“We’ve been wondering for a long time about why, sometimes, earthquakes occur in bunches.”
— Seth Stein, Northwestern University
Examining 22 years of seismic data from a section of the San Andreas fault near the small central Californian town of Parkfield — an earthquake hotspot — the researchers found a substantial upsurge in the frequency of small earthquakes in 2005. These occurred following the Sumatra–Andaman earthquake in late 2004, which caused the deadly Indian Ocean tsunami. A similar increase occurred in the mid-1990s, after a magnitude-7.3 earthquake in 1992 in the Californian desert, hundreds of kilometres away from Parkfield.
It has long been known that earthquakes can trigger new quakes from afar. A magnitude-7.9 tremor that struck Alaska in 2002, for example, caused small earthquakes as far away as Wyoming and California. But these earthquakes occurred within hours of the triggering event, presumably caused when the vibrations induced other faults to give way. Many of the ones at Parkfield, by contrast, occurred months later.
The only way that could have happened is if a distant earthquake somehow weakened the San Andreas fault, says the study’s first author, Taka’aki Taira, a seismologist at the University of California, Berkeley. “Weakening the fault means the fault can store less stress before [it] fails,” Taira explains.
In addition to observing more small earthquakes, Taira and his colleagues also saw changes in a phenomenon called seismic scattering, in which incoming seismic waves are reflected in multiple directions, like sunlight from a wind-rippled pond. Such changes, the team believes, are associated with the movement of groundwater deep within the fault zone. The water would, in essence, lubricate the fault, making it weaker and more likely to move for as long as the water remained. Their research is published this week in Nature1.
How exactly this process occurs has yet to be determined. “We need lab experiments to see how fluids migrate after the ground is shaken,” Taira says.