Energy Budget of Deep‐Focus Earthquakes Suggests They May Be Slip‐Sliding Away

To understand why the Bolivian earthquake of 9 June 1994 has so rapidly become one of the most thoroughly studied seismic events in history, one must know a little about both the event and the significance of when and where it occurred. (See PHYSICS TODAY, October 1994, page 17.) To begin with, the Bolivian quake occurred in the mantle, nearly 640 km beneath the Amazon basin east of the Andes. Such a large “deep‐focus” earthquake—and this one, with magnitude 8.3, was the largest such event ever recorded—can make the planet ring like a bell, exciting overtones of Earth's normal modes of oscillation that are sensitive to inhomogeneities in the core and mantle. Add in the fact that the Bolivian quake occurred after the deployment of several global and local networks of high‐quality digital seismological stations, and it becomes clear that the quake offered researchers an unprecedented opportunity to illuminate the three‐dimensional structure of Earth's mantle and core. (See the box at right.)