Cascadia's Fault - Jerry Thompson [110]
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