Tsunami-generated magnetic fields can lead to quicker warning system

A new study says tsunami warnings can be improved by observing magnetic fields generated by the massive waves which would alert coastal residents a few minutes earlier, as opposed to just observing changes in sea level.

Tsunamis generate a magnetic field with the movement of seawater. Researchers writing in the Journal of Geophysical Research: Solid Earth say previous studies found that “using the tsunami-generated seafloor magnetic field, it is possible to predict the propagation direction and wave height prior to the actual arrivals of tsunamis.”

They had previously predicted, as per a news release, that “the tsunami’s magnetic field would arrive before a change in sea level,” but they “lacked simultaneous measurements” of magnetics and sea level that are necessary to demonstrate the phenomenon.

They note that they found the tsunami-generated magnetic field “arrives earlier than the tsunami sea level change based on analytical solutions and numerical simulations.”

The scientists, Zhiheng Lin, Hiroaki Toh and Takuto Minami, used “the world’s first simultaneous data of sea level change and magnetic field” recorded during the 2009 Samoa and 2010 Chile tsunamis. Having both sets – aka the ‘simultaneous measurements that they lacked’ mentioned earlier – allowed them to research better.

The comparative studies they did studying the relation between the sea level change and magnetic field gave two outcomes. One, that the vertical component of the tsunami magnetic field “arrives earlier than sea level change.” Two, that the horizontal component of the tsunami magnetic field “arrives even earlier than the vertical component.”

Lin et al also say they revealed that the tsunami magnetic field “can be used to estimate the tsunami wave height very accurately.” While how much earlier the tsunami warning based on the magnetic field arrival can be made depends on water depth, the study’s authors say the early arrival time “to be about one minute prior to sea level change” over a 4,800 metre deep sea.

While one minute may not seem much of a pre alert, it may be the difference between life and death during disaster scenarios.

“It is very exciting because in previous studies we didn't have the observation [of] sea level change,” says Zhiheng Lin, senior study-author and a geophysicist at Kyoto University. “We have observations [of] sea level change, and we find that the observation agrees with our magnetic data as well as theoretical simulation.”

What the research team did was to look at simultaneous measurements of sea level change from seafloor pressure data and magnetic fields during the two tsunamis, in Samoa and Chile. They discovered that the magnetic field’s primary arrival, “similar to that of the beginning of a seismic wave,” can be used to warn seaside residents of a tsunami. The field is “so sensitive” that even a wave height of a few centimetres can be caught and used for a tsunami warning.

“They did something that basically needed to be done,” says Neesha Schnepf, a researcher of geomagnetics at the University of Colorado, Boulder who was not involved in the study. “We’ve needed a study that compared the magnetic field data with the sea level change from the pressure data, and I’m pretty sure they’re the first to really compare how well the sea level from magnetic field matches the sea level from pressure, so that’s definitely very useful.”

When the scientists compared the horizontal and vertical components of the tsunami magnetic field with sea level change, they discovered that “both components can precisely predict tsunami sea level change” - yet the caveat is, the models need to include good estimates for ocean depth and the electrical structure below the seafloor.

The relation between the two sets of data, ie magnetic fields and wave height, can be used, the news release notes, to improve tsunami source models, “which estimate the initial sea surface topography of a tsunami and then predict water wave arrival time and wave height” - data that can be vital for disaster preparedness and response.

The problem that future researchers may encounter is limited observational stations, which means tsunami data that includes magnetic field information and wave height are not always at hand. Moreover, the findings apply to deep-sea environments only, and not coastal ones, because deep water in the region “filters out environmental noise to allow the tsunami signal to be detected.”

However, providing warning for these severe events — which have the potential to cause intense damage to large areas — makes the predictions worthwhile, says Lin.

The scientists write in AGU’s JGR Solid Earth that they investigated the observed tsunami magnetic field by the 3-D time domain simulation. But, the currently available tsunami source models were “unable to reproduce” the observation in their research area.

They write that their own high precision tsunami wave height data calculations, based on magnetic field readings, can “improve” existing tsunami source models.

“I think the practical goal would be if your ability to model tsunamis is so improved, … you could come up with much better predictions of what areas might need to be warned [and] how badly it might hit certain places,” Schnepf says.

THUMBNAIL PHOTO: Work crews near the damage resulting from a tsunami in Samoa in 2009. (Wikimedia Commons/ Lorn Cramer)

HEADLINE PHOTO: The aftermath of a 2010 tsunami in Chile, which was analysed in a new study in JGR Solid Earth. (International Federation of Red Cross and Red Crescent Societies)