Why are Japanese researchers looking to use undersea cables to speed up tsunami warning?

Hirogawa, Wakayama Prefecture –

When a massive earthquake struck off the coast of the Kii Peninsula, the residents of Hiroo had no smartphone alerts or loudspeakers to warn them of the wall of water rushing toward the shore.

They didn't even have flashlights to guide them in the dim light.

Instead, they had bundles of rice.

In the hours following the 8.5-magnitude Ansei Nankai earthquake on December 24, 1854, Hamaguchi Jūryō, the headman of a village in what is now Hirogawa, Wakayama Prefecture, realized that the villagers urgently needed to be evacuated to higher ground, and ordered the burning of bundles of rice to light the way up the hill.

Thanks to Hamaguchi's quick thinking, 97% of the village's population was saved. Hamaguchi later played a major role in rebuilding the village and protecting it from future disasters.

Today, a tsunami education center and a museum dedicated to Hamaguchi's life are located on the site of his former residence, and according to museum director Koichi Sakiyama, he remains a highly respected figure among the townspeople 120 years after his death.

Like many quiet coastal communities in Japan, Hirogawa City, Wakayama Prefecture, has a long history with tsunamis. | Joel Tansey

In Japan, tsunami warnings and evacuation plans have made great progress since Hamaguchi's heroic actions.

The waters around the disaster-prone country are now filled with seismometers and ocean-bottom pressure gauges that can detect and transmit information about earthquakes and tsunamis faster and more accurately than ever before.

But gaps remain, and scientists continue to look for new ways to improve tsunami warning systems and ultimately save lives. And for some, including researchers at the Japan Agency for Marine-Earth Science and Technology, a key ingredient for that improvement may already be on the ocean floor in the form of underwater communications cables.

Although much has changed since Hamaguchi's days, the basic principle of reducing loss of life remains the same: time is everything.

Defects

To collect data on the thousands of earthquakes that strike Japan each year, the Meteorological Agency operates 200 seismometers and 600 seismometers across the country, in addition to collecting data from 3,600 seismometers operated by local governments and the National Research Institute for Geoscience and Disaster Prevention.

Even before we feel an earthquake, the agency's early warning system is able to provide crucial information to disaster authorities, the media and the public via smartphones — although false alarms are not uncommon, and areas closer to the epicenter are unlikely to receive a warning before shaking begins.

Fishing boats that were likely washed ashore by tsunami waves following the Jan. 1 earthquake off the Noto Peninsula in Suzu, Ishikawa Prefecture. | Jiji

Within 90 seconds, the agency can provide data on the intensity of earthquakes in areas with at least 3 on the country's 7-point Shindo scale. Three minutes later, the agency issues information on the quake's epicenter and magnitude, as well as an estimate of the arrival time and height of the tsunami.

Later, as tsunami waves accelerate toward the shore, the agency's water pressure gauges can provide more accurate information about the size of the waves and the areas most at risk.

In short, Japan has one of the most advanced earthquake and tsunami detection systems in the world. But these systems are not without limitations.

Highly sensitive seismometers struggle to provide accurate data on the largest earthquakes, those with magnitudes of 8 or greater. This was the case with the Great East Japan Earthquake of March 2011, a disaster in which initial estimates of the earthquake’s magnitude led to an underestimation of the tsunami – the Meteorological Agency now uses qualitative information, classifying waves with terms such as “huge” or “high,” to avoid giving a false sense of security.

“Seismometers are mainly designed to detect small earthquakes,” explains Yusuke Aoki, associate professor at the Earthquake Research Institute, University of Tokyo. “Small earthquakes radiate high-frequency waves, but large earthquakes radiate slow-moving, low-frequency waves, which seismometers can’t really detect.”

By now, the surveillance infrastructure on land is much more advanced than that at sea.

A drawing by Furuta Shomon, a survivor of the 1854 tsunami, depicting villagers evacuating their homes as part of an effort led by Hamaguchi Jōryō.

Although Japan has a large network of seismometers on the ocean floor, covering vast areas of the ocean is not an easy task, and there are spatial gaps between measuring instruments of up to tens of kilometers, which can affect measurements and thus warning systems. In addition, the infrastructure is less dense in the Sea of ​​Japan.

“Offshore surveillance is much weaker than onshore surveillance because of the technical difficulty,” says Oki. “Offshore, the sensors need to be more accurate and you can’t go to the bottom of the sea if something goes wrong. Offshore surveillance costs a lot more.”

“But still, marine monitoring is very important because most large earthquakes occur at sea.”

This is where research by JAMSTEC and Takashi Tonegawa, a senior scientist at the agency, can make a big difference.

new ways

A map of the dozens of submarine communications cables branching off from Japan—with its many colorful lines running here and there to depict connections between Japan's islands and the rest of the world—is a reminder of how interconnected the planet has become.

This ever-expanding data transmission network may also hold the key to a potential new way to detect the most devastating natural disasters, Tonegawa explains.

When a tsunami forms, it changes the water pressure and causes the sea floor to deform, distorting the shape of underwater cables. A system known as distributed acoustic sensing can measure this pressure, and this data can then, in theory, be used to estimate the height of the tsunami.

Tonegawa and his research point to two important advantages that DAS has over traditional instruments such as seismometers and strain gauges.

A coastal area in Suzu, Ishikawa Prefecture, damaged by a tsunami following the Noto Peninsula earthquake on Jan. 1. | Jiji

First, governments and companies around the world have already laid 1.4 million kilometers of cable on the seabed—although Tonegawa points out that DAS cannot be deployed along the entire length of the cable—and partnering with these entities could greatly help ensure widespread deployment. Second, fiber optic cables are split into channels just meters apart, and strain can be measured on each channel, a huge improvement over the spatial gaps of tens of kilometers between traditional instruments.

In short, a network using submarine cables would be much more comprehensive than one based solely on seismometers and strain gauges, and could at least complement existing instruments.

“It may be possible to improve conventional bottom-of-the-ocean seismometers because DAS provides high-density data,” Tonegawa says. “So it may be necessary to fine-tune conventional techniques.”

JAMSTEC has already used experiments with DAS to detect slow earthquakes in the Nankai Trough, which stretches about 900 kilometers from Suruga Bay in Shizuoka Prefecture to the southern coast of Shikoku Island and has been the source of some of Japan's largest earthquakes and tsunamis, including the Hirogawa earthquake in 1854.

Tsunami recordings were more limited, as their observation depended on the timing and location of the DAS experiments.

Tonegawa’s breakthrough came in October last year when small tsunamis were detected near Torishima Island in Japan’s Izu island chain. The data was later matched with pressure gauge data to confirm the discovery.

But now comes the next challenge: converting that pressure data into something quantitative that can predict the height and speed of a tsunami.

Takashi Tonegawa, a senior researcher at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) in his office last month | JAMSTEC

The good news is that a tsunami’s speed—in other words, how long people on land would have to run away from it—can be determined directly from the raw stress data, Tonegawa says. The difficulty lies in predicting a tsunami’s height, for which scientists need to take into account seafloor deformation and water pressure fluctuations.

“Pressure is a bit complicated, and now we researchers around the world are trying to convert pressure data into more accurate physical data,” Tonegawa says.

Aoki, who is not involved in JAMSTEC's research using DAS, sees a lot of promise in the agency's experiments with the technology, believing it could “revolutionize” seafloor seismic monitoring because of the amount of data it can collect and how easily it can be deployed at scale.

He points out that we usually have seismometers on the ocean floor and pressure gauges on the ocean floor every 100 kilometers, but now the DAS system allows us to put them every 5 meters or so.

A fibre optic cable linking Europe and the United States is laid on the beach of Aritar in Spain in 2017. | Reuters

Meanwhile, determining when a tsunami will reach shore may remain an essential part of tsunami warning systems, given the density of the cable network in the Pacific.

“Fiber optic cables are everywhere off the coast,” says Oki, noting that the network is much less developed on the Sea of ​​Japan side due to political sensitivities with Japan’s neighbors. “Using fiber optic cables is a promising tool for quickly understanding when a tsunami will arrive,” he says.

In the event of a massive earthquake in the Nankai region, waves as high as 9 metres (30 feet) are expected to reach Hirogawa in 33 minutes. The situation is much worse on the Kii Peninsula: waves as high as 16 metres (55 feet) would hit the popular tourist town of Shirahama in just three minutes, while residents of the nearby town of Susami would have the same amount of time to escape waves as high as 19 metres (59 feet).

These narrow windows of time that can be the difference between life and death underscore the importance of improved early warning systems that scientists are racing to develop.

But these early warnings must be accompanied by other measures to limit the loss of life. And as with many of Japan’s quiet coastal communities, the lessons of Hirogawa’s past tsunami experiences are evident everywhere.

Large gates near the coast in Hirogawa Town, Wakayama Prefecture, that can be closed in the event of a disaster such as a tsunami | Joel Tansey

Throughout the city, lampposts display evacuation routes and measurements of height above sea level. Near the shore, roads pass through massive gates that can be closed to keep out water in the event of high waves. Parallel to these gates, breakwaters run along the coast, including one commissioned by Hamaguchi in the wake of the 1854 tsunami. (Long after his death, the breakwaters helped protect Hirogawa from a tsunami following another Nankai earthquake in 1946.)

In total, Hirogawa has been hit by eight tsunamis in its recorded history dating back to 1361.

Yet life continues in this coastal community. In the town of about 6,400 people, the residential communities are located close to the coast, a stone's throw from the town hall and the gymnastics center, which often serves as a hub of activities for young people.

“Our ancestors have been telling the people of this city about tsunamis for generations,” says Sakiyama, the museum director.

“We want everyone in Japan to know Hamaguchi’s story, and the next time a tsunami hits somewhere, we want them to evacuate quickly and save their lives first.”

Residents and visitors reenact the tsunami evacuation led by Hamaguchi Jūryō in 1854, in Hirogawa, Wakayama Prefecture, in October last year. | Jiji

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