Cascadia's Fault (34 page)

“He saw the same thing [that Griggs and Kulm had seen], that there were thirteen turbidites above the Mazama ash. So he tried to come up with a method to prove or test the origin of these things. And what he did was this: he noticed that there's a confluence of two channel systems right through here.”

Goldfinger pointed to the map beamed from his computer via an overhead projector to a screen at the front of the ship's lounge. “Well, it turns out that all of these cores have thirteen turbidites.” He pointed to sampling sites on both main channel systems. “So here's his little test, right here. He said, ‘Well, okay—if you have two channels or two canyon systems, 200 kilometers [125 miles] apart, and one has thirteen turbidites and the other has thirteen turbidites—how could you possibly pass the confluence and go downstream and
not
have
twenty-six
turbidites?”

Goldfinger scanned the faces around the room to see fascinated smiles and raised eyebrows. “You
should
have twenty-six turbidites here, right?” Landslides send thirteen turbid currents of sand sloshing down two channels that run together, so there ought to be a total of twenty-six turbidite layers in the mud downstream from the confluence.

“And there are only two ways you could
not
have twenty-six,” he continued. “One is that it just coincidentally dropped out thirteen of them [for unknown reasons, thirteen of the landslides didn't make it past the confluence], which didn't seem very likely. And the other way is that they arrived at the confluence
at exactly the same time,
plus or minus about five minutes—and merged.” In other words, if a big quake triggered a landslide at the same moment at the head of each of the major canyons along hundreds of miles of the continental shelf, then all the mud flows would cascade downhill synchronously and would arrive at the downstream confluence where all the offshore sea channels meet—at the same moment.

Below the confluence there would be a single, merged turbidity flow. No matter how many small tributaries fed into the main channel from the steep slopes above, the total number of turbidites
below
the confluence would always be the same: one for each coastwide landslide. The only reasonable explanation for so many landslides happening synchronously along so many miles of coastline was large subduction earthquakes.

“And that's what John argued,” said Goldfinger. And then he paid one of the highest compliments one scientist can offer another. “This
was done purely with thinking power. And that's out of fashion these days.” The praise seems all the more significant given that Goldfinger and Nelson had initially doubted the simplicity and elegance of the Adams hypothesis enough to ask the National Science Foundation for research money to prove him wrong.

Goldfinger had gone to sea in 1992 and discovered the underwater Elvis, a man-with-a-guitar–shaped mound of folded and fractured ocean sediment, along with eight other new faults in the upper plate along the continental shelf. He wrote that the rough edges of these fractures may limit the size of Cascadia's big subduction quakes by inhibiting the build-up of strain energy and concluded that Cascadia may be the type of subduction zone at which magnitude 9 events “do not occur.” Without magnitude 9 ruptures, the Adams hypothesis had to be wrong. Smaller jolts just wouldn't do what the hypothesis demanded.

But a subsequent research voyage in 1999 turned things around for Goldfinger and Nelson. The evidence in favor of big landslides was very obvious in the offshore mud when examined close up. Not only did they confirm the same thirteen turbidites along 375 miles (600 km) of coastline, but they ventured farther north to the Nootka fault, at the upper end of the Juan de Fuca plate off Vancouver Island, and farther south all the way down to Cape Mendocino in California. Along the way they collected nearly a hundred new cores and added several discoveries of their own, extending the count from thirteen to eighteen events—presumably large quakes—and extending the timeline back to the end of the last ice age, roughly ten thousand years ago. They saw these dark, sandy landslide scars on the ocean floor as “earthquake proxies,” the telltale markers of Cascadia's long and violent past.

 

In the lounge aboard the
Roger Revelle,
Goldfinger explained the challenge of adding the five new turbidites to the series of thirteen already
established. The problem with the new samples was that they came from offshore river channels that were not physically connected to the network of channels flowing primarily from the Columbia River and the Strait of Juan de Fuca up the coast. The new evidence was found in a completely separate, unrelated grid of outflow channels from Barclay Canyon, off Vancouver Island, and from the Rogue River Canyon, midway down the Oregon coast.

How would it be possible to know whether the five additional turbidite flows had happened all the way down the coast at roughly the same time, in the same kind of synchronous gushes that Adams noticed in the main Cascadia channel? The question could be answered using oilfield techniques well known to many of the faculty at OSU, where petroleum geology was a significant part of the academic program. As Chris Goldfinger likes to tell it, oil drillers have been doing this sort of thing for years.

Again he pointed to the overhead map, zeroing in on the offshore region near the Oregon–California border. “These channel systems don't have the same sources and they're even further apart [than the channels that Adams studied]. They don't have anything in common,” he said. Oregon's Rogue River, for example, flows directly from Crater Lake—the former volcano Mount Mazama—to the sea with no downstream connection to the Cascadia channel. There is no confluence of canyon heads and tributaries that would physically link the Rogue turbidites with the others farther north.

The
stratigraphic patterns
in all the samples, however, did look very nearly identical. The relative age, thickness, and spacing of the alternating bands of turbidite sand, silt, and gray-green ocean mud were the fingerprints of Cascadia's history. Goldfinger and Nelson used a process known as wiggle-matching to make a detailed, layer-by-layer examination of all the minute gradations of muck that had been laid down on the ocean floor.

“Correlating the wiggles” in core samples from the entire length
of the Cascadia Subduction Zone took quite a while, but the match-up was pretty convincing. “Even though this hadn't been used before in paleoseismology,” Goldfinger said, “this is basic, subsurface oilfield geology. This is how oil deposits are tracked from one place to another, because turbidites make good oil reservoirs of sand. So correlating turbidites from place to place is something that hundreds of people do on a daily basis all over the world. We're just taking that technique and applying it to a different purpose here.” Instead of chasing oil, they were chasing earthquakes.

By 2003, when Goldfinger and Nelson published another paper based on more turbidite data, the tide of opinion had turned. The number of doubters had dwindled. The onshore record of sunken marshes, drowned trees, and sheets of tsunami sand had been accepted by most as evidence of Cascadia's past quakes. The evidence from offshore turbidites was still circumstantial, although the case was strengthened now that the work of John Adams had been redocumented, confirmed, and extended. Still, there was a lot to be done.

Goldfinger knew there were not enough data yet to establish absolute numerical ages for each of the offshore events—progress was slow because radiocarbon dating was difficult to do with so little plankton or other biotic material available in the deep-sea samples—so it was initially hard to correlate the turbidite record with the land-based data. There were enough similarities in the offshore core patterns, however, to establish “lateral continuity” of the turbidite layers. Meaning turbidite bed number three from a core taken off Vancouver Island was probably in the same regionwide stratigraphic layer as turbidite bed number three from a core taken near the California border.

Whatever triggered the offshore landslide up north presumably also triggered simultaneous landslides hundreds of miles to the south. The exact date may not be known, but in all likelihood the matching turbidites made a synchronized plunge downhill. And the only force strong enough to rattle the sea floor all the way from Vancouver
Island to California would have to be a very large temblor.

As a control sample, to see what the ocean mud looked like beyond the end of Cascadia's fault, Goldfinger and Nelson took their 1999 cruise ninety miles (150 km) south of Cape Mendocino, where they collected three more cores in the Noyo channel, an offshore canyon that drains the northern California continental margin adjacent to the San Andreas fault. They discovered a similar cyclical pattern of sandy turbidite flows interlaced with ocean mud, the main difference being that here the landslides seemed to happen more often. In the last ten thousand years there appeared to be thirty-one events—most likely caused by San Andreas ruptures big enough to trigger offshore landslides. It seemed that California's most famous fault was causing the same kinds of offshore landslides as the Cascadia Subduction Zone.

 

With ninety-six new cores going back nearly ten thousand years, there was finally a long enough history in the mud to look for patterns in the timing of these monster quakes. Suddenly, with the offshore landslide samples, the clock could be rolled back much further than before. At least in theory the new turbidite timelines offered a glimpse of long-term fault behavior. And the Cascadia cores did seem to reveal a repeating cycle. Judging by the thickness of ocean mud laid down between the turbidite layers, and with increasingly precise radiocarbon dates, they could tell roughly how much time had elapsed between events.

The first cycle began with a long quiet period after the Mazama volcanic eruption—more than a thousand years
without
an earthquake. After the first rupture was another moderately long interval of quiet, followed by two more jolts at shorter intervals—the shorter being only 215 years. The recurrence interval, or gap, between jolts was long, short, short. And that same sequence—long, short, short—had apparently repeated three times in the last 7,500 years.

Goldfinger and Nelson wrote that “while it is tempting to expound about earthquake clustering,” it was still early days. They had taken “a
tantalizing look at what may be the long-term behavior of a major fault system” but were careful to point out that their analysis was preliminary and would require confirmation of the radiocarbon age data.

Essentially they had encountered the same problem with radiocarbon dating that Brian Atwater did: there was very little biotic material to work with in deep-sea mud, and it had a way of getting moved around by burrowing sea creatures and sloshed out of the tops of the piston cores as they were wrangled onto ship's decks in heaving ocean swells. Unlike Atwater they could not rely on a ghost forest of ancient cedars conveniently nearby; nor could they use tree rings cored from perfectly preserved roots to help nail down the turbidite dates by other means.

They did, however, feel confident the wiggle-matching technique borrowed from their oilfield colleagues would eventually overcome these problems to help establish a solid and convincing long-term history for Cascadia. By December 2004 the clustering story had become more refined. Their turbidite studies had been updated, peer reviewed, and republished in several different science journals with enough new details that the media relations department at Oregon State decided to issue a news release. A draft prepared just before Christmas was set aside to be polished and sent out after the holidays.

Then, when no one was looking, another subduction zone in the Ring of Fire ripped apart, and the entire planet got knocked for a loop by a temblor so big it made the earth wobble slightly on its axis. The great Sumatra quake and tsunami of 2004 happened the day after Christmas and all eyes shifted instantly to the Indian Ocean. So it's unlikely that any more than a few diligent local reporters paid much attention to the OSU release issued on New Year's Eve. For those who bothered to read it, the release provided the latest chronology of clustered quakes on Cascadia's fault, a historical record with “two distinct implications—one that's good, the other not.”

Looking at the expanded pattern of turbidite beds off the coast of the Pacific Northwest, Goldfinger and Nelson concluded that the Cascadia Subduction Zone had experienced “a cluster” of four massive ruptures during the past 1,600 years. If the trend continued—if the pattern repeated—“this cluster could be over and the zone may already have entered a long quiet period of 500 to 1,000 years, which appears to be common following a cluster of earthquake events,” noted the OSU release. That was the good news.