Federica Riguzzi

Static stress drop as determined from geodetic strain rates and statistical seismicity

Accepted for publication in Journal of Geophysical Researches. Copyright (2010) American Geophysical Union

The tectonic puzzle of the Messina area (Southern Italy): Insights from new seismic reflection data

The Messina Strait, that separates peninsular Italy from Sicily, is one of the most seismically active areas of the Mediterranean. The structure and seismotectonic setting of the region are poorly understood, although the area is highly populated and important infrastructures are planned there. New seismic reflection data have identified a number of faults, as well as a crustal scale NE-trending anticline few km north of the strait. These features are interpreted as due to active right-lateral transpression along the north-eastern Sicilian offshore, coexisting with extensional and right-lateral transtensional tectonics in the southern Messina Strait. This complex tectonic network appears to be controlled by independent and overlapping tectonic settings, due to the presence of a diffuse transfer zone between the SE-ward retreating Calabria subduction zone relative to slab advance in the western Sicilian side.

Evidence of active crustal deformation of the Colli Albani volcanic area (central Italy) by GPS surveys

Abstract Colli Albani is a volcanic complex located in central Italy, very close to the city of Rome. Its last eruption is dated at 0.03-0.02 Ma. Now it displays a recurrent seismicity, sporadic gas emissions from soils, wells and springs, and post volcanic hydrothermal circulation. Moreover an uplift of about 30 cm over the last 43 years was recently detected by comparing the height differences between some vertices of repeated leveling surveys, thus suggesting a recent active magmatic process beneath the volcano. In 1995 a first-epoch GPS campaign was carried out with last generation receivers, the repetition campaign was performed in 1996. Data coming from the two repeated surveys were carefully processed and analyzed. A statistical analysis rigorously applied between the 1995 and 1996 adjusted coordinates shows significant coordinate differences. Some sites of the network, in particular, the sites of Vivaro (VVR), Cava di Ciampino (CVA) and Istituto Nazionale di Geofisica (INGR) display a significant subsidence of 2.6 ± 0.7, 2.7 ± 0.8 and 2.0 ± 0.8 cm, respectively, with a confidence interval at 95 percent level. This subsidence seems to be induced by a water table level lowering in Vivaro, while in the area where the sites of Cava di Ciampino and INGR are located, this motion is also confirmed by historical levelling data that show a subsidence of ~ 2 cm/yr during the last 45 years and could be due to local tectonics. Moreover, significant planimetric deformations are exhibited by the site of Castel Romano (CSR). It is remarkable that the stations of CVA, INGR and CSR are located in the area that was struck by the June 12, 1995, M d = 3.8 earthquake, although the displacement vectors are quite small. The motion of the Capranica Prenestina (CPR) station, which shows an uplift of 2.7 ± 0.7 cm, must be considered separately, being located outside the volcanic structure and placed over the carbonatic outcrops of the Prenestini Mts. Furthermore, the accuracy obtained in height determinations does not allow us to confirm if the central part of the GPS network corresponding to the area investigated by Amato and Chiarabba is uplifting within the analyzed time span.

The coseismic and postseismic deformation of the L’Aquila, 2009 earthquake from repeated GPS measurements

We analyze more than 100 GPS time series of continuous and discontinuous GPS stations located in the Abruzzi region (Italy) surrounding the epicentres of the L’Aquila 2009 seismic sequence. The purpose of this work is to reconstruct the coseismic displacement field caused by the 6 th April (Mw 6.3) main shock from a dense network of survey-mode stations surrounding the epicentral area and to characterize the early postseismic deformation field. In the months following the main shock, an extensive GPS survey was carried out on the existing Central Apennines Geodetic Network (CAGeoNet), with the intention of collecting a robust data set and to study the co- and postseismic deformation field of this Apenninic normal faulting earthquake. The analysis is carried out with two independent procedures and software (Bernese and Gamit) in order to provide reliable and validated geodetic solutions. The analysis of the postseismic transients and the knowledge of long-term inter-seismic velocities at all GPS stations, issued from permanent and CAGeoNet sites, allow us to derive a dense co- and postseismic displacement field for the L’Aquila Mw 6.3 main shock in a wide area around the epicentre. The highest deformation rate occurs during the first 4–5 months after the main shock and persists in the following at slightly slower rate throughout the whole monitoring period. Fast deformation rates imply that most of the observed deformation is due to a process different from a pure viscoelastic relaxation of the stress perturbation. Since the observed rates would imply a too low effective viscosity value (below 10 17 Pa s), we rather suggest that most of the observed deformation in the first months after the earthquake is due to different processes, most likely frictional afterslip possibly modulated by the presence of fluids. The new coseismic displacement field is used to invert for the main shock fault geometry, analysing the consistency among the different geodetic solutions and the combined one, with the goal of validating the two data sets.

Macroseismic study of the Potenza (southern Italy), earthquake of 5 May 1990

The aim of this paper is to define the macroseismic field of the 5 May 1990 Potenza earthquake, analysing about 3000 questionnaire forms together with the results of the direct inquiries carried out in the field. The study evidences the seismic vulnerability of the Potenza district and of the whole of southern Italy in general, due to the low resistance characteristics of the old constructions and a lack of seismic prevention. Besides, geomorphological failure increases hazardous situations at the earthquakes occurrence. This earthquake provided a good test for checking and improving the ING macroseismic data collection procedures.

Normal fault earthquakes or graviquakes.

Earthquakes are dissipation of energy throughout elastic waves. Canonically is the elastic energy accumulated during the interseismic period. However, in crustal extensional settings, gravity is the main energy source for hangingwall fault collapsing. Gravitational potential is about 100 times larger than the observed magnitude, far more than enough to explain the earthquake. Therefore, normal faults have a different mechanism of energy accumulation and dissipation (graviquakes) with respect to other tectonic settings (strike-slip and contractional), where elastic energy allows motion even against gravity. The bigger the involved volume, the larger is their magnitude. The steeper the normal fault, the larger is the vertical displacement and the larger is the seismic energy released. Normal faults activate preferentially at about 60° but they can be shallower in low friction rocks. In low static friction rocks, the fault may partly creep dissipating gravitational energy without releasing great amount of seismic energy. The maximum volume involved by graviquakes is smaller than the other tectonic settings, being the activated fault at most about three times the hypocentre depth, explaining their higher b-value and the lower magnitude of the largest recorded events. Having different phenomenology, graviquakes show peculiar precursors.

Geodetic Surveys across the Messina straits (Southern Italy) Seismogenetic Area

Abstract The Messina Straits, southern Italy, unfortunately became famous after the occurrence of the great earthquake of December 28, 1908, Ms = 7.5, that caused thousands of deaths and severe destruction over a wide area along the Sicilian and Calabrian coasts. After that time many geophysical and geological studies were performed to evaluate the seismic characteristics of the 1908 earthquake, the seismic risk and the geological evolution of this region in the framework of Mediterranean geodynamics. In 1970, a geodetic network was set up across the Straits and was repeatedly measured with terrestrial techniques until 1980, showing a northward displacement of the Sicilian sites with respect to the Calabrian ones, between 1970 and 1971. In 1987, the old terrestrial network was surveyed again for the first time by the GPS technique, improving the accuracy of the coordinate determinations. Finally, in 1994, a wider network was established and surveyed again to collect additional GPS observations from a larger area across the Straits. In this paper, an analysis of the results obtained from the two GPS surveys with respect to those achieved by the terrestrial surveys (from 1970 to 1980) is given. This analysis shows there has not been significant crustal horizontal deformation across the Straits in the last 15 years. Although this crustal tectonic ‘quiescence’ corresponds to a low seismic activity level in the Straits area, terrestrial and GPS geodetic results would agree with Straits geophysical models excluding any aseismic deformations acting perpendicular to the Straits axis.

Current motion and short-term deformations in the Suez–Sinai area from GPS observations

We analyze data from four GPS campaigns carried out between 1997 and 2002 on a network of 11 sites in the Suez-Sinai, the area of collision between the African and the Arabian plates. This is the key area to understand how and in which way Sinai behaves like a sub-plate of the African plate and the role played between seismic and geodetic (long term) deformation release. Our analysis shows that, on average, the Suez-Sinai area motion (in terms of ITRF00 velocities) matches African plate motion (NNR-NUVEL-1A model). However, the baseline length variations show transient deformations in Sinai and across the Gulf of Suez, reaching up a maximum value of about 1.5 cm in five years. Since current geodynamical models do not predict significant tectonic deformation in this area, we worked under the hypothesis that a contribute may be due to post-seismic relaxation. Under this hypothesis, we compared the baselines length variations with the post-seismic relaxation field associated with five major local earthquakes occurred in the area, testing two different viscoelastic models. Our results show that the transient deformations are better modelled for viscosity values of 1018 Pa s in the lower crust and 1020 Pa s in the asthenosphere. However, since the modelled post-seismic effect results modest and a certain amount of the detected deformation is not accounted for, we think that an improved modelling should take into account the lateral heterogeneities of crust and upper mantle structures.

Re-evaluation of minor events : The examples of the 1895 and 1909 Rome earthquakes

The city of Rome is subjected to moderate seismic risk due to both local and external seismicity. Up to now, the maximum intensity felt has never exceeded VIII MCS. The 1 November 1895 (Io = VII) and 31 August 1909 (Io = VI) earthquakes demonstrate that small local events can also cause damage in a large old city. In the present work, we have re-evaluated the intensity values of those two events by means of automatic processing. A comparison between the present results with geological evidence and previous studies is shown, especially for the historical centre of Rome. For the first time, the 1909 earthquake instrumental magnitudeML = 3.6 has been calculated from original recordings.

Software available for analyzing GPS deformation

A FORTRAN 77 software package that analyzes Global Positioning System (GPS) deformation control networks is available free of charge. This package is able to simulate and adjust GPS networks. It can also perform onedimensional, two dimensional, and three-dimensional statistical analysis to study the significance of coordinate differences coming from two repeated surveys of the same network. In addition, the package computes displacement vectors with error and reliability ellipsoids, and it is suited for both engineering and geophysical applications. It has been used successfully in Italy to analyze small and large networks established for high precision engineering surveys and geodynamic deformation control [Acker et al., 1998; Anzidei et al., 1996].