Science1 publisher2 min readPublished
Florence geoscientists trace most Apennine earthquakes to lower crust dropping into the mantle
Stefano Tavani's group combined Italy's earthquake record with GPS and satellite ground motion to argue that the lower crust beneath the Apennines is peeling away and sinking, a late-stage process he calls unzipping.
The Scientist · Science desk
What happened
- Stefano Tavani's group concludes that delamination, not the subduction of one plate beneath another, causes most of the seismic activity in the Apennine Mountains.
- The paper, titled "Lower-crustal unzipping drives active orogenic deformation", was published this month in Communications Earth & Environment, with Tavani of the University of Florence as lead author.
- Tavani said the lower crust will finish dropping away in a few more million years, after which the two plates will weld together.
- He said similar processes are under way elsewhere, including at the Hellenic trench south of Greece.
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Why it matters
- constraint The inputs are a seismic catalogue and surface ground motion, so a detaching lower crust is the model that best explains those measurements; the described data do not on their own exclude a different geometry that fits the same strain field.
- decision Anyone parametrising Apennine earthquake sources has to pick which structure to represent, because a peeling lower crust and an active plate interface are different shapes at different depths feeding the same recorded deformation.
- precedent If the same reading transfers to other dying subduction zones, surface geodesy combined with catalogues becomes an accepted basis for claims about what the lower crust is doing underneath a mountain belt.
Subduction and delamination place the moving mass in different layers. Subduction, in Tavani's description, is a slab of oceanic crust moving under a neighbouring piece of continental crust [6]. Delamination is the lower crust beneath the Apennines peeling away and dropping into the mantle [3].
The evidence is indirect. The team combined the earthquake record with ground movements from GPS and satellite measurements, then read all of it against tectonic models of the plate motions [5]. Ground motion is recorded at the top of the crust, while the layer the paper holds responsible lies at its base [3].
The Mediterranean between Africa and Eurasia is a patchwork of tectonic microplates and mountain ranges [18]. The African plate, to the south, has been pushing north for about 50 million years, driving the old Tethys ocean down into the mantle beneath the Apennines [8]. While that went on, the boundary of the subduction zone migrated toward the African plate, stretching and thinning the crust on the Eurasian side and opening two basins [9]. The older one sits west of Corsica and Sardinia; the Tyrrhenian, east of them, formed roughly 10 million years ago [10]. The younger basin therefore belongs to about the last fifth of that convergence history [16].
Tavani said the driver behind both the extension that opened those basins and the compression that built the Apennines had not been entirely clear before this work [11]. "We are experiencing the very, very late-stage to the subduction, and the tectonics is driven by a different engine, which is this unzipping," he told Live Science [7].
For hazard, the difference is geometric. A source model that hangs Apennine seismicity on a subducting interface is using a structure this paper says is no longer the driver [4]. The Live Science account reports no revision to Italian hazard estimates [17]. Hazard maps are built on the same catalogue the study used as input, so reinterpreting the engine does not by itself move one [5].
Tavani expects the approach to apply elsewhere. He said the findings "could be applied to other several systems," adding, "because in the end, it is a kind of very late-stage plate tectonics" [14].
What to watch
- Whether independent imaging of the lower crust beneath the Apennines shows a detaching layer where the model puts one.
- Whether Italian hazard modellers change the source geometry they use for the Apennines in response to the paper.
- Whether the same analysis applied to the Hellenic trench reproduces the delamination attribution or contradicts it.