SEISMOLOGY : Bolivia’s ‘Big One’ a Break for Scientists

The earthquake that occurred 400 miles underneath Bolivia on Wednesday night, and was felt thousands of miles north in Canada, was upgraded by scientists Thursday to at least a magnitude 8.2 and called a unique opportunity for study of Earth’s structure.

Hiroo Kanamori, director of the Caltech Seismological Laboratory in Pasadena, said the quake was possibly the strongest of its kind this century and that it indicated the interior of Earth may be very heterogeneous, with some regions capable of propagating earthquake waves more efficiently for great distances than others.

The quake occurred in one of the world’s most seismically potent deep subduction zones, stretching along the eastern edge of the great Andes mountain range from Colombia through Bolivia.

A subduction zone is an area where one of the great tectonic plates floating on the surface of Earth’s mantle is pushing under another. Periodic great quakes are associated with this phenomenon, as is volcanic activity.

But, as UC San Diego geophysicist Guy Masters noted Thursday, “Such events are reasonably benign as earthquakes go.” Masters and other scientists said that the reason the great quake caused no major damage anywhere and no reported casualties was that it was so far beneath the surface that it was quite distant from any human habitation.

“It’s like having an 8-point earthquake in San Francisco and feeling it in L.A.,” said Joel Ita, a research associate at Caltech. “Being 400 miles away, it wouldn’t do any appreciable damage here, even if it was that big.”

By contrast, a similarly sized quake, forecast to occur at some time in the much more shallow subduction zone off the coast of Washington, Oregon and Northern California, would be closer to the surface and prove far more destructive, the scientists said.

In an analysis prepared for The Times, Ita noted that past observations that deep quakes appear similar to more shallow ones, despite scientists’ belief that there are no faults at such depths, “has long confounded seismologists.”

“The problem lies in the fact that at the pressures that the (deep) earthquakes occur, the rocks should flow like Silly Putty in response to changes in stress and strain, instead of breaking in a brittle way” as occurs with faults near the surface, he said. So the deep quakes should be fundamentally different than the shallow ones.

Ita said recent experiments indicate that rather than the brittle fractures that take place along fault lines close to the surface, the material in the deep subduction zones “actually collapses on itself in a manner exactly opposite of such cracks.”

The 400-Mile-Deep Quake

The quake lightly damaged buildings as far south as Sao Paulo, Brazil, and was felt as far north as Toronto. There were also reports of tremors in California and throughout the Midwest. The quake occurred in a zone far beneath Earth’s surface. This had the odd effect of increasing the temblor’s range while reducing its destructiveness.