Large quakes don’t trigger others far away, study finds

Big earthquakes don’t trigger others far away, a new study from the Royal Geographic Survey and the University of Texas has found.

While large quakes do, clearly, set off aftershocks in the immediate vicinity – up to about 600 miles away – they have no effect further off.

Earlier work had indicated that small aftershocks could be triggered all over the planet following a large quake. But the team wanted to investigate whether the same applied to larger quakes.

The team examined data on earthquakes over the last 30 years, of which 205 were big, with a magnitude of over seven. There were 25,222 moderate quakes with magnitudes of between five and seven. Earthquakes of this size can be picked up with instruments anywhere on the globe.

The team checked the timing to see if the moderate quakes could have been triggered by larger ones. And while there was an increase in moderate quakes in the 24 hours following a major one, these all took place within 600 miles of the original quake – indeed, nearly all were within 375 miles.

“The regional hazard of larger earthquakes is increased after a mainshock, but the global hazard is not,” the team concludes.

Writing in Nature Geoscience, the scientists point out that this isn’t big news to seismologists, who would have already noticed if there were a major knock-on effect from large quakes.

But with many laypeople suggesting recently that there might have been a connection between the Christchurch earthquake and the subsequent one in Japan, the report has rather good timing.