How an earthquake becomes a tsunami

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A wave glider with GNSS and acoustic measurement devices for seabed measurements. Credit: Todd Ericksen

The movement between the continental and oceanic plates on the seabed, the so-called megathrust earthquakes, generates the strongest tremors and the most dangerous tsunamis. How and when they occur, however, has so far been poorly understood, since the ocean floor is difficult to access for measurements.

Thanks to the new technologies, an international research group, which also included Prof. James Foster from the Institute of Geodesy of the University of Stuttgart, was able for the first time to carry out measurements to the nearest centimeter in an underwater seismic area at off Alaska. The researchers reported their findings in the journal The progress of science.

The Chignik earthquake of July 28, 2021 occurred 32 km under the seabed off the coast of Alaska and, with a magnitude of 8.2, was the seventh strongest earthquake in US history. It has occurred because the Pacific oceanic plate is sliding past the North American continental plate, thus causing a huge thrust.

In the sparsely populated region, the damage caused by the earthquake was limited. In general, however, such megathrust earthquakes have enormous destructive potential in the so-called subduction zone, i.e. the zone where the oceanic and continental tectonic plates meet. In particular, tsunami waves can be generated. These are not very high in their place of origin, but hours later and many 100 or 1000 kilometers away, they can hit the coasts like a catastrophic tsunami and endanger many lives.

Despite the magnitude of these natural hazards, the relevant physical processes involved in megathrust earthquakes are still only to a limited extent understood. It is therefore difficult to estimate the spatio-temporal evolution of coupled earthquake and tsunami hazards in subduction zones.

In order to better predict the likelihood of an earthquake triggering a tsunami, the research team led by Benjamin Brooks of the United States Geological Survey examined the seabed off Alaska shortly before and about 2.5 months after the Chignik earthquake. using a global navigation satellite system (GNSS), an acoustic positioning system and a robotic ship.

Autonomous gliders allow measurements to the nearest centimetre

In the project, a key role was played by autonomous boats operating on the water surface. These so-called wave gliders, in the development of which Prof. James Foster of the University of Stuttgart’s Institute of Geodesy was also involved, are equipped with both GNSS and acoustic measuring devices.

Modern technology has made it possible to measure the movements in the subduction zones to the nearest centimeter and thus an accurate picture of the complicated slip and faulting processes. Particular attention has been paid to the shallow portions of the slip zones, as these are critical to whether or not a tsunami will occur.

The measurements were made at a water depth of between 1,000 and 2,000 metres. “It would be even better if we could take measurements at water depths of 3,000 to 4,000 meters, directly above the lowest part of the fault system,” Foster says.

However, geodetic systems currently used on the seabed cannot be used at these depths. The tsunami researcher is all the more pleased that he will soon be able to purchase a device whose sensors allow geodetic measurements at these depths. ‘With this system, we will be able to directly measure the movement of the seabed in these deepest sections of the tsunamigenic faults.’

More information:
Benjamin A. Brooks et al, Rapid shallow megathrust fall from 2021 Chignik, Alaska M8.2 earthquake revealed by seafloor geodesy, The progress of science (2023). DOI: 10.1126/sciadv.adf9299

About the magazine:
The progress of science

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