Earlier tsunami warnings via magnetic fields

Tsunami warnings: A giant wave heading toward a residential area.
The 9.1-magnitude undersea earthquake that struck off the coast of Japan on March 11, 2011, was the most powerful earthquake recorded in Japan since record-keeping began in 1900. It caused a powerful tsunami, which in turn caused a meltdown of 3 reactors at the Fukushima Daiichi nuclear plant. The meltdown released radioactive water in Fukushima. Hundreds of thousands of residents were forced to evacuate the area. Official figures released in 2021 reported 19,747 deaths, 6,242 injured, and 2,556 people missing from this tsunami disaster. Would earlier tsunami warnings have saved some of them? This image shows the March 11, 2011, tsunami engulfing a residential area in the city of Natori, Japan. Image via Kyodo/ AP/

A new insight into magnetic fields, generated by tsunamis in seawater, might lead to earlier tsunami warnings for people who would otherwise be in danger. Scientists said December 21, 2021, that the magnetic fields generated by tsunamis arrive before the waves themselves. They said the magnitude of a tsunami’s magnetic field can help determine the potential height of the tsunami wave before it strikes land. Scientists can detect a tsunami’s magnetic fields only up to a few minutes earlier than previous methods. Yet those few minutes might provide precious time for people to seek higher ground.

The peer-reviewed Journal of Geophysical Research: Solid Earth published the scientists’ study on magnetic fields and tsunamis on October 18, 2021.

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Animation of the Indian Ocean, a huge wave that starts in Indonesia reaches India and Somalia.
The deadliest tsunami on record took place on December 26, 2004. According to the U.S. Geological Survey, a total of 227,898 people died. Indonesia was the worst affected area, with most death toll estimates at around 170,000. This animation of the tsunami is from the NOAA Center for Tsunami Research/ Wikimedia Commons.

Tsunami warnings via magnetic fields

Scientists have known for years that the force of tsunamis generates electric currents in conductive seawater, creating magnetic fields. They’ve speculated that these magnetic fields would arrive before the tsunami wave and that, if they could detect the magnetic fields from tsunamis, that detection could lead to earlier tsunami warnings. The 2009 tsunami in Samoa and the 2010 tsunami in Chile provided them with the evidence in the form of simultaneous measurements of magnetics and sea level to make their case.

How much earlier do the magnetic fields arrive? The answer depends on the depth of water along a particular coastline. These scientists found that the early arrival time is only about one minute before sea level change over a 15,700-foot (4,800-meter) deep sea. Author Zhiheng Lin of Kyoto University said:

It is very exciting because in previous studies we didn’t have the observation [of] sea level change. We have observations [of] sea level change, and we find that the observation agrees with our magnetic data as well as theoretical simulation.

People beginning to run, on a beach, with a huge wave striking behind them.
A photograph of the December 26, 2004, tsunami in Ao Nang, Krabi Province, Thailand. Image via David Rydevik/ Wikimedia Commons.

Issuing tsunami warnings

Using data from the two tsunamis (2009 in Samoa and 2010 in Chile), the scientists compared simultaneous measurements of sea level change from seafloor pressure data and magnetic fields. They found that the primary arrival of the magnetic field is similar to the beginning of a seismic wave, which is currently what triggers tsunami warnings. The scientists were able to detect a wave height of only a few centimeters in the magnetic field. explains how current tsunami warnings occur:

To provide alerts as early as possible, initial tsunami warnings are usually based only on seismic information that detects the earthquake’s location and magnitude. Information from coastal sea-level gauges and offshore buoy stations confirms the existence or absence of a tsunami. Then, information from seismic data, sea-level gauges, and buoy stations are used to generate models that forecast tsunami arrival times and estimate coastal impacts. The models help warning centers update or cancel warnings as they learn more about the earthquake and resulting tsunami.

Blue and red cargo trailers scattered all over near a harbor.
In 2010, a tsunami wreaked havoc in parts of Chile. A new study – which used data from this event – might offer an earlier tsunami warning system through the monitoring of magnetic fields. Image via AGU/ International Federation of Red Cross and Red Crescent Societies.

Some shortcomings

The magnetic field data can be used to improve tsunami source models, which predict the arrival time and wave height of the tsunami. Unfortunately, the new insight won’t help in all tsunamis. Limited observation stations limit the data from tsunamis. Also, their finding relates only to environments where deep water filters out other noise and allows the detection of the tsunami signal. Shallow coastal areas can’t be used for these models.

But even if the data are not useful in all locations, Lin says that providing warnings makes the predictions worthwhile.

Two vehicles partially submerged in water and surrounded by debris.
The 2009 tsunami in Samoa also provided data that the researchers used in their study of magnetic fields and tsunamis. Image via NOAA/ NCEI.

Bottom line: Tsunamis create magnetic fields, which can be used to provide early warnings in certain situations.

Source: Direct Comparison of the Tsunami-Generated Magnetic Field With Sea Level Change for the 2009 Samoa and 2010 Chile Tsunamis


Read more: Ancient tsunamis on Mars?

January 5, 2022

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