Cascadia's Fault - Jerry Thompson [112]
“Right now, using all of the land evidence and all of the marine evidence that we have, we can put together a story going back about ten thousand years that shows a total of thirty-four earthquakes, plus or minus one or two,” Goldfinger intoned somberly. The clustering story was there in the data but still difficult to verify. “It’s something that’s totally dependent on the radiocarbon ages and those have a lot of slop in them,” he continued. “So it appears that events seem to cluster in time and then sort of die back and are quiet for a time—and then cluster again. I would say there’s probably a 70 percent chance that there’s some statistically meaningful clusters in the earthquake record.”
There was no obvious explanation for why quakes might cluster. “We’re just starting to really investigate the relationship between the Cascadia Subduction Zone and the faults that surround it,” Goldfinger said. “It’s of course connected to the entire Ring of Fire by other faults. We have the Queen Charlotte fault going off into Canada and the San Andreas fault going off into California, and all of these faults are all physically connected. So when you move one, it affects the others.”
I had read a study by Ross Stein of the USGS in Menlo Park that showed how one earthquake could trigger another. The magnitude 7.5 Landers quake in southern California in 1992, fifty miles (80 km) north of Palm Springs, had triggered swarms of smaller jolts—at least sixty thousand aftershocks—as far away as Mount Shasta at the northern end of the state and even in Yellowstone National Park, more than eight hundred miles (1,300 km) away. Goldfinger told me the same thing was probably happening here in the Pacific Northwest.
“Even if they don’t trigger each other, Cascadia and the San Andreas have stress relationships so that when one moves, it’ll affect the stress state of the other. And so as the entire Pacific plate moves—and the whole Ring of Fire has its ring of subduction zones—everything that happens in one affects the whole system,” he explained. “So hypothetically the Cascadia Subduction Zone, when it has its large event—it slips something like twenty meters [65 feet] or so—the energy from that is transferred to other places. And one of the places it could be transferred to is the northern San Andreas.”
The thought of earthquake dominoes was hard to avoid. “It looks like somewhere around 80 to 90 percent of the San Andreas events have a Cascadia event associated with them,” he said. “Almost every event on the San Andreas has come fairly closely in time—within forty years or so—of a Cascadia Subduction Zone event. So we could be looking at a scenario where Cascadia goes off, and then some relatively short time later the San Andreas goes off as well. And if that relationship is true, that has some implications for planning for the future.”
In the summer of 2010, an even more comprehensive package of data from Cascadia boosted the earthquake count yet again. Now it’s forty-one events in total, nineteen of which have been full-margin ruptures, and the average length of time between megathrust disasters has been recalculated at roughly every 300 years.
It sounded to me as though Goldfinger’s turbidite timeline—perhaps combined with those ETS events that come every fourteen months—could eventually help emergency planners by shedding new light on the dark art of earthquake prediction. I would soon learn, however, that there are those who vehemently believe any time or money spent trying to predict a disaster is a complete waste.
CHAPTER 20
When’s This Going to Happen? The Problems with Prediction
Boiled down to its essence, the argument against earthquake prediction is that nature is simply too complex and chaotic. There are too many variables, too many things happening deep under ground where we cannot easily see what’s