A Seismic Jigsaw Puzzle Takes Shape

     Jeff Babcock should have heard his future calling him back in grade school. When the teacher told the class it was free drawing time, he was the kid sketching volcanoes.

     But it wasn’t until he was an undergraduate student at UC San Diego, taking an earth sciences class from a Scripps professor, that he considered making geophysics a career.

     Today, Babcock is sketching out a different kind of image—four years after earning his doctorate at Scripps. Play has become work aboard R/V Robert Gordon Sproul as Babcock maps in detail an offshore earthquake fault system. He uses an innovative seismometer developed by his team at Scripps.

     “Call it foreshadowing, but I guess it’s in my blood,” he said.

     On this October day, Babcock, fellow geophysicists Graham Kent and Alistair Harding, and graduate students Renee Bulow and Jeff Dingler are retrieving eight of the instruments from the ocean floor.

     There’s still some time to play, though. In the hour it takes to get from one drop location to another, the scientists improvise a game of ring toss in the shipboard lab with a roll of tape and a door latch. Last one to ring the tape on the latch fetches drinks at dinner.

     Earthquake-producing faults, like the one the team is cruising over off Oceanside, California, and volcanoes are the most concentrated expressions of global tectonic activity. Such activity is also manifested in areas of the seafloor that move like conveyor belts and in continental plates that merge into each other like slow-motion car crashes, turning the “crumple zones” into mountains.

     Many details of these faults have been effectively invisible to science because the action takes place under the ocean. Underwater seismic observations have been hampered by a lack of affordable technology and the expense of getting instruments to places of tectonic activity. A team of Scripps geophysicists and engineers at the Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics (IGPP), however, has achieved successful seismic monitoring with its creation—a fleet of ocean-bottom seismographs (OBS) that records ground movement and changes in acoustic pressure. These seismographs have been designed and built at a cost that is no longer prohibitively expensive, allowing scientists to make more frequent tectonic observations. The OBS fleet goes by the name Low-Cost Hardware for Earth Applications and Physical Oceanography, L-CHEAPO for short.

     “The seismographs can fill in the part of the story that complements the surveys you have on land,” Babcock said. “Tectonics doesn’t observe borders, whether they’re political or the ones between land and water.”

     Scientists have tended to stop at such boundaries, but the unknown fault might harbor even more destructive power than realized. Just as the IGPP team, which includes institute Director John Orcutt, was getting funding to build the seismographs, other scientists suggested that the Oceanside Fault could produce quakes with magnitudes exceeding 7.0 on the Richter scale, the same size as temblors that damaged San Francisco in 1989 and Northridge, California, in 1994.

     The Oceanside Fault, located about 32 kilometers (20 miles) off the southern California coast, scuttles across silent seas in the shadow of the San Andreas Fault. The fault occupies the farthest reaches of the friction zone between the Pacific and North American plates, an area geophysicists know as the borderlands. A few of its neighboring faults are the San Clemente Fault farther out to sea and the inland Rose Canyon Fault. Scientists consider the latter to be probably the most dangerous in San Diego County because the fault travels through a densely populated area of the city of San Diego; it runs in a north–south direction across Mission Bay where two pieces of earth headed in opposite directions are colliding. The fault slithers in an S-curve around Mt. Soledad, which owes its elevation to the extra compression at the center of the curve.

     The fault then disappears into the Pacific at La Jolla Cove after following a path studded by pricey hillside houses. Once underwater, the Rose Canyon Fault, the Oceanside Fault, and several others form a nebulous network that may link to other faults in Orange and Los Angeles counties.

     L-CHEAPO, developed by Scripps engineers Crispin Hollinshead and Dave Willoughby, not only can trace the paths of these faults but will also provide their dimensions and temblor frequency as researchers accumulate data over time. Additionally, researchers using L-CHEAPO can illuminate a number of geophysical problems: from the structure of continental margins to the traits of volcanic hot spots. Babcock hopes that this newfound knowledge will create an appreciation for the potential danger of earthquakes, at least in southern California.

     “We don’t have the major fault lines like San Francisco or Los Angeles,” said Babcock, a Mission Viejo native who has lived most of his life in shaking distance of the Oceanside Fault, “but San Diego does have a certain amount of seismic risk.”