L. Margheriti

The 1997 Umbria‐Marche, Italy, Earthquake Sequence: A first look at the main shocks and aftershocks

A long sequence of earthquakes, six with magnitudes between 5 and 6, struck Central Italy starting on September 26, 1997, causing severe damages and loss of human lives. The seismogenic structure consists of a NW-SE elongated fault zone extending for about 40 km. The focal mechanisms of the largest shocks reveal normal faulting with NE-SW extension perpendicular to the trend of the Apennines, consistently with the Quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions. Preliminary data on the main shocks and aftershocks show that extension in this region of the Apennines is accomplished by normal faults dipping at low angle (∼40°) to the southwest, and confined in the upper ∼8 km of the crust. These normal faults might have reactivated thrust planes of the Pliocene compressional tectonics. The aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km-long fault segments, adjacent and slightly offset from one another.

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.

Complex Normal Faulting in the Apennines Thrust-and-Fold Belt: The 1997 Seismic sequence in Central Italy

A long sequence of moderate-magnitude earthquakes (5 M 6) struck central Italy in September and October 1997. At the end of the sequence a year later, the seismogenic area extends for about 60 km along the Apennines. The analysis of historical seismicity suggests that this seismic sequence filled a 700-year gap in this portion of the chain. Other historical sequences in the same area are characterized by prolonged seismic release on adjacent fault segments, probably due to the in- volvement of shallow and complex structures inherited by the compressive tectonics. The distribution of seismicity and the fault-plane solutions show that the extension in this region is accomplished by normal faults dipping at relatively low angles (40) to the southwest. The focal mechanisms of the largest shocks reveal normal faulting with extension perpendicular to the Apenninic chain (northeast-southwest), consistently with the Quaternary tectonics of the internal sector of the northern Apen- nine belt and with previous earthquakes in adjacent regions. Three mainshocks oc- curred on distinct 5- to 10-km-long fault segments, adjacent and slightly offset be- tween each other. High-quality aftershock locations show that seismicity is confined within the sedimentary Mesozoic cover in the upper 8 km of the crust and that most of the aftershocks are shallower than the largest shocks, which nucleated at 6-km depth. Faults evidenced by aftershock locations have a planar geometry and show increased complexity toward the surface. Most of the aftershock focal mechanisms are dominated by normal faulting. Several strike-slip events occurred at shallow depths, reactivating portions of pre-existing thrust planes that segment the normal fault system. The spatiotemporal evolution of seismicity shows a peculiar migration of hypocenters along the strike of the main faults with multiple ruptures and the activation of fault segments before the occurrence of the main rupture episodes.

Spatio-temporal distribution of seismic activity during the Umbria-Marche crisis, 1997

We present the spatio-temporal distribution of more than 2000 earthquakesthat occurred during the Umbria-Marche seismic crisis, between September 26and November 3, 1997. This distribution was obtained from recordings of atemporary network that was installed after the occurrence of the first two largest shocks (Mw =, 5.7, Mw = 6.0) of September 26. This network wascomposed of 27 digital 3-components stations densely distributed in theepicentral area. The aftershock distribution covers a region of about 40 km long and about2 km wide along the NW-SE central Apennines chain. The activity is shallow,mostly located at less than 9 km depth. We distinguished three main zonesof different seismic activity from NW to SE. The central zone, that containsthe hypocenter of four earthquakes of magnitude larger than 5, was the moreactive and the more complex one. Sections at depth identify 40–50°dipping structures that agree well with the moment tensor focalmechanisms results. The clustering and the migration of seismicity from NW to SE and the generalfeatures are imaged by aftershock distribution both horizontally and at depth.

Seismic anisotropy beneath the Northern Apennines (Italy) and its tectonic implications

We examined shear wave splitting in SKS and S phases from 22 teleseisms at 10 temporary stations on a transect across the Northern Apenninic arc. The array, near 43°N, spans from Corsica Island across the Tyrrhenian region and the Apenninic belt to the Adriatic coast. We applied particle motion, covariance matrix decomposition, and cross correlation methods to estimate the polarization direction of the fast split - shear wave ( ?) and the delay time between split phases (δt). Most of the analyzed shear waves show clear evidence of splitting. The ? in the Adriatic foreland and in the Apennines are approximately parallel to the strike of the mountain belt (NW-SE). The largest δt correspond to the highest elevations, suggesting that anisotropy is related to the compressional tectonics which built the Apennines, and that this tectonic compression involved at least the entire lithosphere. In the Tyrrhenian area we observe ? oriented about E-W, suggesting a reorientation of the mantle fabric due to astenospheric flow, responsible for the E-W post-orogenic extension observed at the surface.

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.

Eurasia‐Africa Plate Boundary region yields new seismographic data

The tectonic plate boundary between Eurasia and Africa is complex, in that it cannot be characterized as a single discrete plate boundary Deformation near this plate boundary varies from trans-tensional in the Azores archipelago, through strike-slip in the eastern Atlantic basin, to overall compressional between the European and African continents, with extensional sub-domains in the Mediterranean Sea. This complex pattern of deformation, related plate motion, and underlying driving forces leads to strong variations in seismic hazard throughout the region. A better understanding of the plate boundary processes requires knowing crust and upper mantle structure in the region, which is best investigated with three-component, broadband seismic data. To investigate the regions three-dimensional crust and upper mantle structure, we are carrying out a multiinstitutional project (MIDSEA) involving seismologists from 10 countries on the northern, southern, and western sides of the plate boundary.

Anisotropic seismic structure of the lithosphere beneath the Adriatic coast of Italy constrained with mode-converted body waves

Received 7 May 2002; revised 26 August 2002; accepted 3 September 2002; published 26 November 2002. [1] PS converted waves observed near Ancona on the Adriatic coast of central Italy, as revealed by teleseismic receiver functions (RFs), vary with earthquake backazimuth and epicentral distance in a manner consistent with a 1-D anisotropic seismic structure. Using reflectivity calculations, we develop a profile of anisotropic seismic velocity through the Adriatic lithosphere at this locality. We infer crustal thickness of � 45 km. Anisotropy within the crust appears at � 15-km depth, suggesting a decollement between the subducting Adriatic lithosphere and the overriding crustal wedge. Lineation of inferred rock fabric is compatible with simple shear in ENE-WSW direction. In the upper mantle, we infer an anisotropic layer at 80–90 km depth. If caused by olivine crystals alignment, the nearly north-south lineation of the inferred rock fabric would be consistent with some nearby shear-wave splitting observations. This anisotropic layer may be related to mantle deformation induced by the rollback of Adriatic lithosphere. INDEX TERMS: 7203 Seismology: Body wave propagation; 7218 Seismology: Lithosphere and upper mantle; 8102 Tectonophysics: Continental contractional orogenic belts. Citation: Levin, V., L. Margheriti, J. Park, and A. Amato, Anisotropic seismic structure of the lithosphere beneath the Adriatic coast of Italy constrained with mode-converted body waves, Geophys. Res. Let t., 29(22), 2058, doi:10.1029/ 2002GL015438, 2002.

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.