Today, our guest, geologist Peter Copeland tells us about rocks, air
and memory. The University of Houston presents this series about the
machines that make our civilization run, and the people whose ingenuity created them.
The consequences of natural disasters are devastating --
earthquakes, volcanic eruptions, floods. If we hope to be ready for them, we
need to understand their magnitude, frequency and effects. We also need to
understand the different circumstances that bring them about.
An earthquake occurs when stresses that have built up in rocks are more than
they can resist. The frequency of earthquakes in a given region depends on how
rapidly tectonic forces accumulate, and on the character of the geologic formations
present. If the stress doesn't abate, then the chances of an earthquake increase.
Here's an example of the relationship between time and likelihood of an earthquake:
the segment of the San Andreas Fault in central California near the town of Parkfield.
The fault marks the boundary between the North American plate and the Pacific plate.
It experienced six, moderate, magnitude-six, earthquakes from 1857 to 1966. Their
average recurrence was 22 years. Scientists figured this segment of the fault was
pretty predictable, so they deployed an array of instruments in and around Parkfield
in the late 1980's and waited for the next earthquake.
They hoped to gain a better understanding of the processes of rock mechanics by having
their equipment near the source. Each year without an earthquake, the likelihood of one
increased. That's because systems like these have a memory. Stress in the rocks
accumulates, making failure of the rock more and more likely. Finally, in 2004, Parkfield
again was shaken. (In this case, it took 38 yers for an earthquake to repeat.) Now,
just a few years after the last quake, the chance of another magnitude-six earthquake near
Parkfield is quite low. But, in two or three more decades, we can reasonably expect another.
The chances of an eruption from an active volcano similarly go up as time since the last
event increases. We should be much more concerned about an eruption from Mt. Ranier near
Seattle, whose last significant eruption was about 2,200 years ago, than we are of Mt. St.
Helens, 50 miles southwest of Ranier, which erupted in 1980.
However, the chances of disasters whose source is in the sky -- tornadoes, hurricanes, floods
-- do not increase as time passes without an event. Unlike the rocks, the sky has
no memory; yesterday's weather may have some residual effect today; but
today will have no effect on conditions a year from now.
In 2005, Hurricane Katrina devastated New Orleans. A
few weeks later, Hurricane Rita bore down on, but ultimately missed Houston.
Does this mean that New Orleans is off the hook with regard to tropical cyclones, but Houston
is in some way "due"? No to both! The Big Easy cannot breath easy nor should Houston be any
more concerned than it was in 2004. Hurricanes are worrisome, but we have to remember that
the likihood of future of storms is independent of the past. Houston should plan for hurricanes
because of where it is -- adjacent to the Gulf of Mexico -- not because of what did or did not
happen last year. The rocks have a memory of earthquakes and volcanoes. Storms are another matter.
The air quickly forgets what happened, just the other day.
I'm Peter Copeland, at the University of Houston,
where we're interested in the way inventive minds
For more on the last Parkfield earthquake, Mt. Ranier, and Mt. St. Helens, respectively, see the U.S.G.S. pages on:
The Parkfield earthquake,
Mt. Ranier eruptions,
and Mt. St. Helens
Peter Copeland is an Associate Professor in the Geoscience Dept. at the University of Houston,
where he has taught for 17 years. His research has focused on the evolution of the continental
crust with particular emphasis in the Himalaya and Caribbean. From 2001-2004 he was editor of
the Geological Society of America Bulletin.