Tropical Punch

Using a 200-foot-high rock hammer, a marine geologist looks at how an underground river helps to trigger temblors in the Caribbean.

Illustration by Molly Brown

EARTHQUAKES HARDLY ever happen on Barbados, but when they do they can be devastating. If one of these magnitude six or seven temblors were to strike today, it would capsize boats, flood the beaches, and possibly topple a hotel or two.

Quakes occur here because Barbados, a small island in the West Indies, has the misfortune of sitting atop the junction of two massive plates of the earth's crust, which are crunching past each other at the rate of about an inch per year. In this geological battle of wills, the small Caribbean plate scrapes over the top of its eastern neighbor, the North American plate, like a bulldozer blade, pushing and bending it down in a process called subduction.

So great is the pressure of this unrelenting collision that it wrings buried sediments like a washcloth, squeezing water out molecule by molecule. Driblets of this water collect in channels; the channels become a stream. Finally, the stream finds the crease between the crushing plates and escapes through it up to the sea floor. Gradually, the pressure of this underground river pushes the two plates apart, and every so often they lurch, like giant sumo wrestlers slipping in their own sweat. With that subterranean movement, the surrounding rocks throb, and an earthquake is born.

Although scientists know that earthquakes will occur at these junctions, they don't understand the hidden machinery setting them in motion.

J. Casey Moore, professor of earth sciences at UCSC, believes he and his colleagues can help change that. Moore studies plate collision zones to deduce their form and function. "I'm into plate boundary faults," he says. "They're exciting." As he speaks, his enthusiasm escalates, and he cants forward in his rolling chair at a precarious angle.

Moore is addicted to his research. He has been to sea seven times in search of fault zones, and three times he has cruised to the plate boundary at Barbados, most recently this past summer. Much of this work, including the latest trip to the Caribbean, took place on the research vessel Resolution, commissioned by the Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES). The JOIDES Resolution is the flagship of the Ocean Drilling Program, an international effort to discover the inner workings of the sea floor and unravel the history of the planet.

Moore's particular passion is the arrangement of sediments deep under the sea floor. Each layer tells a story, he says. Neat arrays of sand and silt are typical of ocean floors, but in the zone of a fault the sediments may be turned, bent, upside-down, or completely jumbled. These deviations chronicle the tortured history of rocks that have been squashed and scrambled by an over-riding plate.

Pulling sediments up from the ocean depths is no easy task. To do the job, scientists like Moore need the drilling power of the JOIDES Resolution, which can work in water depths of up to five miles. The ship itself is a football field and a half long. In the center of the top deck sits a stout version of the Eiffel Tower, a 200-foot-high metal derrick.

A circular drill lowered from the derrick can cut through ten yards of ocean floor in less than two hours. Winches crank up the surgically sliced sediment in cylinders 30 feet long and about as big around as a coffee mug. These drill cores are laid out on deck, wrapped in clear plastic tubing like so much slice-and-bake cookie dough.

Half of each core length is stored away as reference material, but the other half goes immediately to the ship's international complement of scientists, who probe, prod, chop, grind, and dissolve it, hoping to reveal the history of the sea floor. Work is constant. The researchers churn out data in alternate twelve hour shifts, and drilling continues throughout the day and night. "It happens really fast," says Harold Tobin, a UCSC doctoral student who accompanied Moore on two cruises. "It's real-time science. You can literally go to sleep with one hypothesis of what's going on, and by the time you wake up, the story's changed significantly."

During this flurry of research, Moore sits below deck at his computer, drinking copious cups of coffee and punching in the information as it arrives. Day by day, a picture of the fault boundary emerges: neat bands of deposits with a diagonal break running up to the ocean floor from deep under Barbados. And in that break runs the channel of water that lubricates the earthquake-causing slippage.

Moore discovered this subterranean stream during his first cruise to Barbados, in 1981. Since then, he has carefully mapped its course. Because earthquakes typically occur only after years of pressure buildup, Moore is eager to find a way to monitor this underground stress. The fluid pressure of the fault-zone river will provide such information, he believes. "We're studying the plumbing of the system," he says. "If we can understand how it transports these fluids, and can monitor them, potentially we can plug into the earthquake cycle."

On the latest Barbados cruise, Moore's doctoral student Gretchen Zwart helped install a permanent probe in the drill's bore hole to monitor fluid flow and pressure. The probe, or "instrument string," is made of black electrical wire, over a quarter-mile long, and hung with cylindrical silver monitors like knots on a rope swing. It hangs down from a funnel-shaped plug on the ocean floor, which will serve as a landing spot for a research submarine within a few months. Scientists in the sub will hook their computers to the probe's funnel and tally any changes in the fault zone.

For the moment, Moore sits safely back in his chair and hashes out the data from this summer's cruise. But sooner or later, the urge to drill the depths will hit him again. "A lot of marine geology involves going down to get some mud," he says, "but that's less satisfying than drilling a hole and getting real rock." And with the JOIDES Resolution, he says, "you've got the biggest rock hammer in the world."

Science Notes / Summer 1995 / Science Communication Program
University of California, Santa Cruz