Patrizia Tosi

Recurrent patterns in the spatial behaviour of Italian seismicity revealed by the fractal approach

Three are the essential parameters needed to describe seismicity: the b-value of the Gutenberg-Richter relation and the spatial and temporal fractal dimensions (Ds and Dt). Several cases have been reported when a significant decrease in the b-value or in the spatial fractal dimension preceded major earthquake sequences or aftershocks. Here we use the fractal method based on the correlation integral to study the temporal changes in the spatial (2-D) distribution of earthquakes in three important seismic zones of Italy. In all three zones Ds shows significant variability, which correlates well with the major events and clearly marks the beginning and the end of an earthquake cycle. Finally, the nucleation of most major events is associated with a fractal dimension value which corresponds to the topological dimension of a plane.

Self-Affine Asperity Model for Earthquakes

A model for fault dynamics consisting of two rough and rigid brownian profiles that slide one over the other is introduced. An earthquake occurs when there is an intersection between the two profiles. The energy release is proportional to the overlap interval. Our model exhibits some specific features which follow from the fractal geometry of the fault: (1) non-universality of the exponent of the Gutenberg-Richter law for the magnitude distribution; (2) presence of local stress accumulation before a large seismic event; (3) non-trivial space-time clustering of the epicenters. These properties are in good agreement with various observations and lead to specific predictions that can be experimentally tested.

Time clustering properties of seismicity in the Etna Region between 1874 and 1913

Time properties of seismic activity of the Etna Volcano (Italy) from 1874 to 1913, are investigated by fractal dimension analysis. Period selection was due to the interesting volcanic and seismic activity which had occurred within this time interval. A catalogue was adopted, revised by the reexamination of historical sources and with the addition of new material. Time evolution of the temporal fractal dimension Dt, calculated on a 40 seismic events moving window, revealed the following correlations with the eruptive processes: long term fractal dimension variations are due to the magma rise from the depth; mid term variations are closely related to stress conditions at the volcano edifice, and short term variations are induced by the onset of eruption. The spatial localisation of data allowed interpretation of seismic activity in relation to the main structural systems outcropping on the volcano edifice and in the surrounding areas.

Spatial patterns of earthquake sounds and seismic source geometry

Earthquake sounds are heard during or immediately before an event up to several kilometres around the epicentral area. Sound data as heard by people have been collected by way of macroseismic questionnaires, showing that sound audibility does not follow isotropic patterns. The explanation of this effect is found in the behaviour of the particular radiation pattern of the P-waves for each earthquake, in connection with its focal mechanism. This result opens the possibility to get more information about the earthquake source parameters in the future and past events, making use of non-instrumental observations.

Application of Kriging Technique to Seismic Intensity Data

Spatial analysis, involving experimental semivariogram evaluation and kriging interpolation, is performed on macroseismic intensity data assumed to represent a regionalized variable. A semivariogram is modeled, showing that data components act at different scale levels. Interpretation of the semivariogram in terms of fractal dimension allows separation of the error component from other scale-dependent components. Use of an objective best spatial-range determination for filtering eliminates the subjective choice that is usually based on data-sampling density, permitting the reconstruction of the smoothed interpolated intensity field. Results are given together with error estimation due to local variability and sampling-density distribution. The method is first applied to synthetic macroseismic data with controlled variable error content and sampling density: the ability to rebuild the original, error-free intensity field is demonstrated. Then macroseismic data from an Italian medium-intensity earthquake are analyzed and spatial intensity attenuation re-evaluated.

Macroseismic effects highlight site response in Rome and its geological signature

A detailed analysis of the earthquake effects on the urban area of Rome has been conducted for the L’Aquila sequence, which occurred in April 2009, by using an online macroseismic questionnaire. Intensity residuals calculated using the mainshock and four aftershocks are analyzed in light of a very accurate and original geological reconstruction of the subsoil of Rome based on a large amount of wells. The aim of this work is to highlight ground motion amplification areas and to find a correlation with the geological settings at a subregional scale, putting in evidence the extreme complexity of the phenomenon and the difficulty of making a simplified model. Correlations between amplification areas and both near-surface and deep geology were found. Moreover, the detailed scale of investigation has permitted us to find a correlation between seismic amplification in recent alluvial settings and subsiding zones, and between heard seismic sound and Tiber alluvial sediments.

Web-based macroseismic survey: fast information exchange and elaboration of seismic intensity effects in Italy

A new method of macroseismic survey, based on voluntary collaboration through the internet, has been running at Istituto Nazionale di Geofisica e Vulcanologia (INGV), Italy, since June 2007. The macroseismic questionnaire is addressed to a single nonspecialist person and reported effects are statistically analysed to extrapolate the Mercalli-Cancani-Sieberg (MCS) scale and the European Macroseismic Scale (EMS) intensity referred to by that observer. Maps of macroseismic intensity are displayed online in almost real time and are continuously updated. The aim of the questionnaire is to evaluate seismic effects as felt by the compiler. The final result is a definition of a particular degree of intensity with an evaluation of an associated uncertainty. Results of medium to low magnitude earthquakes are presented here, showing the ability of the method to give fast and interesting results. Effects reported in questionnaires coming from towns are analysed in-depth and assigned intensities are compared with those derived from traditional macroseismic surveys, showing the reliability of the web-based method.

Preliminary results of a macroseismic survey of the Colfiorito sequence (Central Italy)

Abstract Umbria-Marche region (Central Italy) has been interested by a seismic sequence, which caused a large amount of damage within an extensive area (around 5000 km2). The sequence produced eight shocks with magnitude higher than 5.0, the largest of them occurring on September 26, UTC 09:40, M1 = 5.8. The incidence of many shocks with a magnitude higher than 4.0 contributed to the creation of a damage pattern that was in evolution for more than one month. Such seismic behavior motivated field operators to perform a real-time macroseismic survey to update the data set. One of the major objectives was to differentiate the effects pertaining to each of the largest shocks. In this paper we present the macroseismic survey performed during the sequence, along with some preliminary results inferred. Intensity points of seven of the main shocks are shown, together with the filtered macroseismic fields produced following the application of specific statistical methods. Finally, consideration is given to the comparison of these study results with corresponding tectonic and geological data.

Clustering properties of Etna seismicity during 1981-1991

Space and time clustering properties ofseismic activity, affecting Etna Volcano (Italy)during 1981–1991, are investigated by fractaldimension analysis. Very interesting volcanic andseismic activity occurred within this time interval.Temporal evolution of the time fractal dimension Dt calculated on a moving window, revealscorrelation with the eruptive processes at differenttime scales confirming results obtained for a differenttime span (De Rubeis et al., 1997). Spatial fractaldimension Ds shows to be negativelycorrelated with the time fractal dimension Dt, suggesting a peculiar dynamic patternassociated with volcanic processes.

Seismotectonic characterization of Italy, using statistical analysis of geophysical variables

Abstract After having calculated the average seismic energy per year, for the Italian territory, research was then carried out on the relationships of some geophysical and geological variables felt to be important for a seismotectonic characterization. Using multiple correlation on the whole territory a model of seismicity was developed, that significantly reproduced the actual seismicity observed. A regionalized study of Italy was then conducted, whose final aim was to pinpoint possible different effects of the geophysical variables on the seismicity from zone to zone. The variables that best contribute to the definition of the model are: the depth of Moho, the topographic altitude, the fault length and the Bouguer anomaly.