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Conference " Tsunami and earthquakes : from past to the Present"

par Flavien Choulet - publié le

What do we really know about past tsunamis ?
James Goff

This talk looks at how well we are doing palaeotsunami work and why we are essentially not doing a very good job. Identifying historical events is easy, although we are still filling in the gaps in historical databases and will continue to do so, but identifying palaeotsunamis is far more complicated than we first thought. We have moved on now from the early “wow” days of finding big events that meant an instant Nature paper, to now trying to work out how big they are, how often they happen and where do they come from ? Useful science if you wish. This is a very young discipline but in the wake of events such as the 2004 Indian Ocean and 2011 Tohoku-oki tsunamis huge demands are being placed on earth scientists to determine the tsunami hazard for our coastlines. This is no small task. There are fundamental questions that still haunt us such as, is it a storm or a tsunami deposit ? Also, there are so many techniques available to us that in reality we must be familiar with as many of them as possible in order to do justice to the question being asked – what is the tsunami hazard for our coastline ? If we do not use all the tools in the toolbox that we have, then we run the risk of missing the evidence in front of us. This talk gives some examples, primarily from the Southern Hemisphere, of some of the problems, pitfalls, and wider “earth science” evidence in the ongoing search for past tsunamis. In essence, the devil is in the detail.

Earthquakes in central Apennines, Italy : did clay assist seismic slip propagation ? Insight from field and laboratory studies
Luca Smeraglia

The central Apennines of Italy is one of the most seismically active portions in the Mediterranean area. In fact, in the last century, three devastating earthquakes struck the central Apennines causing infrastructures damage and fatalities : the Avezzano, 1915, Mw 7.0 earthquake (30,519 fatalities), the L’Aquila, 2009, Mw 6.3, earthquake (299 fatalities), and the Amatrice-Norcia, 2016, Mw 6.0 and 6.5 earthquakes (309 fatalities). These earthquakes nucleated at 8-10 km depth and upward propagated through the carbonate-hosted brittle crust causing extensive surface faulting and ruptures, in places up to 2 m high. To understand the mechanisms that facilitated earthquake slip propagation at shallow depths (< 2 km) and surface faulting a set of exhumed carbonate-hosted extensional faults from the central Apennines has been extensively investigated, combining both field and laboratory approaches. Field and microscopic observations show that decimeter- to micrometer-thick layers enriched in weak phyllosilicates can occur along carbonate-hosted fault surfaces, thus reducing the expected fault strength. At the nano-scale, instead of their common platy-lamellar texture, phyllosilicates consist of welded nm-thick nanospherules similar to phyllosilicates deformed in rotary shear apparatus at seismic velocities or altered under high hydrothermal temperatures (>250 °C), thus suggesting a co-seismic origin. Eventually, laboratory simulations demonstrate that, at seismic slip rates (1 ms-1), calcite gouges with pre-existing phyllosilicate-bearing (clay content ≤ 3 wt.%) micro-layers weaken faster than calcite gouges gouges. Therefore, the occurrence of ultra- low clay content localized along micro-layers within carbonate-hosted extensional fault of central Apennines can facilitate seismic slip propagation during earthquakes, possibly enhancing surface faulting and associated damages.