New Zealand’s tsunami tracking system aces its first test

Within a span of six hours on 4 March, three earthquakes —of magnitudes 7.3, 7.4, and 8.1—shook the waters northeast of New Zealand. Fortunately, the earthquakes and the mild tsunamis they generated caused little damage and no loss of life. Government officials had early confidence that coastal communities would be safe thanks to data from a newly installed network of buoys called the Deep-ocean Assessment and Reporting of Tsunamis (DART). Both the swiftness of the forecast and a retrospective analysis of the measurements demonstrate the importance of modeling tsunami origin and propagation using direct observations in the open ocean rather than relying on the adaptation of seismic data. “DART data has been a game changer for us,” says seismologist Bill Fry of GNS Science, a New Zealand government research institute focused on the geological sciences.

The New Zealand government began in 2019 to install a dozen latest-generation DART buoys in the southwestern Pacific . The buoys were developed by NOAA’s Pacific Marine Environmental Laboratory . Unlike tide gauges that hug the coastline, the DART instruments can be placed in the open ocean and can measure both sea-surface heights and pressure on the seafloor. Multiple buoys were placed in the Kermadec subduction zone, where the Pacific Plate is plunging under the Australian Plate at a rate of about 6 cm per year. The epicenters of the earthquakes on 4 March straddled the Kermadec Trench.

Two of the newly installed buoys recorded tsunami waves within 20 minutes of the earthquakes that caused them. Directly measuring the tsunami waves allowed researchers to quickly determine the amplitudes of the propagating waves rather than wait anxiously for readings from coastal tide gauges. As a result, Fry and his colleagues on the government’s Tsunami Expert Panel decided that the waves generated by the third and strongest quake posed no threat to coastal communities about four hours earlier than they would have if they had been relying solely on tide-gauge data. Those gauges were even less reliable than usual that day, because their readings encompassed residual wave energy from the day’s prior earthquakes.

In the weeks following the trio of seismic events, Fabrizio Romano of Italy’s National Institute of Geophysics and Volcanology and colleagues used data from five DART buoys and seven tide gauges, which were located near the magnitude 8.1 quake’s epicenter, to characterize the seafloor movement that generated the tsunami. By inverting the tsunami waveforms, the researchers determined that a slab of crust located about 100 km northeast of the earthquake’s focus slipped roughly 5 m. Crucially, the substantial depth of the slip, 20–30 km, ensured a vertical deformation of no more than 1.1 m on the seafloor, which limited the severity of the resulting ocean waves. Using a numerical tsunami wave simulation, the researchers showed that their DART-derived tsunami source effectively described the wave heights measured at four distant Pacific tide gauges.

The success of the DART buoys in March illustrates their value for local and regional forecasts, Fry says. More than 60 have been deployed in the Pacific, Atlantic, and Indian Oceans, with plans for denser networks like New Zealand’s in the works. (F. Romano et al., Geophys. Res. Lett., 2021, doi:10.1029/2021GL094449 .)

Editor’s note, 13 September: This article was updated to correct an error regarding the movement of the Pacific and Australian plates in the Kermadec subduction zone.