Tsunami-generating earthquakes being investigated in a laboratory

Understanding the link between earthquakes and tsunamis and, in particular, what are the conditions that amplify the risk of a tsunami, it is of fundamental importance to assess the risk to which many coastal areas are exposed. This topic is the subject of an international study, led by Paola Vannucchi of the Department of Earth Sciences, published in Nature Geoscience.

An international study (Past seismic slip-to-the-trench recorded in Central America megathrust), recently published in Nature Geoscience (DOI: 10.1038 / s41561–017–0013–4), investigated this field: the research was financed by two European Union projects called USEMS and NOFEAR (Uncovering the Secrets of an Earthquake: Multidisciplinary Study of Physical-Chemical Processes During the Seismic Cycle and New Outlook on seismic faults: From Earthquake nucleation to Arrest).

We talk about it with Paola Vannucchi, the first author of the article, who is a researcher at the Department of Earth Sciences of the University of Florence and a professor at the Royal Holloway of London.

What is the novelty factor of your research, which was carried out together with the National Institute of Geophysics and Volcanology (INGV) and the University of Padua, as well as with international organizations such as the Royal Holloway University of London, Manchester and Durham University (United United), Tsukuba and Kyoto University (Japan)?

To understand it, we need to make a couple of premises. First of all, the fact that earthquakes occur along fracture surfaces that cross the earth’s crust, called faults. The boundaries between the tectonic plates are “super” faults, and particularly large earthquakes occur where the plates run one on top of the other (subduction). These faults are underwater, and the energy produced by an earthquake can, in turn, generate a tsunami. The magnitude of a tsunami, that is the measurement of the energy released by the event, usually reflects the magnitude of the earthquake that produced it. However, in some episodes, as in the earthquake of 1992 in Nicaragua or in 2006 in Java, tsunamis occurred with a magnitude far greater than that of the earthquakes that caused them.

Let’s move on to the second premise.

An earthquake consists of a break, which propagates with a certain speed, followed by the displacement of rock blocks at the side of the fault. Generally, tsunami-producing earthquakes have a slower propagation rate (1–2 km/s) than earthquakes affecting the continental crust (2–4 km/s). Moreover, they allow large displacements of fault blocks very close to the seabed and have an epicentre not far from the oceanic pit.

In other words, that ocean depression that can be found alongside a continental margin.

Yes. Now, if the earthquake is a fracture, it is necessary that what is fractured has a certain rigidity. But at the exact point where the subduction begins, there are sediments deposited on the ocean bed, practically muds, which flow under other muddy sediments. So far it has been thought that an earthquake, which usually originates at about 15–30 km of depth along these faults that mark the limit between the two plates, could not propagate to the ocean floor, where the material is so little petrified, not rigid, consisting of clay and limestone shells of marine microorganisms. But this was not the case in the 2011 Tohoku earthquake in Japan, which was associated with a tsunami that violently flooded the northern coast.

What happened?

Thanks to the monitoring of the seabed operated by the Japanese as well as other scientific evidence, it was understood that the break has spread to the seabed with devastating consequences. Until now it was believed that the coefficient of friction of unconsolidated materials increased with the sliding speed along a fault, stopping the break before it reached the seabed. So we decided to study — with some perforations of the ocean floor — the area off the Pacific coast of Costa Rica, where the tectonic plate of the Cocos flows under the plate of the Caribbean. Here we sifted the sediments that are involved and cut by the fault that marks the limit between the two plates.

What was the result?

We have discovered that in the past, some earthquakes have spread to the ocean floor. The geometry of the fault, experiments on the friction of materials and the balance of energy involved in the breaking of carbonate mud and clays sheds new light on the fact that, during earthquakes with magnitude greater than seven, which is relatively small for this type of tectonic environment, the propagation of the rupture can reach the surface and stress the water column which in turn can produce very large tsunamis.

And this leads to “atypical” tsunamis, whose intensity would not be otherwise explained?

Yes, because the breaking of the ocean floor is associated with the raising of the seabed itself and the consequent energization of the overlying seawater column. Since the water column is several kilometres in height in the deep sea trench area, lifting the seabed in these particular oceanic environments involves the generation of massive and violent tsunami waves, up to 20–30 metres high (a 10-storey building) when they break on the coast, as in the case of the Tohoku earthquake.

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The University of Florence is an important and influential centre for research and higher training in Italy

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University of Florence

University of Florence

The University of Florence is an important and influential centre for research and higher training in Italy

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