Wondering why northern Italy got hit twice in short order with earthquakes? I asked Jeffrey Park, coauthor of Dynamic Earth (Read it. It's about the earth you live on. The one we return to, call mother, mine, fight over, and, sadly, the one that puts the terror in terroir). He's a Yale seismologist who works on the seismology of northern Italy. Here’s what he had to say. A warning, there’s strong earth science language in this report, a suggestion that plates are getting stuffed in the mantle, plus a reference to a quake in the 1500s. Reader access to a geological dictionary is advised. ~ Steph

The earthquakes both hit very close to Bologna, where I spent a year's sabbatical while collecting seismic data. I once spent a night in Carpi, where the cathedral roof fell in.

The standard interpretation for Italy is that the Apennines formed atop a subduction zone, leading to earthquake hazard all the way from Piacenza in the Po Valley to Sicily. Bologna lies where the deepest part of the oceanic trench would be if this subduction zone corresponded to an ordinary subduction zone, such as those offshore Japan and the Pacific Northwest. The nominal trench parallels the Adriatic Coast and the southern margin of the Po Valley.

The standard interpretation for Italy is a little misleading, because Italy lacks the large thrust earthquakes that characterize Japan or the Pacific Northwest, and we have 2000+ years of Italian/Roman history to confirm this. There is no evidence for a M=7 earthquake in the Bologna area, and all measured thrust quakes in the region (e.g., since 1900) have been M=6.0 or smaller. There are no earthquakes deeper than 100 km, but seismic tomography suggests that subducting lithosphere extends to 300km without causing earthquakes. SO it’s a curious subduction zone. I prefer a different model, in which the lithosphere is not actually being consumed by subduction (which would swallow the rocks beneath the feet of the Bolognesi and Modenesi). My seismic data suggested that the downgoing lithospheric rock detaches from the overlying crust at 40-km depth or so, effectively peeling off the bottom of the tectonic plate beneath the Apennines. This process resembles subduction in some ways, but does not lead to massive shallow thrust earthquakes.

The {recent} Italian quakes were not at the nominal plate boundary, but were located within the plate that is being "subducted." The faults have been mapped and are thought to have been responsible for an earthquake in the 1500s that severely damaged the city of Ferrara. Despite the damage, the quakes were both only moderate-sized. Damage was enhanced by the shallowness of the quakes.

The ruptured faults are thrusts that are deforming the "downgoing" plate prior to its arrival at the main thrust in the Apennine foothills. The model would be that the sedimentary rocks atop the downgoing plate are starting to crumple before they get stuffed into the mantle. In my interpretation, the thrusting of this region is being forced by a detachment of lithosphere that is active, but largely free of earthquake activity, 40km beneath the surface. Because the tearing is deeper, the surface faulting associated with it should occur over a wide belt, rather than be focussed at a single thrust fault in the Apennine foothills.

The INGV has produced this fact sheet on the quakes.

Jeffrey

Thanks Jeff! You've given us amateur rock heads good info to mine.