"...There was a horrifically destructive Pacific earthquake in New Zealand on Feb. 22,
and an even more violent magnitude-8.8 event in Chile almost exactly a year before.
All three phenomena
involved more or less the same family of circum-Pacific fault lines and plate boundaries
—and though there is still no hard scientific evidence to explain why,
there is little doubt now that earthquakes do tend to occur in clusters:
a significant event on one side of a major tectonic plate is often
—not invariably, but often enough to be noticeable
—followed some weeks or months later by another on the plate’s far side.
...Now there have been catastrophic events at three corners of the Pacific Plate
—one in the northwest, on Friday;
one in the southwest, last month;
one in the southeast, last year.
That leaves just one corner unaffected—the northeast.
And the fault line in the northeast of the Pacific Plate is the San Andreas Fault,
underpinning the city of San Francisco.
...the San Andreas Fault is due to rupture one day—it last did so in 1906,
and strains have built beneath it to a barely tolerable level.
To rupture again, ...some triggering event has to occur.
Now three events have occurred that might all be regarded as triggering events."