Seismologists learn the warning signs of a 'big one.' When should they tell the public?

Brian Atwater, emeritus scientist with the USGS, shows mud deposited by the tsunami.

COPALIS BEACH, Wash. – When Japan issued its first-ever “super-earthquake” warning last week, Washington State seismologist Harold Tobin was watching closely.

The warning came after a 7.1-magnitude earthquake struck the southern island of Kyushu. While the quake didn't cause major damage — the largest tsunami it caused was knee-high — it wasn't the main concern.

But seismologists were concerned that the quake could cause stresses that could trigger a bomb to explode off Japan’s coast: the Nankai Trough, perhaps the country’s most dangerous fault. The Japanese government estimates that the subduction zone has the potential to generate tsunamis up to 100 feet high, killing nearly a third of a million people.

But does the smaller quake mean the “big one” is imminent? No one can say for sure, but the odds have suddenly gone up — if only by a few percentage points.

“This is exactly what would keep me up at night,” Tobin said, if it happened on the West Coast.

In Japan, the warnings prompted officials to close beaches, cancel fireworks celebrations and slow trains. People rushed to stock up on emergency supplies.

Police stand outside a damaged building after an earthquake in Miyazaki, western Japan, on August 8.

In the United States, Tobin said, “We don’t have such a protocol.”

But we have a similarly dangerous fault: the Cascadia subduction zone.

According to the Federal Emergency Management Agency, a magnitude 9 earthquake on the Cascadia Fault, and the resulting tsunami, could kill about 14,000 people in Oregon and Washington.

But if a smaller earthquake like the one just hit Japan near Cascadia occurs, seismologists will have to decide immediately whether and how to alert the public.

This is a scenario Tobin has been thinking about for years: If he finds evidence that a devastating earthquake is even slightly more likely, why sound the alarm? If the odds are that you’ll cry wolf—should you?

“You don't want an unwarranted mass panic and evacuation, but you do want people not to continue on their merry way,” Tobin said.

Tobin’s predicament is partly due to the field’s strange times: Researchers think they are on the cusp of identifying the triggers or factors that precede earthquakes in the world’s most dangerous regions, but the science is not settled. Even when the odds of an earthquake are higher, the odds are still small. That leaves big questions about when a warning should be issued.

A “ghost forest” of cedar trees near the Copalis River has helped researchers pinpoint the date of a tsunami off the U.S. West Coast in 1700.

These cedar trees likely died 324 years ago, after a tsunami caused the ground level to drop and flood the field.

On a cool summer day in Washington state, Tobin and dozens of other scientists took a boat ride down the Copalis River to a cemetery of cedar trees that had been killed 324 years earlier.

The kingfisher was chirping, and the wind was sending chills through the tall golden grass. It was a quiet place, a mile from the Pacific coast, and it told the story of a violent day.

On January 26, 1700, an earthquake on the Cascadia Fault caused the forest to slide down more than three feet. Shortly after, a tsunami that may have been a hundred feet high hit at 20 or 30 miles an hour.

Scientists would visit the forest to see the geological evidence of the Cascadia earthquake for themselves. Sometimes, they would get out of their boats, dig through the mud and extract a 300-year-old pine cone as evidence.

Experts know the quake measured at least 8.7 on the Richter scale, because that's how strong it would have been to send the tremors documented in Japan across the world.

“Some of the best written records of the 1700 tsunami come from Nankai,” said Brian Atwater, a USGS geologist emeritus who led the small boat flotilla. Atwater used those Japanese records, along with plants buried in the sand deposited by the tsunami and dates from cedar tree rings in Washington, to piece together the tsunami story.

Brian Atwater, an emeritus scientist with the USGS, points to a layer of sediment deposited by a tsunami in 1700.

Research by geophysicist Danny Brothers of the U.S. Geological Survey suggests that there have likely been at least 30 major earthquakes over 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. A major earthquake there is expected to occur at least once every 450-500 years, on average.

But for years Cascadia has been quiet; some scientists say that’s because much of it is “bound” and under increasing pressure. When it explodes, a chunk of seafloor will be pushed forward—perhaps dozens of feet or more. The vertical displacement of the seafloor will send a tsunami toward shore.

“This will be the worst natural disaster in our country’s history,” said Robert Ezell, director of Washington State Emergency Management.

For seismologists, the key question now is how to predict such violence in the future. Rapidly developing research suggests that faults like the Cascadia and Nankai may send out warning signals: a smaller earthquake as a foreshock, or a faint groan that can only be detected by sensors, which scientists call a slow slip event.

In Tobin's nightmare scenario, the Cascadia Fault suddenly lets out this kind of groan. So – what do we do?

A major earthquake in Cascadia is expected to injure more than 100,000 people, assuming the quake occurs when few people are on the beach. The tremor would last five minutes, and tsunami waves would hit the coast for ten hours.

The inland hills will turn to ice, destroying roads and bridges. About 620,000 buildings will be seriously damaged or collapsed, including an estimated 100 hospitals and 2,000 schools.

“We are not ready,” Ezel said frankly.

Washington state advises residents they may have to fend for themselves against the elements for two weeks.

“It would be like neighbors taking care of neighbors,” Ezel said.

The map of the Pacific Ring of Fire — where tectonic plates converge to form subduction zones and volcanoes — leaves Ezel particularly uneasy.

“Over the past 50 to 60 years, you'll find that every subduction zone fault has undergone major rifting — except Cascadia,” he added.

Tree ring analysis helped determine the date of the tsunami.

Japan lifted its warning of a “massive earthquake” 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, a 7.8-magnitude earthquake struck the eastern side of New Zealand's South Island, killing two people and causing more than $1 billion in damage.

A day later, scientists noticed a few centimetres of movement near the North Island coast via satellite monitoring. The slight tremors were coming from the Hikurangi Margin, a subduction zone and the country’s largest fault, which lies directly beneath the capital, Wellington.

The quake was a slow-moving earthquake, a type of slow-moving earthquake, and was triggered by the Kaikoura tremor. Such earthquakes release energy slowly over weeks or months and cause no noticeable shaking. Scientists first became aware of their existence about two decades ago, thanks to advances in GPS technology.

Some scientists, like Tobin and geophysicist Laura Wallace, believe that these slow-slip events may sometimes precede major subduction-zone earthquakes. Scientists recorded a slow-slip event in 2011, before the magnitude-9.0 Tohoku earthquake and tsunami in Japan, which killed more than 18,000 people and triggered the Fukushima nuclear disaster. A similar pattern occurred in 2014, before a magnitude-8.1 earthquake in Chile.

Wallace, who was working at New Zealand research institute GNS Science at the time of the 2016 earthquake, spent her waking hours trying to track every movement in the quake, model the risks, and answer questions from the government.

“I don’t think I’ve ever felt such a huge amount of responsibility,” Wallace said. “I took my dog ​​to the office because I didn’t want to be separated from my dog ​​in the event of a major earthquake.”

Wallace and her colleagues found that the probability of a large earthquake increased 18-fold, and that the risk within a year ranged from 0.6% to 7%. But the large earthquake never happened.

“Which of these slow-sliding events will lead to the next big one? That's one of the most important problems we're trying to understand,” Wallace said.

For the Cascadia subduction zone, gaining a better understanding of warning signals requires more data on slow-slip events, improved fault zone mapping, and enhanced ability to monitor seafloor faults.

Harold Tobin is a Washington State seismologist, director of the Pacific Northwest Seismic Network, and professor at the University of Washington.

Tobin was part of a team that recently mapped the Cascadia subduction zone in the most detailed detail yet. They found that the fault is divided into four sections, which can break all at once or individually in succession. Individual sections are capable of triggering earthquakes of magnitude 8 or greater.

Meanwhile, researchers are trying to strengthen the Cascadia region's marine monitoring network.

Japan has a sophisticated array of seafloor sensors, but it's “one of the few places that has them,” says David Schmidt, a geophysicist at the University of Washington.

The United States lags behind in seafloor monitoring, but Schmidt and Tobin are part of a group that received $10.6 million in federal funding to add seismic sensors and seafloor pressure gauges to a fiber-optic cable off the coast of Oregon.

These devices will help monitor Cascadia, and if the data can help researchers identify what’s normal for the fault, they may also be able to determine when to worry.

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