When the fault fails, the walls of rock slip against each other, causing an earthquake.

The strength of a fault can't be measured at the Earth's surface and geologists had no way to monitor changes in the strength of a fault.

"Earthquakes are caused when a fault fails, either because of the build-up of stress or because of a weakening of the fault. Changes in fault strength are much harder to measure than changes in stress, especially for faults deep in the crust," said Taka'aki Taira of the University of California at Berkeley, in a statement.

The team of seismologists from the Carnegie Institution, Rice University and the University of California at Berkeley analyzed 20 years of readings from a network highly sensitive seismometers monitoring the San Andreas Fault near Parkfield, Calif. Their research appears this week in the journal Nature.

The fault undergoes repeated small earthquakes, and readings on those earthquakes from an array of seismometers inside boreholes, called the high-resolution seismic network, can be analyzed to reveal the underground structure of the Earth's crust and its faults.

The researchers found that the San Andreas Fault zone was riddled with fluid-filled fractures, and these fractures sometimes shifted slightly. The repeated earthquakes also became smaller and more frequent, indicating a weakened fault, the researchers said.

"Movement of the fluid in these fractures lubricates the fault zone and thereby weakens the fault," said Fenglin Niu of Rice University.

The scientists found that twice, the shifts in the fracture occurred after the fault was rattled by strong, far-off earthquakes, such as the 2004 earthquake in the Indian Ocean near Indonesia.

"So it is possible that the strength of faults and earthquake risk is affected by seismic events on the other side of the world," said Niu.