Unveil the possible physical processes that allow an earthquake to generate a tsunami by uplifting the seabed. This is the goal of a study by INGV, the University of Padua and Florence, Royal Holloway University of London, Manchester and Durham University (United Kingdom), Tsukuba and Kyoto University (Japan), published in Nature Geoscience
There are different types of tsunamis, sometimes generated by the rupture of a fault plane, by the collapse of volcanic systems or by large underwater landslides triggered by earthquakes. A study conducted by a team of researchers from the National Institute of Geophysics and Volcanology (INGV), University of Padua and Florence, Royal Holloway University of London, Manchester and Durham University (United Kingdom), Tsukuba and Kyoto University (Japan), attempts to reveal the physical processes that allow an earthquake to generate a tsunami by lifting the seabed. The research Past seismic slip-to-the-trench recorded in Central America megathrust, has been published in Nature Geoscience (https://www.nature.com/articles/s41561-017-0013-4).
Earthquakes are the result of the propagation of a rupture along a surface that crosses the earth's crust called a fault. Rupture propagation allows blocks of rock at the side of the fault to move relative to each other by up to tens of meters in the case of exceptionally large (magnitude nine) earthquakes. In general, earthquakes that produce tsunamis differ from those affecting the continental crust, such as the recent Amatrice and Norcia earthquakes of 2016, in having a slower rupture propagation speed (1-2 km/s) than the others earthquakes (2-4 km/s), allow for large displacements of fault blocks close to the seabed, and have an epicenter located not far from the ocean trench.
"Until a few years ago", explains Paola Vannucchi, first author of the article and researcher at Royal Holloway of London, United Kingdom - University of Florence, "it was believed that seismic ruptures were not able to propagate through the most superficial and soft marine sediments rich in clay. Furthermore, the presence in these sediments of unconsolidated layers with a thickness of tens to hundreds of meters composed of calcareous shells of marine microorganisms had not been taken into consideration. In general, it was believed that the coefficient of friction of these materials increases with the speed of sliding along a fault, arresting the rupture before it breaks the seabed.
Instead, the study showed that the propagation, during large earthquakes (magnitude greater than seven), determines seismic breaks along faults from the depth where the earthquake originates (about 15-35 km for these earthquakes) to the seabed.
“The great Tohoku earthquake (magnitude 9.0) and consequent tsunami that flooded the northern coast of the Japanese archipelago on March 11, 2011 called this interpretation into question. Seismological, geophysical and geological evidence has shown that in this earthquake the rupture spread to the point of breaking the ocean floor with devastating consequences”, continues Vannucchi.
The breaking of the ocean floor is associated with the raising, even of a few meters for large earthquakes, of the seabed and the consequent energization of the overlying sea water column. Since in an ocean trench area the water column is several kilometers high, the uplift of the seabed in these particular oceanic environments involves the generation of massive and very violent tsunami waves, up to 20-30 meters high (a building of ten floors) when they crash on the coast, as in the case of the Tohoku earthquake.
"The research", adds Giulio di Toro, a researcher at the University of Padua associated with INGV, "combines data from ocean floor drilling carried out in the Pacific near the trench that runs along Costa Rica (Central America), from Integrated Oceanic Discovery Program (https://www.iodp.org/ ), from experiments conducted in Italy on marine sediments composed of clays and shells of marine microorganisms sampled during drilling".
The experiments were carried out with the SHIVA (Slow to High Velocity Apparatus) experimental apparatus which with 300 kW (equivalent to the power dissipated by 100 average Italian apartments) is able to dissipate, in rock specimens the size of a small glass of diameter of 50mm, the most powerful earthquake simulator in the world.
“SHIVA, designed and installed in 2009 at the High Pressure - High Temperature Laboratory of Experimental Geophysics and Volcanology of the INGV in Rome, is an instrument capable of understanding the mechanics of earthquakes. This research was funded by two European Union projects called USEMS and NOFEAR (Uncovering the Secrets of an Earthquake: Multidisciplinary Study of Physico-Chemical Processes During the Seismic Cycle and New Outlook on seismic faults: from earthquake nucleation to arrest)". says Di Toro, responsible for these projects.
"This research", concludes Elena Spagnuolo, INGV researcher, "attempts to reveal the possible physical processes that allow an earthquake to generate a tsunami by lifting the seabed. In consideration of the fact that these calcareous sediments are quite common in and that, based on experimental evidence, their presence facilitates the propagation of a seismic rupture up to breaking the seabed, it is believed that this phenomenon may be very frequent".
The High Pressure - High Temperature Laboratory of Experimental Geophysics and Volcanology is located in the Rome headquarters of INGV. Many of INGV's analytical and experimental activities are concentrated here in support of research and monitoring, but also the development of technologies and new survey methodologies. Leading research by INGV in the seismological, volcanological and environmental fields is carried out in the laboratory, some of which is financed as part of European projects. The experimental activities, also carried out in collaboration with laboratories in other countries, concern simulations and measurements related to the physics of rocks and earthquakes, the chemical-physical properties of magmas, and the analogical modeling of volcanic processes. The laboratory is also a pole of attraction for Italian and foreign researchers.
Extended
Past seismic slip-to-the-trench recorded in Central America megathrust
(https://www.nature.com/articles/s41561-017-0013-4)
Paola Vannucchi (Royal Holloway, United Kingdom – University of Florence, Italy), Elena Spagnuolo (National Institute of Geophysics and Volcanology of Rome), Stefano Aretusini (Manchester University, United Kingdom), Giulio Di Toro (University of Padua), Stefan Nielsen (Durham University, UK), Kohtaro Ujiie (Tsukuba University, Japan) and Akito Tsutsumi (Kyoto University, Japan).
The 2011 Tōhoku-Oki earthquake revealed that co-seismic displacement along the plate boundary megathrust can propagate to the trench. Co-seismic slip to the trench amplifies hazards at subduction zones, so its historical occurrence should also be investigated globally. Here we combine structural and experimental analyzes of core samples taken offshore from southeastern Costa Rica as part of the Integrated Ocean Drilling Program (IODP) Expedition 344, with three-dimensional seismic reflection images of the subduction zone. We document a geologic record of past co-seismic slip to the trench. The core passed through a less than 1.9-million-year-old megathrust frontal ramp that superimposes older Miocene biogenic oozes onto late Miocene–Pleistocene silty clays. This, together with our stratigraphic analyzes and geophysical images, constrains the position of the basal decollement to lie within the biogenic oozes. Our friction experiments show that, when wet, silty clays and biogenic oozes are both slip-weakening at sub-seismic and seismic slip velocities. Oozes are stronger than silty clays at slip velocities of less than or equal to 0.01 m s–1, and wet oozes become as weak as silty clays only at a slip velocity of 1 m s–1. We therefore suggest that the geological structures found offshore from Costa Rica were deformed during seismic slip-to-the-trench events. During slower aseismic creep, deformation would have preferentially localized within the silty clays. The 2011 Tōhoku-Oki earthquake revealed that co-seismic displacement along the plate boundary megathrust can propagate to the trench. Co-seismic slip to the trench amplifies hazards at subduction zones, so its historical occurrence should also be investigated globally. Here we combine structural and experimental analyzes of core samples taken offshore from southeastern Costa Rica as part of the Integrated Ocean Drilling Program (IODP) Expedition 344, with three-dimensional seismic reflection images of the subduction zone. We document a geologic record of past co-seismic slip to the trench. The core passed through a less than 1.9-million-year-old megathrust frontal ramp that superimposes older Miocene biogenic oozes onto late Miocene–Pleistocene silty clays. This, together with our stratigraphic analyzes and geophysical images, constrains the position of the basal decollement to lie within the biogenic oozes. Our friction experiments show that, when wet, silty clays and biogenic oozes are both slip-weakening at sub-seismic and seismic slip velocities. Oozes are stronger than silty clays at slip velocities of less than or equal to 0.01 m s–1, and wet oozes become as weak as silty clays only at a slip velocity of 1 m s–1. We therefore suggest that the geological structures found offshore from Costa Rica were deformed during seismic slip-to-the-trench events. During slower aseismic creep, deformation would have preferentially localized within the silty clays.
Figure 1: Scientists on board the R/V Joides Resolution describing the sediments recovered off the coast of Costa Rica during Exp. 334 – in the Paola Vannucchi centre.
Figure 2. The High Pressure - High Temperature Laboratory of Experimental Geophysics and Volcanology of the INGV in Rome. In the foreground the SHIVA experimental apparatus