COPALIS BEACH, WATERFALL — When Japan issued its first “megaquake” warning last week, Washington state seismologist Harold Tobin was watching developments closely.
The warning came after a magnitude 7.1 earthquake struck the southern island of Kyushu. Although that quake caused little damage – the largest tsunami wave it generated would have been knee-high – it was not the biggest concern.
Rather, seismologists feared that the quake could generate tensions that could trigger a ticking bomb offshore: Japan’s Nankai Trench, probably the most dangerous fault in the country. According to Japanese government estimates, the subduction zone could trigger tsunami waves up to 30 meters high and kill nearly a third of a million people.
Did the smaller quake mean that the “big” quake was just around the corner? No one could say for sure, but the probability was suddenly higher – if only by a few percentage points.
“It’s exactly what would keep me awake at night,” Tobin said, if it happened on the U.S. West Coast.
In Japan, the warning prompted authorities to close beaches, cancel fireworks displays and slow trains, and people rushed to stock up on emergency supplies.
In the United States, Tobin said, “we don’t have such a protocol.”
However, we have a similarly dangerous fault: the Cascadia subduction zone.
According to the Federal Emergency Management Agency (FEMA), an earthquake measuring 9.0 on the Cascadia Fault and the resulting tsunami would kill an estimated 14,000 people in Oregon and Washington.
However, if a smaller earthquake like the one in Japan occurs near Cascadia, seismologists must decide spontaneously whether and how to warn the public.
This is the scenario that Tobin has been pondering for years: If he finds evidence that a devastating earthquake is more likely, even if only slightly, what justifies the alarm? If the odds are that you would raise the alarm, should you?
“You don’t want panic in the form of a mass evacuation, which isn’t justified, but you also don’t want people to just go on their way,” Tobin said.
His dilemma is partly the result of this strange time in Tobin’s field: Researchers believe they have identified the triggers or precursors of earthquakes in the world’s most dangerous earthquake zones, but the science is still far from unanimous. And even if the probability of an earthquake might be higher, it remains low, leaving open the question of when a warning should be issued.
On a cold summer day in Washington state, Tobin and a dozen other scientists canoed up the Copalis River to a cemetery of cedar trees that died 324 years ago.
A kingfisher chirped and the wind rustled the tall, golden grass. It’s a peaceful spot about a mile from the Pacific coast that tells the story of a stormy day.
On January 26, 1700, an earthquake in the Cascadia Fault caused the forest to sink more than a meter. Shortly thereafter, a tsunami about 30 meters high rushed through the forest at 32 to 50 km/h.
The scientists visited the forest to see the geological evidence of the Cascadia quake in person, occasionally jumping out of their canoes, digging in the mud and pulling out a 300-year-old pine cone as evidence.
Experts believe that the earthquake had a magnitude of at least 8.7, which is how strong it would have had to be to send the wave documented in Japan around the world.
“Some of the best written records of our 1700 tsunami come from Nankai,” said Brian Atwater, a USGS geologist emeritus who led the canoe flotilla. Atwater has used those Japanese records, along with plants buried in the tsunami sand and data from Washington cedar tree rings, to piece together the story of that tsunami.
Research by USGS geophysicist Danny Brothers suggests that there have probably been at least 30 large earthquakes in the past 14,200 years in parts of the Cascadia subduction zone, which runs along the U.S. West Coast from northern California to northern Vancouver Island. On average, a large earthquake can be expected there at least every 450 to 500 years.
Yet for years, Cascadia has remained quiet; some scientists say that’s because much of the seafloor is “stuck” and building up stress. When the lake breaks open, a piece of the seafloor will slide forward – possibly several meters or more. The vertical shift of the seafloor will send a tsunami toward the coast.
“It will be the worst natural disaster in the history of our country,” said Robert Ezelle, director of the Washington State Emergency Management Agency.
The key question for seismologists now is how to predict this violence. Rapidly advancing research suggests that faults like Cascadia and Nankai could be sending out warning signals: a smaller quake that precedes the tsunami, or a faint groan that can only be detected by sensors, an event scientists call a slow slip.
In Tobin’s horror scenario, the Cascadia Fault suddenly starts making such noises. What should we do then?
If a major earthquake were to occur in Cascadia, it is predicted that over 100,000 people would be injured – assuming the quake occurred at a time when few people were on the beach. The tremors would last five minutes. Tsunami waves would hit the coast for ten hours.
Inland, the slopes would liquefy and destroy roads and bridges. About 620,000 buildings would be severely damaged or collapse, including an estimated 100 hospitals and 2,000 schools.
“We are not prepared,” Ezelle said frankly.
Washington state is warning residents that they will likely be left to fend for themselves and exposed to the elements for two weeks.
“Neighbors take care of their neighbors,” Ezelle said.
A map of the Pacific Ring of Fire – where tectonic plates converge to form subduction zones and volcanoes – is of particular concern to Ezelle.
“You’ll see that there has been a major rupture in every subduction zone in the last 50 to 60 years – except Cascadia,” he said.
Japan lifted its “megaquake” warning on Thursday after no unusual activity was detected in the Nankai Trough.
In a similar situation in New Zealand in 2016, things went a little differently.
In November of that year, the 7.8 magnitude Kaikoura earthquake struck the east side of New Zealand’s South Island, killing two people and causing over a billion dollars in damage.
A day later, scientists using satellite monitoring noticed a movement of a few centimeters near the coast of the North Island. The subtle vibrations came from the Hikurangi margin, a subduction zone and the largest fault in the country, which lies directly beneath the capital city of Wellington.
It was a slow earthquake, the inertial earthquake of the seismic world, triggered by the Kaikoura quake. Such quakes release their energy slowly over weeks or months and cause no noticeable shaking. Scientists first recognized their existence about two decades ago thanks to advances in GPS technology.
Some scientists, such as Tobin and geophysicist Laura Wallace, believe these slow slides sometimes precede large earthquakes in subduction zones. Scientists recorded a slow slide event in 2011 before the magnitude 9 Tohoku earthquake and tsunami in Japan that killed more than 18,000 people and triggered the Fukushima nuclear disaster. A similar pattern played out in 2014 before a magnitude 8.1 earthquake in Chile.
Wallace, who was working for New Zealand research institute GNS Science at the time of the 2016 earthquake, spent her waking hours tracking the quake’s every move, modelling risks and answering government questions.
“I don’t think I’ve ever felt such a great responsibility,” Wallace said. “I took my dog to the office with me because if there was a big earthquake, I didn’t want to be separated from him.”
Wallace and her colleagues found that the probability of a major earthquake was 18 times higher and the risk within a year was between 0.6 and 7 percent. But the big quake never happened.
“Which of these slow slide events is essentially going to trigger the next big one?” Wallace asked. “That’s one of the most important problems we’re trying to understand.”
To better understand the warning signals in the Cascadia subduction zone, more slow-slip data, improved mapping of the fault zone, and improved monitoring of seafloor faults are needed.
Tobin was part of a team that recently mapped the Cascadia subduction zone in unprecedented detail. They found that the fault is divided into four sections that can rupture either all at once or one at a time. Each section can generate an earthquake of magnitude 8 or greater.
In the meantime, researchers are trying to strengthen the offshore monitoring network for Cascadia.
While Japan has a sophisticated array of seafloor sensors, it is “one of the few places that has such instruments,” says David Schmidt, a geophysicist at the University of Washington.
The U.S. lags behind in seafloor monitoring, but Schmidt and Tobin are part of a group that received $10.6 million in federal funding to equip a fiber-optic cable off the Oregon coast with seismic sensors and seafloor pressure gauges.
The devices will help keep an eye on Cascadia, and if researchers can use the data to figure out what’s normal for this bug, they may also be able to determine when it’s time to worry.
