John Vidale, a professor at the University of Washington, directs the Pacific Northwest Seismic Network and is the Washington state seismologist.
A tragic month along the tectonic subduction zones that surround the Pacific Rim has also been a spellbinding one for seismologists. A magnitude 6.2 quake on April 14 was followed a day later by a magnitude 7.0, together killing at least 49 in the Kyushu region of Japan. Less than 24 hours later, a magnitude 7.8 in Ecuador killed at least 650. Major deep earthquakes in Burma and Afghanistan in April were also deadly, and a series of quakes this month struck Vanuatu, too. So many earthquakes of at least magnitude 6.5 in a week is quite uncommon, even in the volatile tectonic zone known as the Ring of Fire, which encircles the Pacific Ocean. But the dangers of this region remain widely misunderstood, and myths — the notion that animals can predict earthquakes, for instance, or that the government knows they’re coming but hides the information — stubbornly persist.
1. Giant faults such as the San Andreas dominate the danger.
World-destroying films such as “San Andreas” and “Earthquake” feature magnitude 8s and 9s. NBC even produced a miniseries called “10.5: Apocalypse,” about a quake that splits North America into two islands. “The Really Big One,” the New Yorker story for which Kathryn Schulz won a Pulitzer Prize this year, explained the potential dangers of living near the large Cascadia fault in the Pacific Northwest. And the plate collision zones under South America and Alaska’s Aleutian Islands are famous for the monster quakes they spawned in the 1960s: magnitude 9.5 in Chile and magnitude 9.2 in Alaska.
But the bigger threats come from smaller quakes. Some are along the major faults, but even more are from the small faults right underfoot. Only one earthquake larger than magnitude 8.0 is on the list of the 16 deadliest earthquakes; about one-third had magnitudes of less than 7.5. Each year, on average, there are one or two quakes bigger than magnitude 8; 15 bigger than 7; about 150 bigger than 6; and so on.
Christchurch, New Zealand, had to be essentially rebuilt after a direct hit from a mere 6.3 in 2011. Japan was shocked when a magnitude 6.9 decimated Kobe in 1995. And the costliest U.S. earthquake was the magnitude 6.7 Northridge quake that shook Southern California in 1994.
2. Huge tsunamis are the foremost consequence of giant quakes.
Two horrific tsunamis, in 2004 in Indonesia and 2011 in Japan, jointly killed some 300,000 people. No wonder floods consume so much of our imagination. As a quote in Schulz’s New Yorker article put it, “Everything west of Interstate 5 will be toast” when a big quake triggers a mega-tsunami. Summarizing on Fox News, Shepard Smith predicted “Seattle, Tacoma, Portland and Olympia, Salem and Eugene, wiped out, altogether about 7 million people. That’s not including tourists.”
But in the United States and many other countries, the costliest and most deadly faults are inland, whereas the subduction zone coastline is sparsely populated. The third-deadliest tsunami in the past century killed a few thousand people, but in the past 40 years alone, a dozen earthquakes each killed more than 10,000. The scariest likely scenario is a temblor that hits the heart of Los Angeles, the San Francisco bay area, Seattle, Portland or Vancouver, not the nearby Pacific Rim oceanfront.
3. Earthquakes often trigger volcanic eruptions.
According to the laws of geology, where tectonic plates collide, volcanoes rise in the background. Volcanoes such as Mount St. Helens throw a long and deep shadow in the Pacific Northwest. And Charles Darwin said he observed volcanic eruptions triggered by the magnitude 8.5 Chile megaquake during the voyage of the HMS Beagle in 1835.
But the connection is hardly reliable, as this doesn’t happen very often. A 2013 study in the journal Nature Geoscience found that two giant temblors — magnitude 8.8 in Chile in 2010 and 9.0 in Japan the next year — caused no eruptions and that nearby volcanoes even sank, instead of rising in response to the tectonic shifts. Because of the 100 or more miles between most volcanoes and the largest faults, such as Cascadia, the shaking at the volcanoes is relatively weak. Earthquakes result from shifting tectonic plates, not magma flows, and have only a weak effect on volcanoes.
4. The major American Pacific fault lines are “10 months pregnant,” “locked and loaded” or “overdue.”
In “The Really Big One,” the Cascadia fault is on the verge of popping. “Counting from the earthquake of 1700, we are now three hundred and fifteen years into a two-hundred-and-forty-three-year cycle,” Schulz wrote. It is easy to imagine a valve keeping more and more pressure contained until it finally bursts.
Yet earthquakes tend to unfold differently with each iteration on a given patch of fault. Quake recurrence is fairly sporadic because the strength of fault surfaces is highly irregular, and so the complex ruptures are unique each time. Sometimes they break south to north, sometimes the reverse. Sometimes they break in two or three smaller earthquakes rather than one big one.
The chance of a quake on a fault rises slowly after a previous earthquake, not suddenly near a “due date.” After half the average recurrence interval, the chance per year rises to roughly the long-term average. The probability rarely exceeds twice the long-term odds; it is these fairly steady odds that we need to anticipate. The current risk of a magnitude 9.0 earthquake for the Puget Sound, for instance, is 1 chance in 300 per year. Quakes are not like a pregnancy — after a certain amount of time, birth is not inevitable or immediate.
5. The entire Ring of Fire settles down or pipes up all at once.
There have been six earthquakes greater than magnitude 8.5 since 2004, after none between 1965 and 2004. Some scientists say we’ve entered a period of enhanced earthquake activity (“a sign of the times,” as a recent article in Nature had it). One week in April saw five places near the Pacific Rim with greater than magnitude 6.5 quakes. Signs of the Times, a site dedicated to undercovered global trends, drew connections between 10 Pacific Rim volcanoes active at the same time in 2013. One well-circulated reading of scientific data says current conditions — including a spike in carbon monoxide — indicate that it’s unsafe to visit the West Coast.
Our relentless imagination, coupled with our compulsion to spot patterns in the noise, makes us poor statisticians of combinations of widely separated earthquakes. A century of observing large quakes tells us that the biggest events trigger, at most, a tiny number of earthquakes beyond 1,000 miles away. A bad day in Ecuador does not mean a dangerous day in Japan, and vice versa.
It may be frustrating that our estimates of earthquake danger change little from day to day and year to year, except for the temporary threat of aftershocks. Personally, I consider it reassuring. There really is almost nothing to foreshadow big quakes, although we continue to prospect for silver bullets. Relax, build a long-term shaking- and tsunami-resilient society, and play it as it lies.