F. P. Lucente

Tomographic constraints on the geodynamic evolution of the Italian region

In this paper we present P wave tomographic images of the mantle beneath Italy obtained by inverting ∼6000 teleseismic P and PKP wave arrival times, accurately repicked, recorded in the time period 1988–1994 by the stations of the National Seismic Network of the Istituto Nazionale di Geofisica. We pay great attention in the data selection and picking procedure of seismic phases to obtain a very high quality data set. The data were inverted with the well-established Aki-Christofferson-Husebye tomographic technique; different reference models and residuals computation have been tried to verify the stability of the results. The high quality of the repicked arrival times allows us to enhance the definition of the deep structures beneath both the Alps and the Apennines, looking for their lateral and vertical continuity down to 800 km depth. The main finding of this study is a continuous high-velocity body located between 250 and 670 km depth beneath the entire Apenninic system dipping toward the Tyrrhenian area, which continues upward segmented in two main anomalies in the northern Apenninic and the Calabrian Arcs. We interpret this high-velocity feature as the subducted oceanic lithosphere between the Eurasian and African plates, dipping down to the upper-lower mantle boundary beneath the Tyrrhenian Sea. The retrieved images of the lithosphere subducting beneath Apennines are reliable in terms of thickness (about 80–90 km) and P wave velocity contrast (2–4% higher than the normal mantle). Furthermore, our tomographic images, which focus on the deep geometry and continuity of the velocity structures, provide new keys to understanding the geodynamic evolution of the Italian region. The segmentation of the high-velocity slab upward suggests a complex evolution of the arc-trench system and the initially continuous subduction of the Ionian-Adriatic plate progressively developed in subordinate arcs, probably due to lateral heterogeneity of the subducting lithosphere.

Temporal variation of seismic velocity and anisotropy before the 2009 MW 6.3 L'Aquila earthquake, Italy

We describe a set of seismological observations of the foreshock sequence preceding the 6 April 2009 M w 6.3 L9Aquila earthquake. The dense configuration of the seismic network in the epicenter area and the occurrence of a long foreshock sequence provide the opportunity for a detailed reconstruction of the preparatory phase of the main shock. Approaching the earthquake, we observed clear variations in the seismic wave propagation properties. The elastic properties of rocks in the fault region underwent a sharp change about a week before the earthquake. From our observations, we infer that a complex sequence of dilatancy-diffusion processes takes place and that fluids play a key role in the fault failure process.

Passive Seismology and Deep Structure in Central Italy

In the last decade temporary teleseismic transects have become a powerful tool for investigating the crustal and upper mantle structure. In order to gain a clearer picture of the lithosphere-asthenosphere structure in peninsular Italy, between 1994 and 1996, we have deployed three teleseismic transects in northern, central, and southern Apennines, in the framework of the project Geo ModAp (European Community contract EV5V-CT94–0464). Some hundreds of teleseisms were recorded at each deployment which lasted between 3 and 4 months. Although many analyses are still in progress, the availability of this high quality data allowed us to refine tomographic images of the lithosphere-asthenosphere structure with an improved resolution in the northern and central Apennines, and to study the deformation of the upper mantle looking at seismic anisotropy through shear-wave splitting analysis. Also, a study of the depth and geometry of the Moho through the receiver function technique is in progress. Tomographic results from the northernmost 1994 and the central 1995 teleseismic experiments confirm that a high-velocity anomaly (HVA) does exist in the upper 200–250 km and is confined to the northern Apenninic arc. This HVA, already interpreted as a fragment of subducted lithosphere is better defined by the new temporary data, compared to previous works, based only on data from permanent stations. No clear high-velocity anomalies are detected in the upper 250 km below the central Apennines, suggesting either a slab window due to a detachment below southern peninsular Italy, or a thinner, perhaps continental slab of Adriatic lithosphere not detectable by standard tomography. We found clear evidence of seismic anisotropy in the uppermost mantle, related to the main tectonic processes which affected the studied regions, either NE-SW compressional deformation of the lithosphere beneath the mountain belt, or arc-parallel asthenospheric flow (both giving NW-SE fast polarization direction), and successive extensional deformation (~E-W trending) in the back-arc basin of northern Tyrrhenian and Tuscany. Preliminary results of receiver function studies in the northern Apennines show that the Moho depth is well defined in the Tyrrhenian and Adriatic regions while its geometry underneath the mountain belt is not yet well constrained, due to the observed high complexity.

The April 1996 Irpinia seismic sequence: Evidence for fault interaction

The analysis of the Irpinia earthquake of 3 April 1996 (ML = 4.9), based on strong motion and short period local data, shows that it was a normal faulting event located within the epicentral area of the MS 6.9, 1980, earthquake. It was located at 40.67° N and 15.42° E at a depth of 8 km. The local magnitude (4.9) has been computed from the VBB stations of the MedNet network. The moment magnitude is Mw = 5.1 and the seismic moment estimated from the ground acceleration spectra is 5.0 1023 dyne cm. Spectral analysis of the strong motion recordings yields a Brune stress drop of 111 bars and a corner frequency of 1 Hz. The source radius associated to these values of seismic moment and stress drop is 1.3 km. The focal mechanism has two nodal planes having strike 297°, dip 74°, rake 290° and strike 64°, dip 25° and rake 220°, respectively. A fault plane solution with strike 295° ± 5°, dip 70° ± 5°, and rake 280° ± 10° is consistent with the S-wave polarization computed from the strong motion data recorded at Rionero in Vulture. We discuss the geometry and the dimensions of the fault which ruptured during the 1996 mainshock, its location and the aftershock distribution with respect to the rupture history of the 1980 Irpinia earthquake. The distribution of seismicity and the fault geometry of the 1996 earthquake suggest that the region between the two faults that ruptured during the first subevents of the 1980 event cannot be considered as a strong barrier (high strength zone), as it might be thought looking at the source model and at the sequence of historical earthquakes revealed by paleoseismological investigations.

Imaging the subducted slab under the Calabrian Arc, Italy, from receiver function analysis

We use data from the CAT/SCAN seismic deployment in southern Italy to reconstruct the crust and uppermost mantle structure above one of the narrowest active subduction zones worldwide, the Calabrian Arc, where the last fragment of the former Tethys ocean is being subducted. An E-W time-domain profile composed of teleseismic receiver functions shows the geometry of the main seismic discontinuities beneath Calabria. It provides a clear two-dimensional (2-D) image of the subducting Ionian plate at shallow depth where it bends and starts to descend into the mantle. In the profile the Moho of the subducting Ionian plate lies at ~35 km and gently dips westward beneath the eastern part of Calabria. Then the depth increases steeply to ~80 km below western Calabria. The locus where the Ionian plate changes its dip and starts to subduct corresponds to the transition from an uplifted plateau to an extensional basin at the surface. It suggests that the Ionian plate has not rolled back relative to Calabria since it slowed. This implies the uplift of the Sila Plateau is more likely due to under-plating than eastward growth of the mantle wedge. When projected on a map, the depths of the Ionian Moho beneath each seismic station reveal a more complex geometry for the subducting plate, including an unexpected deepening northward. This may be related to a tear in the plate or other tectonic motions between Calabria and the Southern Apennines. Thus the relationship between the imaged geometry of the subducting plate and structural elements at the surface provides new knowledge about the geodynamic evolution of the subduction system and the tectonics of the Calabrian Arc.

Subduction rollback, slab breakoff, and induced strain in the uppermost mantle beneath Italy

Differences in seismic anisotropy revealed by split SKS waves that traverse the upper mantle beneath the Italian region reveal four areas of internally coherent anisotropic strength, providing evidence for mantle strain partitioning. When compared to uppermost mantle structure imaged by tomography, the sequence of these areas displays a straightforward parallelism. Under proper assumptions, the correspondence between areas of coherent splitting degree and areas of coherent velocity perturbations offers new insights into the late evolution of subduction in Italy and their effect on mantle strain.

Investigating the Origin of Seismic Swarms

According to the U.S. Geological Surveys Earthquake Hazards Program, a seismic swarm is “a localized surge of earthquakes, with no one shock being conspicuously larger than all other shocks of the swarm. They might occur in a variety of geologic environments and are not known to be indicative of any change in the long-term seismic risk of the region in which they occur” (http://vulcan.wr.usgs.gov/Glossary/Seismicity/description_earthquakes.html).

Stressing of fault patch during seismic swarms in central Apennines, Italy

Persistent seismic swarms originate along the normal faulting system of central Apennines (Italy). In this study, we analyze the space-time-energy distribution of one of the longer and more intense of these swarms, active since August 2013 in the high seismic risk area of the Gubbio basin. Our aim is to verify if information relevant to constraint short-term earthquake occurrence scenarios is hidden in seismic swarms. During the swarm, the seismic moment release first accelerated, with a rapid migration of seismicity along the fault system, and suddenly dropped. We observe a decrease of the b-value, along the portion of the fault system where large magnitude events concentrated, possibly indicating that a fault patch was dynamically stressed. This finding suggests that the onset of seismic swarms might help the formation of critically stressed patches.

Crustal structure and deformation across a mature slab tear zone: the case of southern Tyrrhenian subduction (Italy)

We compute S-velocity profiles of the crust across the Messina strait (Italy), the tear zone at the southern end of the Ionian subduction zone. Separating Sicily from Calabria, the Messina Strait hosted some of the strongest earthquakes to ever occur in Italy. Here, the motion of the Ionian slab with respect to Sicily creates a complex tectonic setting characterized by lithospheric tearing. We show velocity models of the crust, computed from teleseismic receiver function inversion, outlining the differences between Sicily and Calabria. Strong deformation across the Messina Strait between 10-15 and 30 km depth is expressed by strong anisotropy (up to 10%), developed in a ductile shear zone of the crust. The top of these ductile, weaker layers could limit the depth-extent of future ruptures.