On April 11 of this year, an extraordinary cluster of earthquakes struck off Sumatra. The largest shock, magnitude 8.7, produced stronger ground-shaking than any earthquake ever recorded. And it surprised seismologists by triggering more than a dozen moderate earthquakes around the world. The quakes are also a sign of big changes to come in the Earth’s crust. But chances are you don’t remember the April 11 quake off Sumatra. That’s because it was far offshore, so nobody got severely shaken, and it didn’t produce a killer tsunami. But the event didn’t simply fade away for Ross Stein at the U.S. Geological Survey in Menlo Park, Calif.