The Process of an Earthquake Turning into a Tsunami

Movement between continental and oceanic plates on the sea floor, called mega-quakes, generates the most powerful tremors and most dangerous tsunamis. However, how and when they occur is not yet well understood, as it is difficult to reach the ocean floor for measurements.

Thanks to new technologies, an international research team, in which Professor James Foster of the Institute of Geodesy at the University of Stuttgart took part, was able to take measurements to the nearest centimeter for the first time in an underwater earthquake zone off Alaska. The researchers report their findings in the specialized journal Science advances.

The Chignik earthquake occurred on July 28, 2021, at a depth of 32 kilometers below the sea floor off the coast of Alaska, and it was the seventh strongest earthquake in the history of the United States, with a magnitude of 8.2. It happened because the Pacific oceanic plate was sliding across the North American continental plate, causing a massive eruption.

In the sparsely populated area, damage from the earthquake was limited. In general, however, such massive earthquakes have enormous destructive potential in the so-called subduction zone, that is, the area where oceanic and continental tectonic plates meet. In particular, tsunamis can be generated. These are not very high in their place of origin, but after hours and 100 or 1000 kilometers away, they can hit the coasts in the form of a tsunami and endanger the lives of many people.

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

To be able to better predict the likelihood of an earthquake causing a tsunami, the research team headed by Benjamin Brooks of the USGS examined the seafloor off Alaska as recently as 2.5 months before the Chinyik earthquake, using a global satellite navigation system. (GNSS) voice positioning system and ship automation.

Independent wave gliders allow measurements to the nearest centimeter

In the project, autonomous vessels operating on the surface of the water played a major role. These so-called gliders, co-developed by Professor James Foster of the Institute of Geodesy at the University of Stuttgart, are equipped with both GNSS and acoustic instrumentation.

Modern technology has allowed measurements of motions in subduction zones to the nearest centimeter and thus an accurate picture of complex slip processes and faults. Particular attention has been given to the shallow parts of the slip zones, as they are essential for determining whether or not a tsunami will occur.

Measurements were taken at a water depth of 1,000 to 2,000 metres. “It would be better if we could make measurements at a water depth of 3,000 to 4,000 meters, directly above the shallow part of the fault system,” says Foster.

However, geodetic systems currently in use cannot be used on the sea floor at these depths. A tsunami researcher is pleased that he will soon be able to purchase a device whose sensors allow geodetic measurements at these depths. “Using this system, we will be able to directly measure sea floor motion in these deep parts of the tsunami fault.”