Star Tribune

June 10, 1994

An apparently unprecedented game of "bounce the seismic wave off the interior of the Earth" caused parts of North America, including Minnesota, to tremble Wednesday evening following what seismologists believe was one of the largest and deepest earthquakes ever measured on the planet.

The earthquake, measuring 8.2 on the Richter scale, occurred at 7:36 p.m. Twin Cities time about 200 miles northeast of La Paz, Bolivia. Beyond its intensity, what made the event remarkable was its depth, some 375 miles below the Earth's surface.

It was that depth, at the bottom of the Earth's upper mantle and just above what is known as the "transition zone," that caused seismic waves to be felt a short time later in Minnesota, some 4,500 miles away. The transition zone is an area between the upper and lower mantle where the basic structure of the rock changes.

While there were some reports of the tremor being felt on the West Coast, most of the reports came from Minnesota, South Dakota, Nebraska and Toronto. The strength of the tremors in the Midwest apparently is related to the type of rock under the area and the fact that deep earthquakes tend to focus their seismic waves so tremors appear on the surface about 4,000 miles away.

South America is known for big, deep earthquakes, known to geologists as "deep-focus" events. Indeed, there were three such quakes earlier this year, but they weren't quite as deep or quite as powerful as Wednesday's. (For a comparison of sheer force, the great San Francisco earthquake of 1906 measured an estimated 8.3 on the Richter scale, and the quake that rocked the San Francisco Bay area during the 1989 World Series measured about 7. An increase of one whole number on the scale represents a tenfold increase in power.)

"I don't ever remember one in South America being felt in North America, and I've been here for a number of years," said Waverly Person, a geophysicist with the National Earthquake Information Center in Golden, Colo. While scores of scientists are still gathering data on the quake and sorting through records, Person said, "this may be one of the largest quakes at the depth on record."

The Twin Cities area apparently felt the rebounding waves of the Bolivian quake more strongly than many other North American cities, Person said. Shortly after the quake, his information center received more calls from the Minneapolis-St. Paul area than anywhere in the country.

"We were reluctant to say that what you felt was because of the Bolivian quake," he said. "so we checked regionally in your area to see if you'd had your own earthquake up there. We found nothing in your region, so it had to be in Bolivia."

The reason he was hesitant in connecting calls from Minneapolis to an earthquake in Bolivia is self-evident. "This was rare," he said. "It was so deep and so big." And so far away.

So how does an earthquake in Bolivia cause curtains to sway in Hennepin County? Certainly not through the normal processes associated with more familiar, shallow earthquakes in California, where parts of the earth grinding past each other trigger quakes.

The Earth is made up of layers, and it was the crust and the three layers below it that played a role in bringing the South American earthquake to North America.

While there is some controversy in the geophysics community about the exact mechanisms involved in deep earthquakes, David Yuen, a University of Minnesota geophysicist, explained what probably happened this way:

The surface of the Earth consists of huge plates that ride on underlying layers. Under the Pacific Ocean is a tectonic plate called the Nazca Plate, and it's moving to the east. Millions of years ago it collided with the South American Plate, a big, thick slab that extends well down into the planet. With nowhere to go, the Nazca Plate is pushing almost straight down into the mantle of the Earth.

As the Nazca Plate pushes down, Yuen said, it carries with it the material that makes up the Earth's upper mantle -- mostly a mineral called olivine. Normally, as the olivine nears the "transition zone" between the upper and lower mantle, it changes its crystalline structure and becomes another mineral, called spinel.

Yuen believes that, because of the unusual "straight-down" movement of the Nazca Plate, a lot of olivine moved into the transition zone without changing to spinel. When it finally did change, it did so "very fast" in geologic terms.

In other words, when the olivine changed to spinel, it became denser and took up about 10 percent less space, Yuen said.

Given the immense pressure of the rock at that depth, surrounding rock moved in to fill the space, and, boom, there was a deep earthquake.

Unlike the seismic waves of shallow earthquake, which move through the transition zone and hit the lower mantle (made of denser material called perovskite). The waves bounced off the lower mantle, went below the hot, volcanic area of Central America and Mexico, and surfaced through the relatively cold, dense rock underlying the center of North America.

Geophysicists are excited about the earthquake because they can use seismic waves to try to determine the structure of the Earth's interior. Instead of normal seismic waves traveling into or across the upper mantle, the Bolivian bounce means that seismic waves came up through the North American plate from the bottom, providing scientists with a unique view of the plate.

William Menke, a geophysicist at the LaMont-Doherty Earth Observatory at Columbia University in New York City, said the quake may provide the first direct evidence of the transitions (such as olivine to spinel) that happen inside the Earth.