On July 4, 2019, as America celebrated its independence with explosions in the sky, the ground around the California town of Ridgecrest decided to join the party: A magnitude-6.4 quake rocked the region. Two days later, a magnitude-7.1 temblor topped it, and tens of thousands of aftershocks followed in the ensuing weeks. California, no stranger to shakes, has a dense network of ground-based seismometers—and that summer they were busy. At the same time, a small band of scientists, also in the Golden State, were trying to eavesdrop on these grumbles…
