A magnitude 6.5 earthquake struck Japan on Friday, causing widespread injuries and property damage. Then on Saturday evening, a magnitude 7.8 earthquake hit Ecuador on the other side of the Pacific, collapsing buildings and killing more than 350 people.
With two large earthquakes happening only days apart, it might look like they’re connected. But how earthquakes influence each other is rather complex, particularly over long distances.
Large earthquakes are caused by the rupture of faults that are a few tens of kilometers long for magnitude 7, to more than 1,000 kilometers for magnitude 9 and above.
The local perturbations caused by the sudden shift of rock mass across a large rupture results in a cascade of smaller earthquakes, called aftershocks, at distances roughly equivalent to the length of initial fault rupture. Aftershock rates peak immediately following the mainshock and decay exponentially with time.
However, if nearby faults are locked and already stressed, more damaging earthquakes may be triggered in the short term. The 2004 Sumatra magnitude (Mw) 9.2 and 2005 Mw 8.6 Nias earthquakes are the best known examples of short-term “clustering” of very large quakes.
Sumatran earthquake sequence for the 12 month period from 26/08/2004 to 25/08/2005 encompassing the 2004 Boxing Day Mw 9.2 and 2005 March Nias Mw 8.6 quakes. The y-axis shows each earthquake’s latitude. The initial 2004 Boxing Day ruptured some 1,300 kms northward from the initial rupture point near Banda Aceh (blue arrows), with aftershocks distributed all the way along the ruptured fault segment.
The March 2005 Nias event ruptured an adjacent segment of the plate boundary to the south of the 2004 Boxing Day rupture, inducing a distinct aftershock sequence. In total almost 4,500 aftershocks of magnitude greater than 4 were recorded in the nine month period to late August 2005, of which about 560 were greater than 5. Note also how aftershock intensity tails off exponentially with time.
A clear message from earthquake scientists to the public is that the most dangerous time for future large earthquakes in an affected region is immediately (within days to weeks) following a major earthquake.
The recent Nepal earthquakes in 2015 (Mw 7.8 and 7.3 within 17 days) provide further evidence for this.
Certainly, authorities in Ecuador, Panama, Colombia, and Peru will be well aware of the importance of reiterating future hazards to the region posed by aftershocks, other strong earthquakes, tsunami, and other earthquake-triggered hazards in the aftermath of the Mw 7.8 earthquake on Saturday.
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