The seismic energy from such a quake moves Earth’s surface in long, slow undulations. Humans don’t feel them. We notice only the shorter, sharper shivers from earthquakes, the type that rattle foundations. Nevertheless, the slow “tsunami earthquakes,” as Kanamori calls them, can be as quietly dangerous as a pristine hillside of snow on the verge of an avalanche. The Nicaraguan quake, he estimates, moved a 120-mile-long section of the seafloor more than three feet in about two minutes when part of the Cocos plate, a wedge-shaped slice of the Pacific ocean floor, slid under North America. This slow-motion shift, imperceptible to humans, sent the tsunami on its destructive way.
When Kanamori first suggested this model, seismometers weren’t able to reveal the true nature of these long-period vibrations. Part of the problem was that most seismometers depended ultimately on a pendulum to trace earthquake-generated movements onto a graph. The pendulum typically hangs by a spring from a supporting arm. Nestling right up against it is a recording drum—essentially a revolving scroll of paper—anchored firmly to the ground. During an earthquake, the recording drum bumps up and down. But because the pendulum is suspended in midair by a spring, its inertia makes it lag behind the shaking drum. In a sense, it floats freely, while the earth (and drum) jiggles underneath it. A pen attached to the pendulum scribbles a jagged portrait of the ground motion on the drum’s paper.
In these older seismometers, the long-period waves that Kanamori was looking for tended to get buried under the mountain of squiggles left by the far more numerous short-period waves. Newer instruments also rely on pendulums, but their coils are suspended in magnetic fields and attached to electronic recording systems rather than rolls of paper. When the seismometer shakes during an earthquake, the motion of the coil generates a current that can be analyzed digitally by a computer, revealing even subtle patterns such as the long-wavelength signatures typical of slow tsunami earthquakes.
The newer technology allowed Kanamori to study the deceptive Nicaraguan quake in all its complexity. Any earthquake, large or small, releases a symphony of shock waves. Some of these waves, particularly the jarring, high-frequency tremors that cause most of the damage near an earthquake’s epicenter, die out quickly and don’t travel far through the earth. In the Nicaraguan quake, these tremors never made it to the mainland with enough force to be felt on shore. Lower-frequency waves, however, resonate strongly in the planet’s interior, lingering like the deep boom from a bass drum in a concert hall. These tremors cyclically raise and lower the ground about every 20 seconds or more. Seismometers in Nicaragua and elsewhere in the world were able to pick up some of these signals, but not all of them.
As it turns out, the conventional measure of earthquake magnitude—the Richter scale—isn’t an accurate gauge of all an earthquake’s overtones. “In the 1930s,” explains Northwestern University seismologist Emile Okal, “when Charles Richter designed the scale, he had instruments that recorded 20-second-long seismic waves very well but that didn’t record waves with much longer periods.” As a consequence, standard seismic measuring devices may underestimate the size of an earthquake. The values on the Richter scale are derived primarily from a ratio of two numbers: the amount of ground-surface motion during an earthquake—as recorded on a seismometer, either electronically or on paper—and the number of vibrations per second. Simply put, lots of movement in a short time span means a big earthquake. Some earthquakes, however, like the one in Nicaragua, may release their energy very slowly, generating signals with periods longer than 20 seconds.
Last year, in the British scientific journal Nature, Kanamori argued that the magnitude 7.0 assigned to the Nicaraguan quake was far too low. By including the longer, slower seismic signals in his calculations, Kanamori upped the quake's magnitude to 7.6. Since an increase of one on the scale corresponds to a tenfold increase in the size of an earthquake, the Nicaraguan quake, by Kanamori's calculations, was five times bigger than originally estimated.
Emile Okal often does research at a French seismological station in Tahiti, one of a handful of stations in the world equipped with the technology to detect the longer signals of slow earthquakes. The Tahiti station, says Okal, received signals from the Nicaraguan quake even though it was more than 4,000 miles to the northeast, and immediately recognized the potential for disaster.
“The only problem,” says Okal, “was that by the time we picked up the signal, the Nicaraguan coast had been totally ravaged.” It took about 25 minutes for the signal from the quake to travel through Earth’s crust to Tahiti, Okal explains. And although seismic waves travel 15 to 20 times faster than tsunamis, this tsunami had to cover only 40 miles before hitting the Nicaraguan coast.
Unfortunately, even the network of seismological stations Nicaragua did have had greatly deteriorated during the country’s civil war. Had more modern seismometers been available, and had researchers been aware of the importance of the long-period seismic waves, says Kanamori, there might have been enough warning time to tell people to run for high ground. Kanamori hopes that more countries—especially those on the earthquake-prone Pacific rim—will invest in technology capable of detecting a wider range of earthquake tremors. “No matter what we do, tsunamis are going to happen,” he says. “The question is whether we can have a very effective tsunami warning system.”
The United States has two tsunami warning centers, one near Honolulu and another in Palmer, Alaska, just north of Anchorage. Both were built in the 1960s after tsunamis from two large earthquakes, one in Alaska in 1964 and the other in Chile in 1960, caused millions of dollars’ worth of damage in the two states. The tsunami from the 1964 earthquake also hit Crescent City in northern California, killing 11. The warning centers, staffed around the clock, collect data via satellite from dozens of seismological stations in more than 20 countries bordering the Pacific.
For earthquakes above about magnitude 6.5, says Michael Blackford, a seismologist at the Hawaii warning center, the center alerts the warning systems in other countries around the Pacific. If the earthquake is far away—say in Chile, Japan, or Alaska—the seismic signal, pulsing rapidly through Earth’s crust, will arrive in Hawaii hours ahead of any potential tsunami, giving the center time to alert the state civil defense via a hot line.
In addition to seismographic data, the warning systems also receive readings from tide gauges scattered in harbors in Alaska, Hawaii, and Pacific-rim countries: if, accompanying an earthquake, a harbor’s water level drops a few feet in a few minutes, the quake may have triggered a tsunami. While that information would be too late to help local residents, more distant communities could be forewarned.
Even with seismographic and tide-gauge data, tsunami prediction is a hit-or-miss proposition. False alarms outnumber real tsunamis by more than two to one, and this can cause problems besides simply jading the public. In 1986, after a magnitude 7.7 earthquake in Alaska, the Hawaii center issued a tsunami warning. Television, radio, and even air-raid sirens were used to get people to evacuate coastal areas. The exercise cost some $30 million. Yet the tsunami that arrived was barely three feet high.
“I wasn’t here at the time,” says Blackford, “but I got a lot of feedback from people who were. It’s well remembered. The tsunami arrival time was to be in the late afternoon, so the civil defense just blew the sirens. The word was, everybody should evacuate from the coastal areas. They closed offices and turned everybody loose. This resulted in virtual gridlock on the roads in Oahu. Some of the roads are right next to the sea. People have told me about this chaotic situation: you’re sitting in your car with the ocean lapping alongside you there, and you're wondering, ‘Why am I here? I’m supposed to be away from the ocean and I’m stuck in this traffic jam.’ ”
Part of the reason it’s so hard to issue reliable warnings lies in the inherent difficulty of detecting a tsunami barreling invisibly across the ocean. That’s a problem now being addressed by Eddie Bernard and Frank González, oceanographers with NOAA’s Pacific Marine Environmental Laboratory in Seattle. For the past few years González and Bernard have been working on developing a deep-sea tsunami sensor. Six of their devices now rest on the ocean floor, one 300 miles directly west of the Oregon-Washington border, one 30 miles southwest of Hawaii, and four more spread out a few hundred miles south of the Aleutians. The instruments are remarkably sensitive. “In 12,000 or 15,000 feet of water they are capable of sensing a change in sea level of less than a millimeter,” González says.
These sensors measure the weight of the water column above them. When a wave passes across the surface, it increases the height and therefore the weight of the water column above the sensor. As a trough comes by, the height and weight of the water column decrease. The sensor consists of a small metal tube about four inches long, which floats just above the ocean bottom; it is held in place by an anchor (usually an old iron railroad wheel). Partially enclosed within the metal tube is a small device—called a Bourdon tube—shaped like a comma with a very long tail. The end of the tail sticks out of the bottom of the metal tube, exposed to the ocean, and is open like a straw. The other end of the Bourdon tube is closed. When a wave passes overhead, increasing the weight of the water column, the pressure slightly straightens the Bourdon tube in the same way a paper noisemaker unfurls when you blow into it. When a trough comes by, the tube curls up again. As the Bourdon tube alternately straightens and curls, it pushes and pulls on a sensitive quartz crystal, which in turn produces an electric signal that varies along with the changes in pressure of the water above.
The pressure sensors remain on the bottom for 12 months at a time, storing data electronically every 15 seconds. To recover the sensors—and their data—González and Bernard need to send a ship to the ocean site above the sensor. When the ship reaches the site, it broadcasts a signal telling the sensor to release its anchor. The entire package (which includes an orange marker buoy and a signal transmitter) then floats to the surface.
Tsunamis are easy to spot in the sensors’ records, says González. “Tsunamis typically have periods of anywhere from 3 to 30 minutes or so,” he says. “On the other hand, tides and signals generated by storms have periods on the order of hours and tens of hours.” (Passing waves or ships generate pressure changes too small to reach the ocean floor.)
For now the sensors aren’t tied into any warning system. But with very minor changes, says González, they could be. Instead of being picked up periodically by a ship, they would have to transmit data every few seconds to a receiver on a surface buoy. If a tsunami rolled by, the buoy would relay the steady, maybe half-hour-long increase in pressure to a satellite or directly to a land station, where researchers could quickly work out the wave’s size and heading. Sensors between Hawaii and Alaska could warn Hawaiians several hours in advance of an Alaska-born tsunami heading their way.
“We have experience in all of these components,” says González. “It's just a matter of putting them together to get them to do what we want in this specific application.”
NOAA has proposed setting up seven such stations, four off Alaska and three off the northwest coast. Altogether the stations would cost about $700,000 to install and another $250,000 a year to maintain, according to Bernard.
One of the reasons Bernard is eager to push ahead with a warning system is that he worries a tsunami could catch the West Coast by surprise. Hidden beneath the waves about 50 miles off the northwest coast is the Cascadia subduction zone, where the Pacific floor plunges under North America. The zone stretches from Vancouver Island to northern California, just a few hundred miles northwest of San Francisco. Many seismologists fear that it's just a matter of time before an earthquake convulses the area.
Although no major earthquake or tsunami has battered the Northwest in historical times, some geologists believe they have found evidence of past tsunamis in the region. Sand deposits resembling the debris left by modern tsunamis have been found in a number of places along the northwest coast and appear to have been laid down about 300 years ago, before Europeans settled there. Native Americans in the area, moreover, have legends of great floods from the sea that sound eerily like tsunamis.
At a workshop in Sacramento last spring, Bernard met with a number of seismologists to discuss the risk of a Cascadia zone earthquake and tsunami. “We all talked for an entire day about what is the most probable earthquake, not the worst case, but what is the most probable earthquake you could expect from this area. From those discussions came a scenario earthquake of a magnitude of about 8.4, and its fault dimensions are about 140 miles long by about 50 miles wide. Using this as a basis, we set out to model what tsunami could result from that size earthquake.”
Based on case studies of tsunamis generated by earthquakes of a similar magnitude, Bernard and his colleagues estimate that the earthquake would spawn a tsunami that might be 30 feet high when it hit the coast. A tsunami that size would threaten coastal cities in the Northwest, northern California, and Hawaii. But Bernard cautions that there is no way of predicting whether such a disaster will happen next year or in 300 years.
Does this mean that a tsunami could come rolling through the Golden Gate, drowning Alcatraz, Sausalito, and Fisherman’s Wharf? Probably not. Most seismologists speculate that the headlands outside the Golden Gate would take the brunt of a tsunami’s wrath. Others admit the very remote possibility that a tsunami could funnel right into San Francisco Bay. No one really knows.
“I want you to understand that this is a fairly speculative business. But we do have a public safety issue at hand. We have to play the ‘what if’ game,” says Bernard. “Unlike Alaska and Hawaii, where we have warning systems in place to respond in five minutes, we have no such facility on the West Coast.”
It’s been 30 years since the United States last suffered through a major tsunami, and some seismologists think the lull has fostered a dangerous, false sense of security. Another tsunami is inevitable, seismologists say. And vulnerable coastal areas, like much of Hawaii—where development has skyrocketed since the last tsunami—may be especially hard hit.
Like others, Bernard hopes that NOAA’s proposed warning system will be in place before the next big tsunami breaks on Pacific shores, but he’s not optimistic. “You see, there’s a trend here,” he says. “We always build a warning center after a big tsunami. We built one in Hawaii after the 1964 event. After, I want to underline the word after. The question I have is, Are we going to wait until after the Cascadia subduction zone earthquake to build one in California and Oregon, or are we going to do it in advance?”