Recently there was a rather significant Earthquake in Northern Italy along the Po valley. Rescue and recovery efforts are still underway. With luck there will be no additional injuries due to aftershocks and building/infrastructure failure. Though unfortunate, this quake does afford us the opportunity to look around to see what is going on… geologically.
I would like to thank KarenZ whose plots put me on to this line of inquiry.
There is a lot going on in this region, and the structures there are somewhat complicated (to me) but in essence, the Adriatic or Apulian Plate broke off of the African plate and is wedged between the two. Where it is pushed North , the Alps were formed, to the Southwest, the Apennine Mountains formed and make up the familiar “spine” that runs down the Italian peninsula. The northern section of this range between the Po Valley and the Ligurian Sea is the region of interest. It seems that there is a pretty ancient subduction structure here that has a plate section hanging almost vertically underneath the mountains. (see Fig 1 of Margheriti et al). It is suggested that this is not a classic “subduction zone” but could be some exotic structure made up of continental crust fragments frozen in place in said paper.
Why do I bring that up? Well, the focal mechanisms for the two largest quakes show faulting similar to that of a subduction zone… specifically reverse faulting. The dangling slab in the last paragraph is not it.
In reverse faulting, the headwall is pushed up over the other side of the fault (relative to the other side) or the other side is being pushed under the headwall. (same motion, just different ways of looking at it) For this quake, it is actually oblique reverse faulting since it is pushing off to one side a bit. (the ball isn’t perfectly lined up).
The question about the Bulgarian quakes came up , but those have a completely different solution. They show normal faulting where one side slides down and away from the other. (or up and away). The only things those two quake sets have in common is.. um.. nothing. They were along the northern boundary region of the Agean Sea plate and the Eurasian plate. There may be some regional stress that caused them both, but as for fault lines, totally unrelated.
So.. what is with the Apulian Plate and how did it get there? Well, that’s the really wild thing. It seems (according to diagrams in reference 4) that the toe and heel of Italy, and part of Greece, originated in the gap in the North African coast down around Tripoli. During this drive north the Alps were formed. Massive folding and crumpling occurred as the land was tortured into position. Anticlines and Synclines formed and eroded, and the leading edge of the collision warped and formed a basin…much like the Persian Gulf between the Arabian and Eurasian plate collision or the Ganges valley on the Indian Plate to Eurasian Plate collision. As some of you know, the top of the Matterhorn is African crust. Did you also know that it is upside down? That’s how extreme the collision is. (pg 14 of Ref 5) In fact, one anticline was an island in a shallow northern Adriatic sea during the Pleistocene, the Ferrara Anticline, buried about 20 km northeast of Modena in the Po river plain. (ref 2 and 3).
Okay, enough rambling.
In this document I noticed that the study area covered a rectangle directly covering the quake area. Taking a position on the Northeast end of that box, I was able to calculate the distance to each quake and plot them in relation to the cross sectional strata of the study area. As you can see, the fore shock and mainshock occurred in the Mesozoic era limestone that was laid down when this area was part of the sea. Most of the aftershocks are along the interface of that layer and a lower ancient Tethyan crust. Only one quake in the USGS set shows as being in that part of the crust.
The dangling slab is not shown in this plot, and I did yank the mountains off the top. (They were represented in a different scale).
Thank You for your time.
1) “The subduction structure of the Northern Apennines: results from the RETREAT seismic deployment” Margheriti et al, ANNALS OF GEOPHYSICS, VOL. 49, N. 4/5, August/October 2006
2) “HYDROGEOLOGICAL FEATURES OF THE PO VALLEY (NORTHERN ITALY)” Bortolami et al
3) “A new active tectonic model for the construction of the Northern Apennines mountain front near Bologna (Italy)”, Picotti et al JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B08412, doi:10.1029/ 2007JB005307, 2008
4) “FROM THE TETHYS OCEAN TO THE MEDITERRANEAN SEAS: A PLATE TECTONIC MODEL OF THE EVOLUTION OF THE WESTERN ALPINE SYSTEM” Biju-Duval et al lNTERATlONAL SYMPOSIUM ON THE STIUCTUIAL HISTORY OF THE MEDITERIANEAN BASINS. SPLIT (YUGOSLAVlA) 15.29 OCTOBER 1976.
5) “Tectonic evolution of the Alpine orogen” Jacques Charvet