Figure 1.

The area known as the Calabrian Arc is the last remaining segment where oceanic subduction occurs along the African-Eurasian plate boundary, which extends down the Italian Peninsula through Calabria, and across Sicily toward Tunisia (Figure 1).

The old oceanic crust of the Ionian Sea has been subducting, or plunging, beneath Calabria to depths of over 400 km. The volcanoes of the Eolian Islands mark where this downgoing plate begins to melt. To the north, along the Apennines, and to the west, in Sicily, all of the ocean crust has been subducted, and mountain chains now mark the collision of continental landmasses.

The subduction zone that became the Apennine-Calabrian-Sicilian belt has advanced southeastward across the Western Mediterranean region over the last 25 million years. (click for map showing the history of this advancement). In its wake, extension and sea floor spreading has created the Tyrrhenian Sea.

The rapid southeast advance of the Calabrian Arc, with subduction ahead and extension behind, is believed to be driven by rollback -- the retreat of the subduction zone due to the sinking of the old Mesozoic seafloor of the Ionian Sea.

Today, as most of the arc collides with the Adriatic/Apulian continent along the Apennines, and with a piece of Africa pasted onto Sicily, oceanic subduction continues only at Calabria. Or does it? Some consider the Calabrian subduction to have now been stopped by the collision of the continental landmasses. Others argue that subduction and the advance of the remaining arc continues, with the plate tearing along its margins and Mount Etna lying along one of the proposed tears. Yet another possibility is that Calabria is splitting lengthwise and only part of the peninsula continues to advance to the southeast. Understanding the present-day tectonics of Calabria is important to understanding the earthquake risk in southern Italy.

Figure 3. Researchers are working to deploy 50 portable digital broadband seismographs throughout southern Italy (locations shown above in red). These instruments will record both global and regional earthquakes for 18 months. They are also working to deploy an additional 10 digital broad-band ocean-bottom seismometers (OBS) offshore for a period of 12 months (shown above in blue).

CAT/SCAN Objectives are to use signals from distant earthquakes to develop a catscan, or a three dimensional image, of the Earth's crust and mantle beneath the Italian Peninsula of the earth. Specifically, we will:

-- Map the seismic structure of the Calabrian arc, from Calabria to the southern Apennines.
-- Determine the structure of the entire subduction/collision system, from the subducting plate across the trench to the volcanic arc, the subducting slab and the backarc spreading system.
-- Along strike, map the structure of the transition from oceanic subduction in Calabria to continental collision in the southern Apennines.

Researchers are working to deploy 50 portable digital broadband seismographs throughout southern Italy (see Figure 3, red triangles). These instruments will record both global and regional earthquakes for 18 months. Researchers are also working to deploy an additional 10 digital broad-band ocean-bottom seismometers (OBS) offshore for a period of 12 months (Figure 3, blue triangles).

The instruments are on loan from the Incorporated Research Institutions for Seismology (IRIS) Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL), and the Ocean Bottom Seismograph Instrument Pool (OBSIP), both supported by NSF. Columbia University is a member of both IRIS and OBSIP.

Some outcomes of this project will be:

-- The tomographic images provided by analysis of the earthquake waves recorded on the seismometers, similar to X-ray catscans, will indicate temperature and compositional variations beneath the surface related to the subduction, the continental collision, and extension in the Tyrrhenian Sea.
-- Measurements of anisotropy (shear-wave splitting) will provide information on the flow of mantle rocks at depth. Other techniques, such as receiver function analysis, will be used to constrain the thickness of the crust and other layers. These and other advanced studies of the earthquake waves passing through the earth to the seismometers will provide much needed information on the structure and properties of this region.

EQUIPMENT: The seismometers (above) were shipped to Italy from the PASSCAL (Program for the Array Seismic Studies of the Continental Lithosphere) instrument center in Socorro, New Mexico in November. The approximately 80 boxes of equipment weighed over 5000 lbs (2300kg). Deployment of the instruments started in December, and will continue through the holidays into January, when multiple teams will help complete the deployment. The deployment will take a total of about 6 weeks. Final plans for the OBS deployments are still being formulated. They will be deployed from a ship sometime in the Spring of 2004.

Field Program: The deployment (see Figure 3, above) comprises: (1) A linear array across the active Calabrian Arc. The proposed line extends southwest from Apulia across the Gulf of Taranto (using Lamont Ocean Bottom Seismometers (OBSs)), across Calabria, into the Tyrrhenian Sea with OBSs, ending with stations on the Eolian Islands. The line is chosen to take advantage of land sites at either end of the profile; (2) A more broadly spaced pair of transects of OBSs extending to the volcanic center of the juvenile Tyrrhenian Sea oceanic crust; (3) A dense 2-D array across the transition between the seismically active Calabrian Arc and the Southern Apennines to capture the differences between the two crustal blocks and image the zone of possible mantle flow around the Arc; and (4) a second, more broadly-spaced, transect of the Southern Apennines formed by the OBS and station sites farther east in the foreland of Apulia.