When a massive earthquake struck off the coast of the Kii Peninsula, the people of Hiro didn’t have smartphone alerts or loudspeakers to warn them of the wall of water racing toward shore.
They didn’t even have flashlights to guide their way in fading light.
Instead, they had rice sheaves.
In the hours following the 8.5 magnitude Ansei-Nankai Earthquake on Dec. 24, 1854, Hamaguchi Goryo, a village leader in what is now Hirogawa, Wakayama Prefecture, recognized that villagers needed to urgently evacuate to higher ground and ordered that rice sheaves be burned to illuminate a path up the hillside.
In large part thanks to Hamaguchi’s quick thinking, 97% of the villagers were saved. He 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 stand at the site of his former residence and, according to museum Director Kouichi Sakiyama, he remains a deeply revered figure among the town’s residents 120 years after his death.
In Japan, tsunami warnings and evacuation plans have progressed by leaps and bounds since Hamaguchi’s heroics.
The waters around the disaster-prone country are now dotted with ocean-bottom seismometers and pressure gauges that can detect and relay information about earthquakes and tsunami faster and more accurately than ever before.
But gaps still remain and scientists are continuously looking for new ways to improve tsunami warning systems and ultimately save lives. For some, including researchers at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), a key component of that improvement might already be in place at the bottom of the ocean in the form of submarine communication cables.
And while much has changed since Hamaguchi’s day, a key tenet for limiting loss of life remains the same: Time is everything.
Imperfections
To gather data on the thousands of earthquakes that hit Japan every year, the Meteorological Agency operates 200 seismographs and 600 seismic intensity meters across the country, in addition to collecting data from 3,600 seismic intensity meters run by local governments and the National Research Institute for Earth Science and Disaster Prevention.
Even before a quake is felt, the agency’s early warning system can provide crucial information to disaster authorities, media and the public via smartphones — although false alarms are not infrequent and areas closest to the quake’s epicenter are unlikely to receive a warning before shaking begins.
Within 90 seconds, the agency can provide data on seismic intensity in regions experiencing at least a 3 on the country’s 7-point shindo scale. After three minutes, the agency issues information on the hypocenter and magnitude of a quake, as well as estimated tsunami arrival time and height.
Later, as tsunami waves speed toward the shore, the agency’s water pressure gauges can provide more accurate information on the size of waves and the regions that are in the most danger.
In short, Japan boasts one of the most sophisticated earthquake and tsunami detection systems in the world. But it’s not without limitations.
Highly sensitive seismographs struggle to give precise data for the biggest quakes of magnitude 8 or more. This was the case with the Great East Japan Earthquake in March 2011, a disaster in which initial underestimates of the size of the earthquake led to an underestimation of the tsunami — the Meteorological Agency now uses qualitative information, classifying waves with terms like "huge" or "high," to avoid giving a false sense of security.
“Seismometers are mainly designed to detect small earthquakes,” explains Yosuke Aoki, an associate professor with the University of Tokyo’s Earthquake Research Institute. “Small earthquakes radiate high-frequency waves but large earthquakes radiate low frequency, slow moving waves, which seismometers cannot really detect.”
Even now, observation infrastructure is far more advanced onshore compared with what is in place out at sea.
While Japan does have a large network of ocean-bottom seismographs, covering vast areas of the ocean is not a simple task, and there are spatial gaps between the measurement tools of up to tens of kilometers, which can impact measurements and, in-turn, warning systems. Infrastructure is also more sparse in the Sea of Japan.
“Observation offshore is much, much weaker than observation onshore because of the technical difficulty,” Aoki says. “With offshore, the sensors need to be more delicate and you cannot go to the sea bottom if something is wrong. Observation offshore costs much more.
“But yet, observation offshore is very important because most of the big earthquakes occur offshore.”
And that’s where research by JAMSTEC and Takashi Tonegawa, a senior researcher with the agency, can make a big difference.
New methods
A map of the dozens of submarine communication cables branching off from Japan — with its numerous colorful lines darting this way and that depicting connections between Japan’s islands and the rest of the world — is a reminder of how interconnected the planet has become.
But this ever-expanding network of data transmission may also hold the key to a possible new method of detecting the most destructive natural disasters, Tonegawa explains.
When a tsunami forms, it alters water pressure and causes seafloor deformation, distorting the shape of undersea cables. A system known as Distributed Acoustic Sensing can measure that strain, and that data can then, in theory, be used to estimate a tsunami’s height.
Tonegawa and his research point to two significant advantages that DAS has over more conventional tools like seismographs and pressure gauges.
First, governments and companies around the world have already laid 1.4 million km of cables on the seafloor — although Tonegawa notes that DAS cannot be deployed on the entire length of a cable — and partnering with those entities could significantly help ensure a wide rollout of the system. Second, fiber optic cables are divided into channels that are typically just meters apart, and strain can be measured on each channel, a vast improvement over the tens of kilometers-wide spatial gaps between conventional tools.
In short, a network using submarine cables would be far more extensive than one that relies solely on seismographs and pressure gauges and, at the very least, could complement those existing tools.
“Traditional ocean-bottom seismometers may be improved because (DAS) provides data with high density,” Tonegawa says. “So it may be necessary to fine-tune traditional techniques.”
Experiments using DAS had already been used by JAMSTEC to detect slow earthquakes in the Nankai Trough, which stretches about 900 km from Suruga Bay in Shizuoka Prefecture to the southern coast of Shikoku and has been the source of some of Japan’s largest earthquakes and tsunami, including the one that struck Hirogawa in 1854.
Recordings of tsunami had been much more limited, however, as their observation depends on the timing and location of DAS experiments.
Tonegawa’s breakthrough in that respect came in October last year when small tsunami waves were detected near Torishima island in Japan’s Izu Island chain. The data was later matched with that of pressure gauges to confirm the finding.
But now comes the next challenge: Turning that strain data into something quantitative that can project tsunami height and speed.
The good news is that a tsunami’s velocity — or in other terms, how long people on land have to escape — can be determined directly from raw strain data, Tonegawa says. The difficulty lies in projecting a tsunami’s height, as scientists need to take both seafloor deformation and fluctuations in water pressure into account.
“Strain is a little bit complicated, and now we researchers all over the world are trying to convert strain data to the more accurate physical data,” Tonegawa says.
Aoki, who is not involved in the JAMSTEC 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 how much data it can collect and how easily it could be widely deployed.
“We usually have ocean bottom seismometers and ocean bottom pressure gauges every 100 km, but now the DAS system allows us to put it every 5 meters or so,” he notes.
In the meantime, determining a tsunami’s arrival time on shore could still prove to be an essential part of tsunami warning systems, given the density of the cable network in the Pacific Ocean.
“Optical fiber cable is everywhere offshore,” Aoki says, while noting that the network is far less developed on the Sea of Japan side given political sensitivities with Japan’s neighbors. “Using optical cable is a promising tool to quickly understand the arrival time of tsunami.”
In the event of a massive earthquake in the Nankai Trough, it’s estimated that waves 9 meters high would reach Hirogawa in 33 minutes. The situation is far worse farther out on the Kii Peninsula: The popular resort town of Shirahama would be hit with waves 16 meters high in as little as three minutes, while people living next door in Susami would have the same amount of time to escape waves 19 meters high.
Those narrow windows of time that make the difference between life and death underscore the importance of the improved early warning systems that scientists are racing to develop.
But these earlier warnings need to be allied with other measures to minimize loss of life. And like many quiet, coastal communities in Japan, the lessons learned from Hirogawa’s past experiences with tsunami are visible everywhere.
Lamposts throughout the town display evacuation routes and measurements of height above sea level. Near the shore, roads pass through large gates that can be shut to keep water out in the event of a surge. And in tandem with those, seawalls stretch along the coast, including one commissioned by Hamaguchi in the wake of the 1854 tsunami. (Long after his death, the seawall helped protect Hirogawa from tsunami following another Nankai quake in 1946.)
Altogether, Hirogawa has been hit by eight tsunamis in its recorded history dating back to 1361.
And yet, life in this seaside community marches on. In the town of about 6,400, residential communities lie snug up against the coast, a stone’s throw from the municipal hall and a gymnastics center that is often a hub of activity for young people.
“The people of this town have been told about tsunamis from their ancestors for generations,” Sakiyama, the museum director, says.
“We would like everyone in Japan to know about (Hamaguchi’s) story, and the next time a tsunami happens somewhere, we would like them to evacuate quickly and save their lives first.”
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