CHENGDU, China — The birds suddenly disappeared from the sky. Something also was wrong with the pandas. They were strangely skittish. And then, within minutes, the isolated, verdant mountains above China’s most famous panda reserve exploded as if hit by a megaton bomb. “The pandas were agitated and pacing,” recalled Pamela Capito, 60, a member of a 12-person American tour group visiting the reserve. “When the earthquake hit, we realized they had sensed it coming.” If the pandas did sense approaching calamity, one reason is that the Wolong Nature Reserve sprawls over 772 square miles of rugged terrain directly beside...