What is it about?
Landslides are abundant in the submarine slopes of volcanic islands and around tectonically coasts, such as those of Italy. Around the Azores volcanic islands, they may have produced waves with heights of 1-7 m (link), so they are a concern. (There have been tsunamis recorded within the Azores that do not coincide with earthquakes, hence they may have had other causes, such as submarine slope failure.) Do such slopes fail when shaken by earthquakes or are they strong enough to resist shaking? In areas with muddy deposits, samples obtained by coring can be tested to find out how strong the seabed is (geotechnical testing). But around volcanic coasts the material is friable, so it typically fragments if we try to core it. Working out if it can resist shaking is therefore difficult. Figure 1: 3D view of the upper submarine slope of Terceira Island, Azores, revealing steep head and side walls of landslides. (Image courtesy of Yu-Chun Chang.) In this study, we looked at a seamount in the Azores that had been shaken strongly during an earthquake swarm. From comparing sonar data before and after the swarm, we inferred that no landslides occurred and that the seabed is stronger than expected of unconsolidated material. Perhaps it has been strengthened by calcium carbonate precipited from seawater, which has bonded the particles. Or perhaps past earthquakes have consolidated it.
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Why is it important?
Our study follows similar practice investigating why slopes fail or don't fail during earthquakes in landscapes above sea level. Investigating which slopes fail and which slopes remain intact seems the most realistic way to assess stability of submarine slopes when the material is so "awkward" to recover and characterize in terms of geotechnical properties.
Perspectives
The seismic swarm was originally interesting because I thought that it might have been caused by a submarine eruption. I had previously seen the surface effect of an eruption west of Terceira in the 1990s with Marco Ligi and others on the Italian RV Urania. However, the new data data show no features expected of an eruption and instead our study focused on what the results imply about seabed stability. Adaptability can be important in research!
Dr Neil C. Mitchell
University of Manchester
Read the Original
This page is a summary of: Seabed stability inferred from the 2019–2020 earthquake swarm under a volcanic cone field and slopes of Condor Seamount, Azores, Journal of Volcanology and Geothermal Research, January 2025, Elsevier,
DOI: 10.1016/j.jvolgeores.2025.108279.
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