A magnitude-6.9 earthquake struck yesterday off the coast of Fukushima, Japan, likely along the same fault that ruptured in 2011, unleashing a massive 9.0-magnitude temblor that triggered deadly tsunamis and caused widespread destruction. Over the course of its history, Japan has seen its share of shaking, but what makes this part of the world so susceptible to big earthquakes? The answer has to do with Japan's location. The island nation lies along the so-called Pacific Ring of Fire, an imaginary horseshoe-shaped zone that follows the rim of the Pacific Ocean, where many of the world's earthquakes and volcanic eruptions occur....