It is well known that major earthquakes are often followed by aftershocks, sometimes powerful ones. But it is generally understood that these shocks will occur in roughly the same region of the world as the initial quake.
New research reveals a remarkable addition: such shocks are often also observed on the opposite side of the planet. After analysing 44 years of seismic data – from 1973 to 2016 – and comparing it to usual baselines of earthquake frequency, researchers from Oregon State University (OSU) concluded that large earthquakes (of magnitude 6.5 and above) are inducing shocks on the other side of the world.
‘The test cases showed a clearly detectable increase over background rates,’ says Robert O’Malley, researcher in the College of Agricultural Sciences at OSU. ‘Earthquakes are part of a cycle of tectonic stress buildup and release. As fault zones near the end of this seismic cycle, tipping points may be reached and triggering can occur.’ Crucially, the higher the magnitude of the original earthquake, the more likely it is to trigger others. These secondary quakes would generally occur within 30 degrees of the antipodal point of the original epicentre (the point diametrically opposite it) up to three days after the initial event.
The researchers highlight numerous recent examples of this phenomenon in action, including multiple quakes across Asia in the aftermath of the 2010 Chilean earthquake, and high seismic activity on the San Andreas Fault that they have linked to the 2011 Japanese earthquake. These were all apparently triggered by S-waves from the initial quakes thousands of miles away. ‘The understanding of the mechanics of how one earthquake could initiate another while being widely separated in distance and time is still largely speculative,’ explains O’Malley. ‘But irrespective of the specific mechanics involved, evidence shows that triggering does take place, followed by a period of quiescence and recharge.’
This was published in the October 2018 edition of Geographical magazine
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