Alessandro Amato

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.

Subcrustal earthquakes in the northern Apennines (Italy): Evidence for a still active subduction?

Previously unreported subcrustal earthquakes have been located up to 90 km depth beneath the Northern Apennines (Italy). These earthquakes occur beneath a zone of abundant upper crustal seismicity, mostly confined to the upper 20 km of the crust. Although there are relatively few well located subcrustal earthquakes, there appears to be a general southwestward deepening of the hypocenters, from the Adriatic to the Tyrrhenian Sea, consistent with other geophysical data which suggest that the Adriatic lithosphere is presently subducting beneath the Northern Apennines.

Active stress map of Italy

We present a new map of the present-day stress field in Italy obtained from all the available data. The map reports 200 horizontal stress directions inferred from 109 borehole breakout data, 44 centroid moment tensor solutions, 34 other focal mechanisms, most of which are from polarity distributions, seven stress inversions of microearthquake data, two averages of T and P axes of earthquake focal mechanisms in zones of diffuse seismic activity, and four fault slip data. The integration of breakout data, which yield horizontal stress directions, with fault plane solutions, which reflect the stress regime, allows us to obtain an improved map of the present-day stress in Italy. This stress field map can be used for a better comprehension of active tectonic processes, for seismic hazard assessment, and to foresee the behavior of faults recognized with other methods. Stress directions obtained from different data, although relative to different depth intervals (e.g., 0–7 km for breakouts and 0–20 km for most of the earthquakes) and to different tectonic units, are consistent. Since many regions in Italy are characterized by an extensional stress regime, we report the minimum horizontal stress (Shmin) orientations. The map shows that an extensional regime affects most of the Apenninic belt. Conversely, a compressional (or transpressional) regime characterizes the eastern Alps, the eastern side of the northern Apennines, and the southern Tyrrhenian to northern Sicily zone. An abrupt change in stress directions marks the transition between northern and southern Apennines, suggesting that the two arcs are characterized by a different tectonic setting and recent evolution. In this paper we report all the data analyzed to date, with their geographic coordinates and average stress directions, and we describe the main stress provinces in Italy in the framework of the tectonic evolution of the region.

Recent seismicity and tomographic modeling of the Mount Etna plumbing system

The monitoring of seismic activity in eastern Sicily (southern Italy) has been recently improved, in the framework of the Poseidon Project, to investigate both tectonics and volcanic processes of Mount Etna. This effort has produced a homogeneous and complete data set which we use to image the deep structure of the volcano and to define the space and time distribution of the recent seismicity, encompassing the 1995 eruption, diffuse eruptions between 1995 and 1997, a further magma intrusion started in 1997, and an increase of volcanic activity in July 1998. We inverted P wave arrival times from 307 selected local earthquakes to obtain a three-dimensional velocity model of the volcano, with a simultaneous inversion for hypocenters and velocity parameters. The new tomographic images permit us to define the structure beneath the volcano from the surface down to 18 km depth. The main structural feature revealed by our inversion is a high-velocity body located beneath the central craters whose lateral extent increases from ∼6 km between 18 and 9 km depth to ∼12 km between 9 and 3 km depth. Near the surface the fast anomaly branches in two separate high-Vp regions, which are located below the summit craters and the eastern flank (Valle del Bove), respectively. The high-velocity features are interpreted as high-density cumulates of solidified magma that intruded the shallow crust. We hypothesize that magma ascends the crust within the relatively small high-velocity conduit (below 9 km depth) and is stored at depth shallower than 9 km within the broad high-velocity region, as also suggested by petrological data. A sharp increase of seismicity in 1997, with earthquakes occurring at the border of the high-velocity body, suggests that Mount Etna sustained recent episodes of intrusions, which possibly herald future eruptions. No large low-velocity anomaly interpretable as a melted magma chamber is imaged in the upper 18 km of the crust, while a broad low-velocity anomaly in the uppermost mantle, revealed by regional seismic data, possibly indicates the magma source region at depth. Earthquake occurrence from the deep source to the shallow intrusive area helps to trace the magma migration and the feeding of the volcanic system.

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.

Evidence of active extension in Quaternary volcanoes of central Italy from breakout analysis and seismicity

We present active stress directions obtained from borehole breakout analysis performed on 15 geothermal wells located in the western coastal regions of Central Italy. The study area (a 200 km by 50 km NW-elongated area bordering the Apennines) includes several Quaternary high-K alkaline volcanoes active mainly after 0.6 Ma. We analyzed both paper logs and digital data to detect breakout directions, the two techniques yielding similar results. The breakout results show a predominant ENE direction of SHmin, with local deviations in one region (Sabatini volcano) where no seismicity is observed. The comparison of breakout data with stress directions inferred from inversion of microearthquake (M<4) focal mechanisms computed in three of the four volcanoes suggests that the whole area is presently undergoing NE to ENE extension.

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.