Deep Earthquakes

(SFX: Earthquake shaking shattering)

     A JUNE 1994 EARTHQUAKE IN SOUTH AMERICA SENT AFTERSHOCKS AS FAR
AWAY AS TORONTO.  THE QUAKE WAS SO POWERFUL BECAUSE OF WHERE IT
ORIGINATED--SOME 300 MILES UNDER THE GROUND. MOST EARTHQUAKES OCCUR IN
THE TOP 100 MILES, WHEN GIANT PLATES OF MOVING ROCK CATCH ON EACH OTHER
AND THEN BREAK FREE.  THE RESULTING JOLT CAUSES AN EARTHQUAKE.  BUT
GEOLOGISTS CAN'T EXPLAIN THE 20% OF EARTHQUAKES THAT HAPPEN AS THE
BOLIVIAN EARTHQUAKE. THAT DEEP IN THE EARTH, SAYS GEOPHYSICIST PAMELA
BURNLEY OF THE UNIVERSITY OF COLORADO, TEMPERATURES AND PRESSSURES ARE
SO HIGH THAT ROCKS SHOULD SIMPLY FLOW PAST EACH OTHER.
     
     Burnley: "the rocks inside the earth are not all molten in that you couldn't pour them out, the way
lava comes flowing out of a volcano.  but they are softer. . . they shoudl behave like warm silly putty."

     ROCKS THE CONSISTENCY OF SILLY PUTTY SIMPLY SHOULDN'T CATCH ON EACH
OTHER AND CAUSE QUAKES.  YET THEY DO,  BURNLEY IS ONE OF SEVERAL
RESEARCHERS TRYING TO SOLVE THE MYSTERY OF THESE DEEP EARTHQUAKES.  SHE
AND HER COLLEAGUES SQUEEZED ROCKS WITH INTENSE PRESSURE EXPECTING THEY'D
HEAT UP AND BECOME SOFT. THE ROCKS SHOULD HAVE ALMOST SQUISHED IN THE
PRESS. BUT INSTEAD THEY CRACKED IN HALF.  WHEN BURNLEY EXAMINED HER
SAMPLES UNDER A MICROSCOPE SHE DISCOVERED DIFFERENT BITS OF THE ROCK HAD
TURNED SOFT AT DIFFERENT RATES.

     Burnley: "What we found is that small pods almost the shape of lentils, would transform to the
new structure ahead fo the rest of the sample. . . and these. . .were able to hook up with each other and
create a growing shear zone."

     AS BITS OF THE ROCK BECAME SOFT AHEAD OF THE REST, THEY BECAME WEAK
LINKS IN WHAT HAD BEEN A STRONG CHAIN.  THE ROCK CRACKED IN HALF RIGHT AT
THIS SHEAR ZONE.  BURNLEY AND HER COLLEAGUES SURMISE THAT THE SAME THING
HAPPENS DEEP IN THE EARTH, WHERE INSTEAD OF CAUSING ROCKS TO SMOOTHLY
ROLL BY EACH OTHER, THE WARMED, SOFT ROCK CAUSES AN EARTHQUAKE.