Cascadia's Fault (29 page)

What Eddie Bernard and a team of more than twenty-five PMEL engineers, technicians, and scientists, along with eighty-five partner companies and suppliers, came up with was a four-stage warning system they called a “tsunameter,” which does for wave detection what seismographs do for earthquake measurement.

They started with a device that records pressure changes at the bottom of the ocean. Waves whipped up by storms or hurricanes affect only the surface layer of the sea. A subduction earthquake lifts the entire water column from bottom to top. When a mound of seawater several miles deep is lifted and breaks into a series of waves that start to roll across the ocean floor, the weight of all that water can be measured as a change in pressure when the wave passes over the bottom pressure recorder (BPR) developed by the team at PMEL. The BPR had to be able to function under almost twenty thousand feet (6,000 m) of water without needing maintenance for at least two years.

The second stage of the system involved an acoustical transmission device that could beam the pressure data up to a buoy tethered by cable at the surface. This turned out to be the greatest engineering challenge of all, although they eventually found a way to do it. A deep-ocean buoy technology had already been developed for NOAA's Tropical Atmosphere and Ocean weather forecasting system, but the gear needed modifications to make sure it could survive the frequent and more severe storms of the North Pacific.

In the third stage, the buoys would relay the pressure data from the
BPR to an orbiting satellite that would beam the signal back to land. In the fourth and final stage, the data would be received and processed at the two Pacific tsunami warning centers.

That was the plan. Making it happen was something else. They had to build and deploy a new generation of buoys that could withstand an entire year on the wild and turbulent surface of the North Pacific. The equipment for each tsunameter—the BPR, acoustical transmitter, buoy, and satellite relay—cost roughly $250,000, plus another $30,000 per year for maintenance. The most expensive part of the process, however, would be delivering and anchoring the buoy systems in the deep ocean, using ships that cost roughly $22,000 per day to operate.

A prototype to be deployed two hundred miles (320 km) off the coast of Oregon was ready to go by September 1997. It quickly delivered an accurate stream of data, so NOAA decided to install two more. It would take eight different ships on eighteen cruises—more than ninety days of sea time—to set up this initial three-station array. The good news was that it worked better than expected. It was transmitting tsunami data with a reliability factor of 97 percent—much higher than the 80 percent success rate they had hoped for from a prototype.

That was just the beginning. Since the Ring of Fire's subduction zones constantly eat slabs of sea floor, there was an enormous amount of real estate to cover: more than 5,600 miles (9,000 km) of plate boundaries and grinding trenches that could create large earthquakes and trigger tsunamis. NOAA figured they would need buoys spaced 125 to 250 miles (200–400 km) apart to “reliably assess the main energy beam” of a tsunami generated by a magnitude 8 event. Full coverage would require deployment of twenty-five to fifty tsunameter stations.

NOAA, the USGS, FEMA, and the five Pacific states that funded the project realized early on that if and when the network were finally built to its full length, it would still be too small. With buoys this far apart, some smaller but nonetheless destructive tsunamis could slip through the gaps undetected. This floating line of defense between the
“tsunamigenic zones” and the vulnerable coastal communities of the Pacific would be permeable.

The engineers and scientists at PMEL went to work on a new generation of satellite communications technology that would work in both directions and on demand. If a moderate-size earthquake ruptured somewhere under the sea and the seismometers picked up the signal, the on-duty crew at the warning centers ought to be able to send out a signal to wake up the tsunameter buoys and get a reading instantly on all the waves coming across the system—even those slightly below the 1.2-inch (3 cm) threshold.

This way they could spot smaller tsunamis immediately and issue—or cancel—warnings based on real data rather than “an absence of triggered data.” So the PMEL engineering team continued to plug gaps in the existing buoy system. At the same time, Vasily Titov and his colleagues were fine-tuning their tsunami model software so that it could use the incoming data stream to forecast what the waves would do when they finally made landfall.

Eddie Bernard knew that roughly 900,000 people would be at risk from a fifty-foot (15 m) tsunami, which is what the computer models said might happen if Cascadia ripped apart. Inundation maps were being drawn for California, but work on the Oregon and Washington coastlines had barely begun.

The weakness in the system continued to be the strategic location of the detection equipment. Because the deep-ocean buoys were placed far enough out in the Pacific to give western North America and Hawaii plenty of warning for a tsunami from Japan, the sophisticated new technology is too far away to be of use in a
local
rupture of Cascadia's fault. The buoys are anchored out beyond the subduction zone, so Cascadia's waves would hit the beaches of the Pacific Northwest at almost exactly the same moment that the pressure detectors picked up the signal at mid-ocean and sounded the alarm.

Bottom line: if you're on a beach and the ground starts shaking—and especially if that shaking lasts more than one minute—it's probably a subduction earthquake and there probably will be a tsunami. The shaking is all the warning you're going to get. Head for higher ground immediately and don't wait for any official notification.

 

Defining Cascadia's zone gave scientists a more accurate sense of what they were dealing with. Building the prototypes for a deep-ocean tsunami alarm system in the Pacific gave emergency responders a way to make better decisions about whether to evacuate coastal communities when a distant fault ripped and heaved the ocean floor. But the potential for megathrust quakes closer to home remained a subject of debate, and the implications of giant waves generated not far off the West Coast had still not sunk in. An air of unreality and deniability hung over the whole business. It would stay that way until somebody could pin a specific date and magnitude on Cascadia's last great rupture.

CHAPTER 16

Cracks, Missing Rings, and Native Voices: Closing In on a Killer Quake

Long before Chris Goldfinger sailed the Indian Ocean in search of mud cores from the Sumatra 2004 earthquake, he dropped a fish off the Oregon coast and found Elvis. He and Bruce Applegate, both graduate students at OSU in the late summer of 1989, went to sea in a research ship called
Wecoma
using side-scan sonar to take state-of-the-art pictures of the ocean floor. The ship towed a chirping metal “fish” at the end of a long cable thousands of feet beneath the sea surface, pinging sound waves off the bottom to create a digital map that looked as realistic as aerial photos showing the terrain of the ocean floor in stunning detail.

Gliding across the wide, flat abyssal plain, the fish kept chirping, sound waves echoed back, and a strange new picture emerged from the deep. In
Living with Earthquakes in the Pacific Northwest,
Bob Yeats described Goldfinger's discovery of a fault that had cracked the floor of the sea channel and buckled the sediments into a low hill. The onscreen sonar image looked remarkably like a man with a guitar, so inevitably it became known as Elvis. Later, of course, the fractured sea lump was formally named the Wecoma fault after the university's research ship.

Over the next weeks and months the OSU team discovered nine more strike-slip fractures off the Washington and Oregon coast: cracks that penetrated both the Juan de Fuca plate and the overriding continental plate. The ocean floor, at the point where it dives beneath the continent, was buckled, crushed, and deformed into cracks and folds very much like the mangled terrain Gary Carver and his colleagues had found onshore around Humboldt Bay, just down the coast in California.

This was literally and figuratively the cutting edge—the point of impact between two tectonic plates. For Chris Goldfinger the bottom line in this wealth of data was that the newfound fractures and deformations in the crust might be telling us something about the width of the locked zone and also about the kinds of rough spots—the asperities—that the down-going oceanic plate could get stuck on, preventing the entire subduction zone from rupturing all at once.

Goldfinger and another colleague, Robert McCaffrey, published their findings in
Science
on February 10, 1995, concluding that a series of “smaller” earthquakes—perhaps magnitude 8s along the subduction zone or even magnitude 7s in these newfound cracks in the upper plate—could account for pretty much all the Cascadia geologic evidence to date. Since nobody really knew how big a quake had to be to drown the tide marshes that Brian Atwater had found, since nobody really knew how big a shockwave had to be to trigger the offshore landslides that John Adams had written about, it was entirely possible that smaller ruptures could have done all the damage discovered on this coast.

With a fractured and buckled outer edge, the North America plate might be
incapable
of magnitude 9s simply because it couldn't build up and store enough strain for a long enough period to generate a full-zone rupture. That was the “good news.” The decades of terror scenario seemed to Goldfinger and McCaffrey more likely than a magnitude 9 apocalypse. They suggested that the seismic hazard and public safety implications of Cascadia's fault did not look quite as daunting as they had before.

In the fall of 1995, however, an international team of mud, marsh, and sand diggers thought enough evidence had accumulated to suggest quite the opposite—that a magnitude 9, full-length rupture
had
occurred along Cascadia's fault. And they were willing to speculate that it happened roughly three hundred years ago. A dozen scientists from federal, state, provincial, and university research labs on both sides of the Canada–U.S. border got together and jointly published a summary of all their separate bits and pieces of evidence for Cascadia's most recent quake.

From John Clague and Peter Bobrowsky's samples of dead plants from sunken marshes that had been quickly covered by sheets of sand left behind by tsunami waves sweeping across the western beaches of Vancouver Island near Tofino and Ucluelet, to Gary Carver's similar evidence of drowned trees in northern California, the picture looked remarkably consistent all the way down the coast. What Brian Atwater had found in estuaries along the Washington shore, Alan Nelson and his USGS colleagues had found in Oregon. The dozen scientists spent considerable effort—including eighty-five new radiocarbon-dated samples—to obtain the most accurate timeline possible. They found that all the ghost forests and marsh plants had been killed at roughly the same time as the land dropped down and was covered by tsunami sand—roughly three centuries ago.

Given the long distance between Tofino, British Columbia, and Humboldt County, California, the dozen “marsh jerks” (as they jokingly called themselves after the jerky spikes in a sawtooth graph denoting the quake-induced sinking of land) said it was all Cascadia's fault. The plate boundary was the only fault common to all the far-flung sites. They reported their findings in a paper published in
Nature
in November 1995. But because there was still reasonable doubt about the exact dates, they still had to equivocate about whether all these events had
occurred at exactly the same moment. The paper concluded that a single magnitude 9 earthquake, “or a series of lesser earthquakes,” had ruptured most of the length of the Cascadia Subduction Zone “between the late 1600s and early 1800s, and probably in the early 1700s.”

Any one of these studies viewed in isolation might not have been enough to convince the most stubborn skeptics. Taken as a whole, however, and seeing that they all said basically the same thing, this united front of twelve top-level scientists looked like a critical mass. Having to equivocate a bit by including the phrase “or a series of lesser earthquakes” no doubt rankled those who wanted to make the clearest, least ambiguous statement possible. But counting the slow decay of carbon atoms to find out when something happened hundreds of years ago was just too imprecise. And there were many examples elsewhere of several earthquakes occurring in series, several years apart. So this remained an unsolved mystery and a real debate.

What they still needed was a more precise date and some way to show, convincingly, how big that mega-shockwave had been. If they could say that an earthquake happened in a specific year, or better yet on a specific day, the whole thing would become more real, more believable not only to skeptics in the science community but to elected officials, emergency planners, and the people who live within striking distance of Cascadia's fault.