Cascadia's Fault - Jerry Thompson [127]
“Initially they were going to put six in the entire Pacific Northwest. From northern California to Vancouver Island—six strainmeters.” Dragert sounded more than a little incredulous. “There’s thirty-five now,” he chuckled, “because they want to find out about ETS.”
Like Dragert, Garry Rogers was especially keen to have another, independent set of instruments measure Cascadia’s ground motions during the slip events just to make sure it was really happening. “When you start seeing phenomena with several kinds of instruments seeing the same thing, it becomes very convincing to a lot more people in the science world,” Rogers explained. “In fact now three different kinds of measuring techniques—GPS, strainmeters, and seismometers—they’re all telling us the same thing. And they’re all telling us that stress build-up has a time element to it.”
The old notion that stress build-up along the fault was a slow, steady, constant process caused by tectonic plates always moving against each other at roughly the same speed was apparently not accurate. Or the concept was more complex than early thinkers realized. The plates may be moving at a steady rate, but with an earth made up of all kinds of hard and soft rocks, mud, sand, and messy fluids, the build-up of stress between two plates is jerky.
“ETS events could be essentially like the clicks of a ratchet wrench,” said Chris Goldfinger at Oregon State, continuing the thought. “As you crank it tighter and tighter, you’re adding more and more load—as the Juan de Fuca plate tries to dive into the mantle. But the locking point between the two plates won’t let it go, at this point, so it’s giving—in small, squishy motions that may be cranking up the load for the big earthquake.”
“You actually find that the probability of a megathrust earthquake is larger during one of these slips—or immediately after one of these slip events—than the rest of the time,” ventured Dragert. According to calculations made by his GSC colleagues Stephane Mazzotti and John Adams, the risk jumps by a factor of about thirty. “Right now, roughly three hundred years into the cycle [Cascadia’s last big quake was 311 years ago], the probability of a megathrust earthquake next week is roughly one in 200,000. So it’s a very low probability. During the slip event, or immediately after a slip event, it’s maybe twenty or thirty times that. It’s still only one in four or five thousand, so it’s still a low probability. But the difference is a factor of twenty or thirty.”
To me the numbers or percentages seemed less significant than the idea that Cascadia’s level of risk goes up for about ten days every fourteen months. And if it’s true that a new load of stress gets shifted from lower down in the zone to the higher-up, locked part where the earthquake will eventually be generated, then it probably makes sense that one of these ETS events could eventually trigger the main event. Ruptures on Cascadia’s fault (and the other subduction zones) may not be completely random after all. A new glimmer of hope for prediction optimists.
“That’s what we’ve been looking for,” said Dragert emphatically. “We won’t call it a prediction yet, but I think once we know what the heck is going on here, we might be able to say, ‘One of these slip events has started ... The probability of a triggered event is higher than in the previous fourteen months.’ What the emergency response people do with that—it’s up to them.” Dragert and Rogers have already suggested that emergency responders conduct their annual earthquake training exercises during ETS events just to