About 8 years ago, at the end of a long day of meetings at the University of Hawai‘i at Mānoa, visiting speaker and seismologist Charlotte Rowe walked into Bruce Howe’s office. “He rolled a map out…that showed all the transoceanic telecommunications cables,” she recalled. He described a future in which these underwater cables could include seismic sensors every 50 to 100 kilometers, all around the world.
Howe, a professor in the Department of Ocean and Resources Engineering, asked, “Would this be of interest to you seismologists?”
Rowe responded, “Well, yes!”
During that meeting, Howe asked Rowe to quantify just how useful underwater cables equipped with regularly spaced seismic sensors might be for seismologists. “With a few exceptions, all of our seismic networks are on land,” said Rowe. For Earth beneath the oceans, this means that “we don’t do a very good job of characterizing how seismic waves propagate through it.” After doing some calculations with a summer intern back at Los Alamos National Laboratory, Rowe said, “the [modeled] improvement was stunning.”
Since then, Rowe has been involved with the United Nations’ Joint Task Force on Science Monitoring and Reliable Telecommunications (SMART) Subsea Cables, for which Howe is chairperson. They’re working to outfit the ocean with temperature, pressure, and seismic sensors that would help with a host of problems, including earthquake early warning, tsunami tracking, and climate change.
Rowe will update the scientific community on plans for SMART cables in various locations around the world at AGU’s Fall Meeting 2022 on Tuesday, 13 December.
The Internet might seem like a combination of satellites, the “cloud,” and data flitting through the air. In fact, the cloud consists of buildings full of servers scattered throughout the world, where physical cables connect to one other. Sending data across an ocean simply requires longer, tougher, specially engineered cables.
Spanning thousands of underwater kilometers, telecommunications cables, which look like garden hoses, contain at their center hair-thin optical fibers protected by sheaths of metals and other materials, explained Howe. The optical fibers transmit data at the speed of light, while copper in the cable carries the necessary electrical power. This is how we see cat videos from other continents in real time; how we videoconference overseas coworkers, friends, or family; and how global financial transactions take place. “Without these cables, we would not have the Internet as we know it,” he said.
These cables begin and end at shore stations that provide power. About every 70 kilometers along the entire length of each cable, a long cylinder called a repeater amplifies the throughgoing signal, said Howe. These cylinders, between 1 and 1.5 meters long, have enough space for sensors to sit within the mechanically protected, seawater-flooded end sections. The current plan is to find funds to add SMART sensor packages that measure temperature, pressure, and seismicity to future cable deployments to replace aging transoceanic telecommunication infrastructure.
Portugal’s Once and Future Great Earthquake
One of the first cables likely to include SMART sensors will maintain Portugal’s connection to the Azores and Madeira islands. Existing cables are nearing the end of their lifetimes, said Vitor Silva, an earthquake engineer and risk coordinator at the Global Earthquake Model Foundation. “We’ve been lobbying the government to make sure that the next time they replace the cables, [they’ll] be SMART.”
Silva explained why Portugal is a good test case for the SMART sensors. In 1755, he said, the Great Lisbon Earthquake destroyed the nation’s capital. The earthquake, which may have been greater than magnitude 8.0, began southwest of Portugal’s coast, under the waters of the Atlantic Ocean. Following the rupture and collapse of countless structures, a tsunami inundated the coast as fires raged. Today, Portugal does not have an earthquake early-warning system akin to those in other earthquake-prone regions. In addition, many of Portugal’s buildings are not seismically sound, making them, and their inhabitants, vulnerable to shaking.
By strategically laying the SMART cable where scientists suspect the seafloor will break, any such earthquake would be much more rapidly detected, according to a recent paper led by Silva.
“The closer you can get a seismometer to the earthquake source, the sooner you can transmit the fact that there has been an earthquake,” explained Rowe.
Silva and colleagues have also calculated how much money SMART cables could save Portugal should another offshore earthquake strike. The new cables are likely to cost 140 million euros. Making them SMART should add about 10%, upping the expense to 154 million euros. However, the cost could be recouped by saving peoples’ lives. By simulating various earthquake scenarios and determining how many people might die and using estimates for the cost of losing a person’s life, Silva and colleagues calculate that 170 million euros would be saved. “Including this technology could actually pay for the entire thing,” said Silva.
Though Silva’s calculations didn’t account for tsunami warnings, both the seismic and pressure sensors on SMART cables could more quickly inform authorities of incoming ocean waves.
“Detecting the first arrival of the earthquake waves [earlier] will help us get a more rapid and perhaps more accurate location and depth of the earthquake,” said Stuart Weinstein, deputy director of the Pacific Tsunami Warning Center and member of the joint task force. An earthquake’s location, depth, and magnitude factor into whether a tsunami is a possibility and guide initial tsunami alerts.
Should a tsunami form, it changes sea level as it propagates, Weinstein explained. As the crest of a tsunami wave passes, sea level rises, and pressure increases on the sensor. Likewise, when the trough passes, sea level drops, and pressure decreases. In this way, pressure sensors can track tsunamis (not wind-driven surface waves) and validate forecasts produced from seismic data.
Future Cables, Future Climate
Additional sensors have been considered, said Howe, but “the key problem is many of those are not ready for 25-year life on the seafloor.” Plus, he said, adding too many sensors will turn industry off because most cables are privately owned. “To get implemented on a global scale, we have to keep this concept simple.”
Currently, the joint task force is exploring external couplings, particularly for certain custom cables, said Rowe. These couplings would allow later installations of additional seismic instruments that might be able to measure a greater variety of signals than the small seismic sensors planned for the SMART cables. These external instruments, perhaps installed via underwater robot, could be more solidly coupled to the seafloor, which would enhance the signal’s integrity. This setup would solve two limitations of today’s scant ocean bottom seismometer deployments—power and communication—which would both be supplied by the cable, Rowe explained.
In addition, the U.S. National Science Foundation is considering a cable that would connect New Zealand to McMurdo Station in Antarctica, where researchers lack high-speed Internet connections. The Southern Ocean that surrounds Antarctica is the worst-instrumented part of the world, said Rowe. “There’s so much that we could learn” from a SMART-equipped cable, she said.
Another intriguing avenue for the future, according to Rowe, involves hydroacoustic sensors (hydrophones) that listen to sounds propagating through the water. “The ocean is a very, very noisy environment,” she said. Earthquakes rumble. Underwater volcanoes explode. Marine mammals splash and sing. Icebergs crack and groan. Ships clack and clang. Listening to the sea’s sounds is one way to track changes and patterns in the ocean, though hydrophones aren’t part of current plan for the sensor package.
SMART cables could also keep tabs on climate change. Pressure sensors would measure components of ocean circulation, whereas temperature sensors could inform scientists about how ocean bottom temperatures are changing, Howe explained.
Howe noted that scientists cannot trace a direct line from a person suffering from the effects of drought to a specific measurement. Nevertheless, he said, “the climate measurements [along with other observations] will affect everyone on the planet, albeit indirectly and over longer timescales.”
—Alka Tripathy-Lang (@DrAlkaTrip), Science Writer