Cascadia's Fault - Jerry Thompson [49]
“The main point here,” Goldfinger continued, “is that the rupture of a great earthquake in a subduction zone is almost always completely underwater.” He pointed to a fine line at the base of a row of hills along the sandy bottom roughly sixty-five miles (105 km) offshore, where the Juan de Fuca plate slides beneath the edge of the continent. “For other faults, like the San Andreas, that are exposed at the surface, you can walk right up to it,” again he paused, “get out your rock hammer, and tink, tink, tink. Or get a backhoe and dig a hole across it.” His punchline was that studying Cascadia is a tad more difficult because there is no way to get a backhoe or even a hammer to the bottom of the continental slope beneath thousands of feet of seawater.
Goldfinger changed slides. “So what we’re gonna do on this cruise,” he explained, “is turbidite paleoseismology—based on the idea that if you have a big enough earthquake, you’re gonna generate a lot of landslides on the submarine margins. And you basically can just go to the bottom of the hill, take piston cores,” he clicked the mouse again, “and you should get a vertical record of landslides.” The new image onscreen showed a cutaway view of a core sample sliced open on a laboratory workbench. A vertical slice through the stratified layers of thousands of years of seafloor mud, sand, and silt.
One of the younger researchers raised his hand. “Define turbidite.”
Goldfinger grinned. “A turbidite is a sandy, muddy, high-energy deposit coming from submarine landslides.” He clicked the mouse again to a closer view of the core sample. “It’s an underwater sediment plume, gravity driven.” He waved a laser pointer at the screen. “Because all this material is entrained in the flow, it’s subject to gravity and it’s gonna keep going downhill until it runs out of—downhillness.” He wiggled the laser dot at a series of horizontal lines in the core, each one representing a different landslide.
“The trick is,” said Goldfinger, “how do you determine if the landslides are earthquakes?” Meaning the sediment might pile up for years at the head of an undersea canyon and then simply collapse under its own, unstable weight. Or it could be knocked loose in the relatively shallow water of the continental shelf by turbulence from a big storm passing overhead. But Goldfinger and the team from Oregon State were pretty sure those dark lines in the ocean cores were the physical remnants of Cascadia’s tectonic past, debris from landslides that had been triggered by big seismic shocks.
Out on deck, the boatswain signaled to the winch operator, who swung the boom out over the starboard rail where the steel piston hung in a stiff breeze. He then leaned over the side to confirm that the acoustic “pinger” was powered up, a smaller metal tube now clamped to the cable just above the piston probe. It would send signals to the ship’s multibeam sonar system as the coring rig approached the ocean floor. Satisfied that everything was good to go, the boatswain stepped back again, looked over his shoulder at the crane operator and raised his right arm, thumb down. At the working end of the boom, a pulley block began to spin and cable rumbled off the drum. Moments later the piston and its pinger disappeared with a quiet swish into the choppy black water.
Two hours later, in the main laboratory control room, all eyes focused on flat-panel screens and digital readouts as the piston rig dropped through the dark abyss near the bottom of the Sunda Trench. Numbers clicked over rapidly. More than two and a half miles (4,100 m) of cable had spun through the block when Chris Moser, one of the senior coring technicians from OSU, punched