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)
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.
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.