Great quake in Bolivia rings Earth's bell - June 8, 1994 earthquake's depth has far-reaching impact for seismological research

Science News, June 18, 1994 by Richard Monastersky

Seismologists set the snares 2 decades ago, stringing detectors around the world to catch the next great earthquake deep within the planet. But they had to wait until last week before their prey finally tripped the trap. On June 8, a magnitude 8.2 quake emanated from 600 kilometers below Bolivia, punching the planet hard enough to set it ringing like a bell for months on end.

The Bolivian quake packed more power than any other jolt in the last 5 years. But the tremor has an even rarer pedigree; its focus lay especially deep, which reduces the quake's danger and increases its scientific usefulness. The shock caused damage but no known deaths.

"This must be the biggest of the century for deep-focus earthquakes," says seismologist Hiroo Kanamori of the California Institute of Technology in Pasadena.

The quake was felt in much of North America, even as far away as Seattle -- a fact that has puzzled geophysicists. "This is the first time we know of that a quake in that part of South America was felt in North America," says Bruce W. Presgrave of the National Earthquake Information Center in Golden, Colo. He suspects that the quake's depth contributed to the widespread shaking.

Geoscientists say the quake will prove most important for studies about the structure of inner Earth. Because the core and mantle remain hidden from view, researchers interested in these regions must wait for large earthquakes to provide indirect information about the planet's interior.

Like a bell, Earth has its own natural frequencies -- or normal modes -- which start ringing if the globe is hit hard enough. The most persistent of these modes causes the planet to expand and contract every 20 minutes, almost as if it were breathing. Scientists can detect this mode even 3 months after a great quake.

Deep earthquakes have proved especially useful for normal-mode studies because they don't produce many surface seismic waves, which mask important overtones. Overtones provide information about the density of Earth's innards -- a factor that ultimately controls how currents of rock inside the planet move the continents around the surface.

"An event like this can allow us to sample modes that haven't been observed since 1970," says T. Guy Masters, a seismologist at the Scripps Institution of Oceanography in La Jolla, Calif.

The last great, deep earthquake struck below Colombia in 1970 and led to unprecedented discoveries about Eart's interior. After that jolt, scientists set up networks of seismometers capable of detecting the long-period vibrations of the normal modes. Then they waited.

"It's depressing that you can spend 20 years of your life dealing with shallow earthquakes or small, deep earthquakes and trying to add [up all these data]. And you still don't get as much of a signal as you do out of one earthquake like this. This is really going to change the level of our information about the deep Earth," says Masters.

Seismologists were especially pleased that no large quakes occurred soon after the Bolivian jolt, because that would have drowned out interesting overtones.

Earthquakes typically do not occur deeper than 20 to 25 km, because the hot rock below that level bends rather than breaks. The Bolivian quake took place in a special region where a piece of former ocean floor -- called the Nazca plate -- has sunk into the mantle in a process called subduction.

Scientists have yet to unravel exactly what causes these deep jolts. In fact, researchers met this week on California's Catalina Island to discuss the problem. According to one prominent theory, deep tremors occur when increasing pressures cause minerals in the ocean crust to undergo a sudden structural transformation. For answers to this problem and many others, scientists will now start sifting through the bounty provided by the Bolivian quake.