Cascadia's Fault (14 page)

In October 1972, Robert Crosson, a seismologist and professor at the University of Washington, wrote a paper suggesting that this had probably already happened. Based on data from the Pacific Northwest Seismograph Network, a newly installed, six-station, high-sensitivity telemetry system capable of pinpointing even the smallest tremors, Crosson and his colleagues had shown that nearly all the jolts in the Puget Sound region around Seattle and Tacoma were the result of north–south compression. There was no evidence of eastward pressure from the Juan de Fuca plate at all, as far as they could tell.

The vast majority of the recent Puget Sound earthquakes had been relatively shallow ruptures in the upper crust of the North American continental plate. The absence of a down-sloping zone of much deeper shocks (known as a Benioff zone) along the eastward-dipping plate boundary and the lack of any recent volcanic activity in the Cascades (in 1972) could be seen as further evidence that the Juan de Fuca plate had stopped moving or was in its final phase of subduction. Without that constant eastward shove from the Juan de Fuca Ridge, the dominant
tectonic pressure would have become northerly—and that's indeed what the new seismographic data in Washington State seemed to confirm.

Just across the border in British Columbia, however, two scientists working for the Geological Survey of Canada had looked at their data and come to exactly the opposite conclusion. Robin Riddihough and Roy Hyndman argued in August 1976 that subduction was still happening. They pointed to the “significant eastward dip” of the ocean floor and to the layers of sediment—two and a half miles (4 km) thick, lying on top of the Juan de Fuca plate—that had been dragged sideways into a shallow, less obvious trench at the edge of the continental shelf where they were crumpled, folded, and fractured, all relatively recently.

The continental shelf itself had been recently deformed and uplifted, just like those terraces (former beaches) hoisted up near Cape Mendocino in California. They cited a higher than normal heat flow from the inland Cascade Range that was probably caused by upwelling magma from the melting oceanic slab. All of these were classic symptoms of active subduction, according to Riddihough and Hyndman, although they agreed it would be hard to tell if and when a plate had stopped moving in the recent past. So there remained a degree of uncertainty about whether or not Cascadia posed a clear and present danger.

 

Another possible explanation for the lack of large earthquakes came in June 1979, when Masataka Ando of the U.S. Geological Survey and Emery Balazs of the National Geodetic Survey suggested that the Juan de Fuca plate was still subducting, but doing it
aseismically
—without earthquakes. Given the lack of large ruptures over the 140 years since white settlers had arrived and written records had been kept, two things were possible. Silence along the boundary zone could either mean the two plates were now locked together by friction and strain was building up for a major rupture, or that big temblors simply didn't happen in this subduction zone. Which brings us back to the idea that Cascadia is somehow a special case.

“In some subduction zones, such large earthquakes do not occur,” wrote Ando and Balazs. They had a hunch that friction between the Juan de Fuca and North America plates was too low for the rocks to get stuck together. If, for whatever reason, they don't get stuck—because of a slower than normal rate of motion, perhaps, or a shallow angle of subduction—then movement could keep happening without major quakes. Strain might build up enough to compress and bend rocks in the overlying plate and still not cause a rupture. And they figured the only way to find out for sure would be to measure the rate of deformation along the highways of Washington State.

The first precise leveling survey of Washington's roads had been done back in 1904. By 1974 new surveys had been carried out on ten different sections of highway, some of which ran east–west across the Coast Range mountains. In the time between the first and second surveys, the surveyors' data showed that the outer coast had been lifted upward and the inland areas east of the Coast Mountains had subsided. In other words, the entire mountain range was tilting slightly toward the east and this had to be a result of active, ongoing subduction because it had happened in the past seventy years, not millions of years ago in geologic time.

However, another important detail made Cascadia different, according to Ando, who had recently studied strain accumulation in the Shikoku area along the east coast of Japan. There the Philippine Sea plate is thrusting under the Asian plate—beneath the islands of Japan—along the Nankai Trough. The geologic setting is very similar to the Juan de Fuca Subduction Zone. The dip angle of both subducting plates is a shallow twenty degrees and both oceanic plates are relatively thin. The significant difference is that the outer coastal landmass in Japan is tilting down
toward
the ocean rather than leaning inland as it appears to be doing in Cascadia.

Bending the outer edge of the coast downward as the ocean floor scrapes underneath it is a sure sign the plates are locked together by
friction and building strain for a large quake, according to Ando's analysis. Once the rocks along the locked portion reach their breaking point—when friction between them is no longer enough to keep the two plates stuck together—the strain is released in a massive shockwave. As the two plates rip apart in a typical or “normal” subduction zone like the Nankai Trough, the outer coast snaps free from the down-going oceanic plate and springs back upward. The area slightly inland from the coast subsides at the same time. This is exactly what happened in previous large quakes in Japan, Alaska, and Chile.

In the aftermath of these giant jolts, as the overlying continental plate settled back down to its more or less normal position, some of the coastal uplift remained. In other words, the beach never quite got back to where it used to be because the underthrusting oceanic plate was still down there, still moving below the continent, still causing a certain amount of
residual
deformation. The three-step sequence, according to Ando and Balazs, starts with coastal
down
-
warping
just before the quake, followed by
heaving upward
during the rupture, and then a certain amount of
residual uplift
of the beach zones in the aftermath.

Cascadia, however, seemed to be doing something entirely different. If the aseismic hypothesis were true, then the Juan de Fuca plate would be just creeping down underneath the continent, slowly and continuously, lifting and tilting the Coast Range mountains to the east, and doing so without getting completely stuck and without accumulating enough strain to cause a major rupture. To me this sounded like the good news. The bad news came in the concluding paragraphs.

Studies of other aseismic zones had revealed that temblors are still possible even if the two plates are not completely locked together. Hiroo Kanamori at Caltech found that if you look at the total distance—how much long-term horizontal movement there had been along the subduction zone in the Kuril Islands, for example—and compared that to the movement that happened during large thrust earthquakes, the ruptures accounted for one-quarter of the total slip. In northern Japan
another study showed that quakes accounted for one-tenth of the total subduction rate. Which could mean that even in a mostly aseismic zone—where 75 or even 90 percent of the plate motion is slow, smooth, quake-free creeping—the plates can still get locked together and megathrust events do eventually happen. So Cascadia is not completely off the hook for damages, even if the aseismic theory is true.

The tip-off, according to Ando and Balazs, should occur whenever we see the outer coast of the Pacific Northwest start to dip down and get pulled under by the Juan de Fuca plate. Not surprisingly, they recommended constant vigilance by a team of surveyors with state-of-the-art equipment to spot any change along the beach. In the meantime, because the Coast Mountains are now tilting eastward instead of toward the sea, they assured us that a large thrust earthquake from Cascadia's fault is “not expected in the near future.”

Then along came the eruption of Mount St. Helens one year later. How could a violent explosion like this not be the sign that convinces all and sundry that Cascadia is still very active and that a tectonic disaster is looming? I suppose the first and simplest explanation was that the debate about Cascadia as “a special case” was happening mostly within the confines of the science community. The general public was not reading these new technical papers, not attending the scientific meetings, and therefore they did not know, for the most part, that Cascadia's fault even existed.

But why did so many scientists who had read the new literature still hesitate?

Gary Carver, who was still mapping thrust faults in the rumpled hills around the Humboldt Bay nuclear plant at the time Mount St. Helens blew, knew that the majority of scientists were skeptical of the Cascadia disaster scenario. Why would the volcanic eruption not have been seen as proof positive of active subduction? He told me that an eruption could still happen even after subduction had stopped.

Presumably, if the Juan de Fuca plate stopped moving tomorrow, the
segment of the down-going slab that had already been pulled toward the earth's hot interior would have begun to melt. Plumes of magma would already be rising up beneath the arc of volcanic mountains. So there could be a lag of who knows how many years—hundreds, maybe thousands—between the end of Cascadia's plate motion and the final eruption of Mount St. Helens or one of its neighbors. From that perspective, the St. Helens blast didn't prove anything.

Apparently what everyone needed and wanted was forensic evidence that there had been specific Cascadia earthquakes at specific times in the past. Not just hypothetical scenarios, not just signs that the beaches had been hoisted or the mountains tilted, or even that there had been smaller fractures in the continental crust near the California coast. The only thing that could finally put an end to all the back and forth would be tangible signs of past ruptures along the
entire
subduction zone. And once again, a clue about where that proof might be found was layered in the story written by Walter Sullivan of the
New York Times,
just before Mount St. Helens blew.

 

The so-called missing trench had to be significant somehow. There were trenches at all the other converging plate boundaries, so why would Cascadia's subduction zone be different? From what I'd read in the science journals, there was a bit of a trench off the west coast, although it was shallow compared to the others and filled with sediment. Why was the down-going angle of the Juan de Fuca plate almost horizontal while the others were steeper and deeper? And what did it matter that the crack was full of mud? The never-ending dump of sand and silt from the turbulent Columbia River and many others along the Pacific Northwest coast had all but buried the fault, so as with the Juan de Fuca Ridge, nobody knew the trench was there at first.

Trenches at other plate boundaries in deeper parts of the Pacific are located far enough away from the outflow of big mountain rivers that silt and sediment can't hide the evidence of subduction. Cascadia's close
proximity to the gushing plumes of mountain run-off had created this accretionary wedge, a blanket of muck two and a half miles thick (4 km) that not only filled the crack between the two converging plates but also made the subduction zone nearly impossible to study. There was something else, though—something buried in the sediment—that would reveal the hidden story of Cascadia's past. And it too came from that chain of smoky volcanoes.

Without knowing that Mount St. Helens would soon explode, Walter Sullivan had asked scientists about the eruption of another famous Cascade volcano, just to provide a frame of reference. He wrote about the explosion of Mount Mazama 7,700 years earlier, an apparently world-changing cataclysmic event. Sullivan described it so that readers could visualize what would happen again someday in the Pacific Northwest.

Roughly a hundred miles (160 km) east of the Pacific coast, Mount Mazama, like Mount St. Helens, had been created by Cascadia's oceanic plate subducting underneath North America. This ancient stratovolcano was given its name posthumously, because it had exploded and was long gone before geologists arrived on the scene seven millennia later to piece together what happened. The upper part of the mountain had completely disappeared in a spectacular blast that caused the lower walls of the volcano to collapse inward, creating a huge, circular hole in the ground—a caldera—five miles (8 km) wide. This gradually filled with snowmelt and rainwater to form Crater Lake—with a maximum depth of 1,958 feet (597 m), the deepest lake in the United States.

Magma spilled from cracks along the shattered volcanic rim and surged downhill in avalanches that filled nearby valleys with up to three hundred feet (90 m) of hot rock, pumice, and ash. Somewhere between eleven and fourteen cubic miles (not cubic yards,
cubic miles,
or 46–58 km
³
) of magma was ejected. A towering column of ash thirty miles (48 km) high rained down for several days on eastern Oregon, Washington,
Idaho, Montana, Nevada, and southwestern Canada. An ash layer half an inch (1 cm) thick was measured in Saskatchewan, 745 miles (1,200 km) from its origin.

Sullivan quoted Grant Heiken, a volcanologist at the Los Alamos Scientific Laboratory in New Mexico, who suggested that “a safe distance from which to watch such an event might be the Earth's orbit of a space station.” In more recent times, the U.S. Geological Survey website referred to the Mazama blast as “the largest explosive eruption in the Cascades in the last one million years.” Mazama's blast was forty-two times more powerful than Mount St. Helens' in 1980. Only the explosion of Krakatoa (recounted by Simon Winchester in his excellent book of the same name) off the coast of Indonesia in 1883 could compare to Mazama's magnitude and impact.