Earthquake synchronicity changes concept of seismic hazard

Earthquakes may occur in synchrony with one large quake triggering another tremble along the same fault line according to a new study.

Christopher Scholz, a seismologist at Columbia University’s Lamont-Doherty Earth Observatory tracked earthquake patterns from the last 15,000 years in southern California’s Majave Desert, the mountains of central Nevada and the south of Iceland and found evidence that quakes occur in synchrony.

“All of a sudden bang, bang, bang, a whole bunch of faults break at the same time,” Scholz said, “Now we know that some faults may act in consort, our basic concept of seismic hazard changes. When a large earthquake happens, it may no longer mean that immediate future risk is lower, but higher.”

Scholz identified several related quakes and explained how faults which rupture every few thousand years and are separated by less than 50km may align themselves to rupture simultaneously. When a fault ruptures in a large quake, the movement releases stress that has built up over time. Some of this stress is transferred to nearby faults, which – if close to its breaking point – also causes a quake. Scholz says that for two faults to have been simultaneously near their breaking point, they must have synchronised seismic cycles.

Paleoseismology – the study of physical signs left by earthquakes – show that the Majave fault ruptures every 5,000 years or so; Scholz says the seven-year delay in quakes at Landers and Hector Mine are not random. Paleoseismological records show the faults ruptured together before – 5,500 years ago, and 10,000 years ago. Similar trends were seen in Nevada and Iceland.

Scholz believes his hypothesis of synchronised faults may make it easier to assess some earthquake hazards – faults moving at similar speeds and less than 50km apart may rupture at similar times, while faults moving at different speeds and located far apart won’t.