| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
|
|
| Horizons Three's a Crowd Tourists flock to California's north coast for its whales, redwoods, galleries, and wineries. Earth scientists may also enjoy these diversions, but lately they've been distracted by the geology. This is one of few places on the planet where a convergence occurs of three tectonic plates--the huge crustal slabs that bob and bump atop Earth's more molten layers. And unlike most such convergences, the Mendocino Triple Junction (MTJ) occurs on land and close to shore off Cape Mendocino. This region is known for frequent earthquakes and volcanic activity (think of the majestic peaks in the Cascades) and scientists suspect that the MTJ plays a key role in it all. But just what sort of role no one knows. Says geophysicist Alan Levander of Rice University, "It's a first-class scientific problem." Now experts from four universities and the U.S. Geological Survey (USGS) have, for the first time, taken the seismological pulse of this triple junction in an effort to map, in three dimensions, the faultlines and plate boundaries--or as USGS geologist Sam Clarke puts it, "to try to see the whole beast." In the process, they may discover clues to the birth and growth of the San Andreas fault and to the earth-moving mechanics underlying much of the state. A handful of team members reported some preliminary findings during last December's meeting of the American Geophysical Union in San Francisco. Having the MTJ so accessible made it an ideal choice to study. The MTJ encompasses the Gorda, Pacific, and North American plates. The Gorda, which glides past the Pacific plate along a transform fault, thrusts beneath North America along a region called the Cascadia Subduction Zone, best known as the likely source for a much-anticipated "big one" that could threaten Portland, Seattle, or parts of northern California. The Pacific plate creeps northward three centimeters a year along the California coast and the edge of the North American plate, compressing the Gorda plate to the north and triggering frequent earthquakes. As part of an in-depth, four-year study funded by the National Science Foundation and USGS, geologists and geophysicists deployed nearly six hundred seismometers and fired hundreds of explosions beneath land and sea that shot sound waves deep into the Earth. Using a mix of seismic reflection techniques and velocity modeling, scientists can peer beneath the surface of the planet to probe where and how earthquakes occur. By analyzing the paths sound waves take as they pass down and back through the crust, it is possible to piece together faults and other Earth structures. "The type of imaging we do is analogous to a combination of ultrasound and CATscans," says Oregon State University geophysicist Anne Trehu. Though the MTJ provided a focal point for the research, the study spanned eight counties from the coast east to the Sierra foothills. Scientists continue to sift through the 250 gigabytes of data collected in 1993 and 1994 and will spend this year preparing reports for publication. The MTJ formed 30 million years ago far south of Mendocino in Baja California when an ocean spreading center in the Pacific plate collided with the continent's edge. Because the Pacific plate had a northward relative motion, it reconfigured the previous subduction zone boundary into one that moved north and south--the transform fault we call the San Andreas. Both the MTJ and the San Andreas began an incremental but steady move north, influencing along the way the formation of much of the Basin and Range province east to the Rio Grande. The Gorda plate plays a key role in the present-day MTJ mystery. Riddled with large faults, the Gorda, essentially a chip off the much larger Juan de Fuca plate, is cracking up. An unusually large number of earthquakes occurs throughout the plate; magnitude 4.0 quakes happen there as a matter of course, and several magnitude 6.0 or greater quakes have hit since 1980. As a result, says Trehu, "That plate is just being punched and broken." The southern edge of the Gorda plate also marks the northern terminus of the San Andreas fault, which bends sharply out to sea for hundreds of miles as a deep ocean fault known as the Mendocino Fracture Zone. Trehu makes the analogy that the Gorda plate is like a rug being pulled out from under California. The Gorda subducts beneath the edge of the continent, creating a wedge-shaped boundary to the North American plate. At the same time, the Gorda moves to the northeast, dragging the MTJ northward and leaving a region south of the triple junction where the wedge is presumably being filled by hot rock rising from the mantle below. This model explains the high elevations and recent volcanism in the Coast Range. Some geophysicists have recently challenged this explanation, proposing instead that the sinking Gorda gets trapped under the continent's edge south of the MTJ. The more comprehensive picture now emerging from the new imaging efforts may settle the question. The study showed that north of the MTJ, the largely offshore Cascadia Subduction Zone actually comes ashore in northern California via a deep connection to a pair of thrust faults, the Table Bluff and Little Salmon faults. That means, says geologist Clarke, that the sparsely populated North Coast could face the biggest threat from a potentially massive Cascadia quake. New models of how earthquake faults interact may be needed given some other early results. Seismic reflection and velocity data reveal that at least two small faults south of the MTJ, the Maacama and Bartlett Springs, cut straight down 15 miles to the base of the crust. According to scientists from Stanford and Rice universities, the northern end of the San Andreas--the fault's youngest section--may do the same. If it does, then these faults do not appear to be linked together in the crust, unlike the situation in the San Francisco Bay Area, where horizontal crustal connections are thought to exist between the San Andreas, Hayward, and other faults. The Mendocino area faults may share links deeper down in the mantle, but just what such differences in the two regions mean for quake behavior and risk is one of many questions needing an answer. Pesticides Plummet in Sea Lions Although a causal link between the DDT decline and the boost in sea lion numbers cannot be established, the result points out the importance of strict environmental regulations. In 1972, federal law prevented the Montrose Chemical Company from dumping DDT into the ocean off the Palos Verdes peninsula. Before the ban, an estimated 91 tons of the poisonous pesticide entered the water each year for two decades, creating the largest offshore DDT dump and wreaking reproductive havoc with several bird species, especially brown pelicans (see Pacific Discovery, Summer 1990). Sea lions also suffered from contamination of their breeding area. Beginning in the late 1960s, nearly half the pups at some rookeries died from being born prematurely. A study of sea lion blubber done in 1971 found DDT levels approaching and equaling those in beleaguered brown pelicans. The more optimistic report followed-up on this earlier work by University of California, at Santa Cruz biologist and Academy Fellow Burney Le Boeuf and his colleague Michael Bonnell. Le Boeuf, a leading authority on elephant seals, was a co-author of the new paper. For the new analysis, the researchers obtained blubber from male sea lion carcasses that had washed ashore from Ano Nuevo Point to the mouth of the Pajaro River over a four-year period. Males were chosen because they migrate down the coast to the Channel Islands and islands along Baja California for the late spring and summer breeding season. As in the first study, the team used gas chromatography to measure amounts of DDE and other DDT by-products in the blubber. Two decades ago, sea lions each contained 760 parts per million, or roughly a quarter pound, of DDT. The more recent levels, which appeared last November in Marine Pollution Bulletin, were a mere 5.2 parts per million of DDT. A vast improvement, to be sure, but not a complete recovery for the sea lions or their habitat. A large area of coastal sediment remains laden with DDT, perhaps as much as a hundred tons, so the pesticide will still be freed indefinitely as currents churn the seafloor and could continue to accumulate in marine mammals.
Blake Edgar is Associate Editor of California Wild. |
Summer 1996
|
