Cascadia's Fault - Jerry Thompson [18]
Far above the valley floor, it was also easier to see why the prediction experiment was being conducted along this particular segment of the fault. The San Andreas is not a straight line. Far from it, especially in Parkfield, where the shadow line zigzags ever so slightly and in a stretch northwest of town actually kinks. There’s a five-degree bend along a segment 1.2 miles (2 km) long that was the epicenter of the 1966 quake.
In their paper, Bakun and Lindh referred to this as a “geometric discontinuity” and suggested the bend probably controls how much of the fault moves when it ruptures. I imagined it as a kind of plug or doorstop jammed in the crack, causing the fault to slow down or temporarily stop moving. Not only that, with all this zigzagging, there are rough patches in the rocks—seismologists refer to these as asperities—that create friction and also slow the creeping motion along the fault. While the rest of the San Andreas is ripping along at almost 2 inches (5 cm) per year, the Parkfield segment is lagging behind at only 1.4 inches (3.5 cm) per year.
With tectonic plates as big as these, however, it’s obvious the little snags—those Parkfield asperities—can’t hold up progress forever. The stress builds to a point where the rocks fail. The rough spots finally shear away. In the span of less than a minute, roughly twenty-two years of “lost motion” along the fault is recovered as the Parkfield segment catches up with the rest of the San Andreas in a shuddering leap to the north. An earthquake.
Another important reason for clustering so many instruments here is that just before the 1966 main shock, the earth may have given off subtle warning signs. Twelve days before the temblor, fresh cracks appeared in the ground near the center of the rupture zone. Nine hours before the main shock, an irrigation pipe that crossed the fault broke and separated. Were these true precursors? Maybe. That was still the subject of vigorous scientific debate. But even the outside chance of a successful prediction was enough to motivate the USGS team to do everything they could to spot reliable symptoms of the next San Andreas quake.
CHAPTER 3
The Alaska Megathrust: Cascadia’s Northern Cousin
Even before the earth hammered Mexico City there were telltale signs of what to expect from Cascadia’s fault. One of the first clues arrived by sea at ten minutes past midnight on Good Friday, March 27, 1964. A tumbling ball of water moving southbound from Alaska at an estimated 330 miles (530 km) per hour passed beneath the hulls of ships at sea without causing a stir.
At surface level in the open ocean, it felt like just another swell in the North Pacific. But this was a “seismic sea wave,” what used to be called a tidal wave (now known as a tsunami) and it was very different from the chop and rollers left by a storm that had passed through a few days earlier. To a sailor’s wary eye, only the three-foot (1 m) crown of this monster’s head would have been visible that night, just another hump in a sea of thousands, with all its furious strength hiding in darkness below.
Unlike ripples, whitecaps, and windblown breakers that churn only the surface, this rolling mountain of brine reached all the way to the ocean floor and traveled at the speed of an airliner. When it reached the western side of Vancouver Island, the front of the wave began to slow as it scraped over the shallow bottom of the continental shelf, forcing the back half to mound up eight feet (2.4 m) above a normal high tide. An edge of the wave then sheared away and made a fast left turn into a fjord called the Alberni Inlet.
As the turning flood crashed over rocks at the Bamfield lighthouse on the outer coast, a keeper on duty grabbed the phone and placed an urgent call to Port Alberni, a mill town at the head of the inlet.