Bolivian Quake Gives a Rare Glimpse of Earth’s Interior

The most challenging paradox of the magnitude‐8.2 earthquake that struck the Bolivian Alteplano (literally, “high plane”) on 9 June is not that it caused minimal damage while still being strong enough to be felt throughout most of the Western Hemisphere, to excite normal modes of vibration of the Earth never before seen and potentially to enable geophysicists to map the Earth’s interior with 50 km resolution. Rather it is that the earthquake could occur at all. That is because the depth of the Bolivian quake (640 km) puts it in a rare class of earthquakes called “deep focus” quakes, whose foci lie between 75 km and 670 km beneath the Earth’s surface. How such deep‐focus quakes occur in the mantle, where high pressures and temperatures cause rock under stress to flow rather than fracture, has been a subject of intense interest and controversy ever since their existence was established in the late 1920s. The importance of the Bolivian quake is further increased because it is the first really large, very deep event to occur since the deployment of modern global networks of digital seismographs. Three networks of portable broadband seismographs recently deployed in South America promise to provide an unprecedented close‐up view of the quake. (The color relief map on the cover of this issue shows the positions of the stations in these networks in relation to the quake’s epicenter.) Researchers hope that data from the earthquake will yield a wealth of information on the Earth’s interior as well as shed light on how deep‐focus events occur.

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