Scientists Gauge Heat of ’94 Bolivian Quake

The 400-mile-deep Bolivian earthquake of June 9, 1994, which was felt as far away as Canada, released as much heat in 40 seconds as is generated by the entire United States over two months, according to Caltech scientists who have studied it.

The researchers, writing about the magnitude 8.3 temblor in the journal Science, say the 35 billion megawatts of thermal energy from this, the largest deep quake recorded, partially melted tectonic plates.

This allowed the plates to slide by each other more easily and facilitated movement in the subduction zone, where one plate was diving under the other, the scientists concluded.

Lead author of the study was Hiroo Kanamori, director of the Caltech Seismological Laboratory. He was joined by two other Caltech geophysics professors, Thomas Heaton and Don Anderson.

Kanamori said that it was impossible to determine how hot things got at the frictional points of the plates, but plate surfaces could have quickly reached 50,000 degrees Celsius, or hotter than the surface of the sun.

Before the shaking began, the temperature at this depth, with pressure equivalent to about 200,000 atmospheres, would have been 1,200 to 1,800 degrees Celsius.

“There are some fundamental mysteries of how an earthquake can occur at such depths in the first place,” Heaton said. With so much pressure, one would think the plates would be packed together so tightly that they wouldn’t be able to slide, he said.

But once rupture began, melting apparently could occur, reducing friction and promoting fault slip. It would all have to happen very quickly.

The scientists computed the minimum-strain energy change in the materials and then subtracted the amount of seismic energy detected as ground shaking by surface sensors. They concluded that most of the energy that could not be accounted for in the seismic waves could only have been converted to heat energy.