Tonga Tsunami a Reminder of Need for Better Global Detection Network

When a 50-foot tsunami struck the Tonga island nation in the South Pacific Ocean on Jan. 15, the waves scattered horizontally like layered ripples in a pond that ultimately extended from the surface to the seafloor.

A global seismic network of seafloor stations in the open ocean could quickly identify such developing tsunami waves and help warn threatened coastal areas, say researchers in a new study. John Orcutt of UC San Diego’s Scripps Institution of Oceanography and 11 colleagues published a review paper describing how such a network would function in the journal Oceanography.

The researchers describe how technological advances have made it possible to transmit data collected on the seafloor to autonomous surface vehicles that relay the data to an orbiting satellite constellation, and then to computer screens around the world.

“The transmitted data usually arrive less than a minute after the event occurs,” said Orcutt, a distinguished professor of geophysics at Scripps Oceanography. “These new approaches to bringing near-real-time data from the seafloor to your computer screen are becoming more and more popular and effective.”

The U.S. Geological Survey can identify the epicenter of a large earthquake rumbling beneath continents and oceans within minutes, then notify the public of its location. Doing likewise for tsunamis generated by earthquakes under the seafloor or by submarine volcanoes poses a greater challenge since the existing seismic network is on land, Orcutt said.

Many tsunamis originate in undersea trenches where two plates of Earth’s crust and the top of the mantle slowly collide with each other during tectonic processes that dictate the planet’s surface geology. Earthquakes at such intersections trigger most tsunamis, unlike the Tonga event, which was triggered by a volcanic explosion, or the huge eruption of Indonesia’s Krakatoa volcano in 1883. The Tonga event demonstrated the importance of community engagement to prepare for tsunamis, noted the Oceanography article’s lead author, Danielle Sumy, a geophysicist at the Incorporated Research Institutions of Seismology (IRIS) in Washington, D.C.

“For Tonga, one of the most interesting questions is how did a volcanic explosion like this cause such a large tsunami with no warning and yet the casualty numbers could have been much worse,” said Sumy.

One reason may be the drills held in Tonga following a damaging tsunami in 2009.

A global network of observatories like the one discussed in the paper can record data about a tsunami as the tsunami is in progress, said co-author Monica Kohler, research professor of mechanical and civil engineering at the California Institute of Technology. “Although the volcanic eruption itself may escape detection as a tsunami-producing event, the network would record data for the tsunami that can be monitored in real-time or near-real-time for issuing warnings on distant shores.”

Although NOAA’s global network of tsunami-detection buoys currently provides valuable data that are cabled to pressure recorders on the seafloor, 20 to 35 percent of the network’s buoys may not operate at any given time because of severe weather and breaks in the fiber-optic and data cables, Orcutt said, based on a 2011 National Research Council report he chaired.

“It’s often possible to detect and track tsunamis over that network, but if you’re running cable from the surface to the seafloor, the opportunities for failure are huge,” he said.

Japan has avoided that problem by connecting its land-based instruments to large, more reliable cables on the seafloor near its eastern coast, a high-risk tsunami area. Canada also operates an extensively cabled tsunami detection network off its northwestern Pacific coast.

“Not every country has the resources to do that,” Sumy said. “Tsunamis are relatively rare events that occur about twice per decade. Through international collaboration, we have an opportunity to make global tsunami warning systems a possibility and save lives in the process.”

More than a decade ago, the National Science Foundation funded a project led by Kohler that placed 34 seafloor seismometers spaced 50 to 75 kilometers (31 to 46.6 miles) apart in the Pacific Ocean. The seismometers collected data during Japan’s catastrophic March 2011 Tohoku earthquake and tsunami. The data documented the ocean wave behavior that Kohler compares to supersized ripples in a pond.

After the 2011 event, Kohler and her colleagues used the data to trace the various arrivals to their point of origin. Some of the waves had traveled from Tohoku off the coast of Japan to the coastline of Antarctica, then to western North America. Other waves reached the west coast of South America, then bounced over to western North America.

Better warning systems are part of a new international initiative intended to improve overall ocean observations. The United Nations Decade of Ocean Science for Sustainable Development’s scientific priorities include developing a comprehensive observation system for all major ocean basins and an integrated multihazard warning system.

“Tsunamis do not know political boundaries,” Sumy said. “The technology for a global, reliable tsunami early warning system exists and it matters. That’s why I am excited about the UN Decade for Ocean Science.”

“One of the most important parts of any global seismic network would entail developing autonomous stations that could be deployed almost anywhere in the ocean,” Kohler said. “This includes areas that have no cabled systems, including the Southern Hemisphere and parts of the North Atlantic. Without seismic and tsunami data from those regions,” she warned, “we’re going to have really poor resolution results for those parts of the world.”

In recent centuries the United States has managed to escape most damaging tsunamis. An enormous tsunami inundated what is now northern Washington state on Jan. 27, 1700. The date and event were noted in Japan and recorded for posterity. 

“It gives you pause about the threat today,” Orcutt said. “It’s still an issue.”

Study co-authors include Sara K. McBride of the U.S. Geological Survey, Christa von Hillebrandt-Andrade of NOAA, Shuichi Kodaira and Takane Hori of the Japanese Agency for Marine-Earth Science and Technology, Kate Moran and Benoît Pirenne of Ocean Networks Canada, independent researcher Danial McNamara, Elizabeth Vanacore of the University of Puerto Rico/Puerto Rico Seismic Network, and John Collins of Woods Hole Oceanographic Institution.