Wednesday Morning: Make-Your-Own Earthquakes

Print version E-mail to a friend
Text Size: Change text to small (default) Change text to medium Change text to large
Scientists explain their research at the AGU meeting
Enlarge Image
Sometimes explaining research takes two scientists at once. Margaret Boettcher and Greg Hirth (both standing and pointing) explain an earthquake experiment to two colleagues. (photo by Hugh Powell, WHOI)


Related Links

» Peering into the Crystal Fabric of Rocks
More of Greg Hirth's research, from Oceanus magazine.

» Rewriting the Story of Earth's Formation...Slowly
More on the way the Earth's crust moves, from WHOI Currents




On any visit to San Francisco, the subject of earthquakes can be avoided only for so long. Considering that AGU brings together thousands of geophysicists, making it to Tuesday afternoon must be some kind of record.

As the world-famous San Andreas fault creaked beneath us, I learned about lesser known but equally significant (geologically speaking) faulting and earthquakes that occur along ocean ridges and ripple through a common jade-green mineral called olivine. My teachers were Margaret Boettcher, a recent MIT/WHOI Joint Program Ph.D., and Greg Hirth, a WHOI associate scientist. They described their attempts to create tiny earthquakes in a tube of olivine the size of Hirth's pinky finger.

Oceanic faults in olivine are the kind of simple system that scientists prize. Although they don’t threaten to topple any buildings, they do move in ways similar to other strike-slip faults, like the San Andreas. Better yet, they’re made of pretty much one kind of mineral (olivine) instead of a hodgepodge of different materials, layers, and particle sizes. They’re sort of the earthquake version of the worms you dissected in ninth grade to learn about animals: the simple analog that helps with understanding of more complex systems.

Boettcher and Hirth, along with Brian Evans of MIT, wanted to know how fast you could push on a slab of olivine before it broke loose and slid. Finding out would let them compare to a curious observation along oceanic faults: Earthquakes in olivine happen only when it is cooler than 600ºC.

So the team packed a vial full of olivine grains, put it in a machine best described as a precision squeezer, heated the whole setup to around 800ºC, and started squeezing. As the force on the olivine increased it simulated different amounts of pushing along a fault.

When the team converted its measurements to real-Earth equivalents, they found that pushing on olivine when it’s hotter than 600ºC only makes it more resistant to slipping. At cooler temperatures, Boettcher recorded sudden weakening in the rock as the force pulverized individual olivine grains. Voilá: very small earthquakes in the lab.

“People have spent a lot of time studying how olivine flows, but they haven’t spent a lot of time on how it breaks,” said Boettcher, who is submitting the results to the Journal of Geophysical Research this week.



 

WHOI logo

rss headlines

Last updated December 9, 2005
© Woods Hole Oceanographic Institution. All rights reserved