Abstract: We model a fault cross-cutting the brittle upper crust and the ductile lower crust. In the brittle layer the fault is assumed to have stick-slip behaviour, whereas the lower ductile crust is inferred to deform in a steady-state shear. Therefore, the brittle-ductile transition (BDT) separates two layers with different strain rates and structural styles. This contrasting behaviour determines a stress gradient at the BDT that is eventually dissipated during the earthquake. During the interseismic period, along a normal fault it should form a dilated hinge at and above the BDT. Conversely, an over-compressed volume should rather develop above a thrust plane at the BDT. On a normal fault the earthquake is associated with the coseismic closure of the dilated fractures generated in the stretched hangingwall during the interseismic period. In addition to the shear stress overcoming the friction of the fault, the brittle fault moves when the weight of the hangingwall exceeds the strength of the dilated band above the BUT. On a thrust fault, the seismic event is instead associated with the sudden dilation of the previously over-compressed volume in the hangingwall above the BDT, a mechanism requiring much more energy because it acts against gravity. In both cases, the deeper the BDT, the larger the involved volume, and the bigger the related magnitude. We tested two scenarios with two examples from L'Aquila 2009 (Italy) and Chi-Chi 1999 (Taiwan) events. GPS data, energy dissipation and strain rate analysis support these contrasting evolutions. Our model also predicts, consistently with data, that the interseismic strain rate is lower along the fault segment more prone to seismic activation. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: The study of geodynamics relies on an understanding of the strength of the lithosphere. However, our knowledge of kilometer-scale rheology has generally been obtained from centimeter-sized laboratory samples or from microstructural studies of naturally deformed rocks. In this study, we present a method that allows rheological examination at a larger scale. Utilizing forward numerical modeling, we simulated lithospheric deformation as a function of heat flow and rheological parameters and computed several testable predictions including horizontal velocities, stress directions, and the tectonic regime. To select the best solutions, we compared the model predictions with experimental data. We applied this method in Italy and found that the rheology shows significant variations at small distances. The strength ranged from 0.6 +/- 0.2 TN/m within the Apennines belt to 21 +/- 6 TN/m in the external Adriatic thrust. These strength values correspond to an aseismic mantle in the upper plate and to a strong mantle within the Adriatic lithosphere. With respect to the internal thrust, we found that strike-slip or transpressive, but not compressive, earthquakes can occur along the deeper portion of the thrust. The differences in the lithospheric strength are greater than our estimated uncertainties and occur across the Adriatic subduction margin. Using the proposed method, the lithospheric strength can be also determined when information at depth is scarce but sufficient surface data are available. Citation: Carafa, M. M. C., and S. Barba (2011), Determining rheology from deformation data: The case of central Italy, Tectonics, 30, TC2003, doi: 10.1029/2010TC002680.
Abstract: Two critical items in the energetic budget of a seismic province are the strain rate, which is measured geodetically on the Earth's surface, and the yearly number of earthquakes exceeding a given magnitude. Our study is based on one of the most complete and recent seismic catalogs of Italian earthquakes and on the strain rate map implied by a multiyear velocity solution for permanent GPS stations. For each of 36 homogeneous seismic zones we use the appropriate Gutenberg-Richter relation, which is based on the seismicity catalog, to estimate a seismic strain rate, which is the strain rate associated with the mechanical work due to a coseismic displacement. We show that for each seismic zone, the volume storing most of the elastic energy associated with the long-term deformation, and hence the seismic strain rate, is inversely proportional to the static stress drop. The GPS-derived strain rate for each seismic zone limits the corresponding seismic strain rate, and an upper bound for the average stress drop is estimated. We show that the implied regional static stress drop varies from 0.1 to 5.7 MPa for catalog earthquakes in the moment magnitude range [4.5-7.3]. The stress drop results are independent of the regional a and b parameters and heat flow but are very sensitive to the assumed maximum magnitude of a seismic province. The data do not rule out the hypothesis that the stress drop positively correlates with the time elapsed after the largest earthquake recorded in each seismic zone.
Abstract: The present-day tectonic setting of the Italian peninsula is very complex and involves competing geodynamic processes. In this context, southern peninsular Italy is characterised by extension along the Apenninic belt and in the Tyrrhenian margin and by transpression in the Apulia-Gargano region. The extension is well defined by means of geological, seismological, and contemporary stress data. For the latter only few data are available in the Apulia-Gargano region, leaving the state of stress in that area unresolved. Here we develop a finite-element model of the southern Italian region in order to predict the contemporary stress field. Our model predictions are constrained by model-independent observations of the orientation of maximum horizontal stress (S-Hmax), the tectonic regime, and the horizontal velocities derived from GPS observations. We performed a blind test with 31 newly acquired S-Hmax orientations in the Southern Apennines. These new data come from the analysis of orehole breakouts performed in 46 deep oil exploration wells ranging in depth from 1300 to 5500 m. The model results agree with the stress data that define a prevailing NW-SE S-Hmax orientation along the Apenninic belt and foredeep and thus are capable to predict the stress field where no stress information is available. We first analyse how much model predictions, based on older data, deviate from present-day stress data and then recalibrate the models based on our new stress data, giving insight into the resolution of both models and data. In the studied region, which is affected by low deformation rates, we find that geodetic data alone cannot resolve such low levels of deformation due to the high relative measurement errors. We conclude that both GPS and stress data are required to constrain model results. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: We develop new approaches to calculating 30-year probabilities for occurrence of moderate-to-large earthquakes in Italy. Geodetic techniques and finite-element modelling, aimed to reproduce a large amount of neotectonic data using thin-shell finite element, are used to separately calculate the expected seismicity rates inside seismogenic areas (polygons containing mapped faults and/or suspected or modelled faults). Thirty-year earthquake probabilities obtained from the two approaches show similarities in most of Italy: the largest probabilities are found in the southern Apennines, where they reach values between 10% and 20% for earthquakes of M (W)>=6.0, and lower than 10% for events with an M (W)>=6.5.
Abstract: By means of a 2D finite-element procedure, we tested how heterogeneities at the scale of seismogenic fault affect the displacement. We defined one or more slip distributions for two typical normal-faulting earthquakes in the Central Apennines, computed the displacement occurred within different structures including lateral heterogeneities, and compared the different displacement profiles to isolate the effect of the crustal structure. To understand at what magnitude the heterogeneities affect the observation significantly, we compared the predicted coseismic displacement with GPS and DInSAR data for the Colfiorito 1997 earthquake. We find that heterogeneities significantly affect the observable horizontal coseismic displacement for the larger magnitudes, whereas for smaller quakes, they affect horizontal displacement close to the fault trace only.
Abstract: We designed a new seismic source model for Italy to be used as an input for country-wide probabilistic seismic hazard assessment (PSHA) in the frame of the compilation of a new national reference map.
We started off by reviewing existing models available for Italy and for other European countries, then discussed the main open issues in the current practice of seismogenic zoning.
The new model, termed ZS9, is largely based on data collected in the past 10 years, including historical earthquakes and instrumental seismicity, active faults and their seismogenic potential, and seismotectonic evidence from recent earthquakes. This information allowed us to propose new interpretations for poorly understood areas where the new data are in conflict with assumptions made in designing the previous and widely used model ZS4.
ZS9 is made out of 36 zones where earthquakes with M-w >=5 are expected. It also assumes that earthquakes with M-w up to 5 may occur anywhere outside the seismogenic zones, although the associated probability is rather low. Special care was taken to ensure that each zone sampled a large enough number of earthquakes so that we could compute reliable earthquake production rates.
Although it was drawn following criteria that are standard practice in PSHA, ZS9 is also innovative in that every zone is characterised also by its mean seismogenic depth (the depth of the crustal volume that will presumably release future earthquakes) and predominant focal mechanism (their most likely rupture mechanism). These properties were determined using instrumental data, and only in a limited number of cases we resorted to geologic constraints and expert judgment to cope with lack of data or conflicting indications. These attributes allow ZS9 to be used with more accurate regionalized depth-dependent attenuation relations, and are ultimately expected to increase significantly the reliability of seismic hazard estimates.
Abstract: What forces control active deformation in the Central Mediterranean? Slab-pull has long been debated, but no other hypothesis has been generally accepted. Here we analyze the role of shear basal tractions. By using a thin-shell modeling technique, we generated a large number of models that span different sets of boundary conditions from the literature; we then explored acceptable ranges of model parameters. We computed residuals between model predictions and several datasets of stress directions, GPS measurements and tectonic stress regimes that have been produced in recent studies, and then compared the best models obtained in the presence of tractions with those obtained in the absence of tractions. For all tested boundary conditions and all considered datasets, our results show that the only successful models are those with significant basal shear traction exerted by eastward mantle flow.
Abstract: Is compression across the northern Apennine fold-and-thrust system (Italy) still active? To address this question, we quantified the long-term rates of migration and shortening of the system along with the measurement errors. Our approach integrates structural geology, seismicity patterns, and statistical treatment of tectonic activity. On the basis of recently published surface and subsurface data, we found a migration rate of 8.85 +/- 0.61 mm yr(-1). The inception age of individual fold structures follow closely this average rate, indicating that the system has been migrating at a constant rate for the past 17 Myr. Cumulative shortening of the system also increases linearly through time at 2.93 +/- 0.31 mm yr(-1). The location of the youngest structures in the easternmost portion of the system coincides with a significant peak of seismic moment released by historical earthquakes. We conclude that not only these easternmost thrusts are still active, but also that they generate earthquakes.
Abstract: Seismic data users and people managing a seismic network take a great interest in the potentiality of the data, with the difference that the former look at stability, the latter at improvements. This work measures the performances of the Italian Telemetered Seismic Network in the years 1985-2002 by defining basic significant parameters and studying their evolution during those years. Then, we deal with the geological methods used to characterise or to plan seismic station deployments in a few cases. Last, we define the gain of the network as the percentage of well-located earthquakes with respect to the total recorded earthquakes. By analysing the distribution of non-located (<< missed >>) earthquakes, we suggest possible actions to take to increase the gain. Results show that completeness magnitude is 2.4 in the average over the analysed period, and it can be as low as 2.2 when we consider non-located earthquakes as well. Parameters Such as the minimum recording distance and the RMS of the location decrease with time, reflecting improvements in the location quality. Methods for geologic and seismological characterisation of a possible station site also proved effective. Finally, we represent the number of missed earthquakes at each station, showing that nine stations control more than 50% of all missed earthquakes, and suggesting areas in Italy where the network might easily be improved.
Abstract: We compute displacement and stress due to a normal fault by means of two-dimensional plane-strain finite-element analysis. To do so, we apply a system of forces to the fault nodes and develop an iterative algorithm serving to determine the force magnitudes for any slip distribution. As a sample case, we compute the force magnitudes assuming uniform slip on a 10-km two-dimensional normal fault. The numerical model generates displacement and stress fields that compare well with the analytical solution. In fact. we found little difference in displacements (< 5%), displacement orientation (< 15 degrees). and stress components (< 35%, half of which due to slip tolerance). We analyze such misfit, and discuss how the error propagates from displacement to stress. Our scheme provides a convenient way to use the finite-elements direct method in a trial-and-error procedure to reproduce any smooth slip distribution.
Abstract: In this work, the control exerted by the stress axes orientation on the evolution of seismic sequences developing in compressive and extensional regimes is analysed. According to the Anderson fault theory, the vertical stress is the minimum principal stress in compressional tectonic regimes, whereas it is the maximum principal stress in extensional regimes. Using Mohr diagrams and discussing the present knowledge about the distribution of vertical and horizontal stress with depth we show that, in absence of localised fluid overpressure, such changes imply that thrust and normal faults become more unstable at shallower and greater depths, respectively. These opposite mechanical behaviours predict, in a rather isotropic body, easier rupture at shallower level in compressional regimes later propagating downward. On the contrary, a first deep rupture propagating upward is expected in extensional regimes. This is consistent with observations from major earthquakes from different areas in the world. We show that the exceptions to downward migration along thrusts occur along steeply inclined faults and probably imply localised supra-hydrostatic fluid pressures. Moreover, we show that the inversion of the meaning of the lithostatic load has consequences also for the role of topography. High topography, increasing the vertical load, should inhibit earthquake development in compressional environments and should favour it in extensional settings. Although several factors, such as geodynamic processes, local tectonic features and rock Theology, are likely to control earthquake locations, stress distribution and tectonic regime, these model predictions are consistent with seismicity distribution in Italy, central Andes and Himalaya. In these areas, large to medium compressional earthquakes occur at the low elevation borders of compressional mountain belts, whereas large extensional earthquakes occur in correspondence to maximum elevations.
Abstract: Recent efforts to relate the style of seismicity in the Central Apennines to the stress build-up induced by active geodynamic processes by means of simple viscoelastic dynamic models have shown that seismicity in this region is likely to be controlled by two major processes: the westward underthrusting of the Adriatic plate under the Apennines and the asthenospheric upwelling underneath Tuscany, In this work, using these results as a starting point, we evaluate, by means of 2-D dynamic models, the effects of structural heterogeneities (major crustal-scale faults: this improvement is made possible by the recent publication of the results of the CROP-03 seismic line) and of temperature-dependent rheologies within the crust and the lithospheric mantle on the stress field induced by the two processes mentioned above. Modelling results are compared with detailed seismicity data.
We show that, for all models, the predicted stress fields are controlled? at large wavelengths, by an upward state of flexure in Tuscany and a downward state of flexure in the Apenninic foredeep! in agreement with previous studies. When activating crustal-scale discontinuities, however, strong local rotations of the major axes of the stress tensor occur in the proximity of the faults, generating complex local stress fields characterized by the coexistence of compressive and tensional styles. This could explain the coexistence, as observed within certain regions of central Italy, of compressive and extensional events. When detailing the rheology using plastic behaviour for the lithospheric portions of the models, a complex profile of yield stresses is considered, the materials characterized by higher strength being the sedimentary cover and the lithospheric mantle. Model-predicted stress concentrates in these materials. The lateral variability of the thickness of the sedimentary cover induces strong local rotations, at shallow depths, of the stress eigenvectors. All of the models predict a stress field roughly compatible with the distribution and style of seismicity of the region. The use of progressively more realistic rheologies and geometries, however, gives slightly better results when compared to seismicity data. This paper shows that sensible results may be obtained by combining active and natural source seismicity with finite element modelling.
Abstract: To contribute to the understanding of the relationships between moderate earthquakes and the faults that are recognizable in the geological record, we analysed seismological and geological data related to the 1997-1998 Umbria-Marche (Central Italy) earthquake swarm. The seismological recordings, collected by local networks, allowed accurate location of about 1000 events, whereas the geological field observations provided a picture of the structural features and the ground-surface deformations. We also re-examined and used some published data and results, mostly about the fault plane solutions and the geology.
On the basis of earthquake locations, fault plane solutions, and geological mapping we explored the possible correlation between the earthquake causative fault planes and the normal faults exposed in the area. Our results show that the two main shocks that occurred on 1997 September 26 (M-W = 5.7 and M-W = 6.0) originated on the same structure, reactivating at depth the Colfiorito normal faults. Neither rupture propagated up to the ground surface, but both triggered gravitational sliding that occurred along pre-existing fault scarps. The earthquake that occurred on 1997 October 14 (M-W = 5.6) originated on another fault branch at a much shallower depth. In spite of its lower magnitude, this earthquake produced tectonic ruptures where the fault plane projects to the surface in an area where no faults were previously mapped.
By comparing the palaeostress reconstruction, based on slickenside lineation analysis, and the focal mechanism solutions, we suggest a possible correlation between the long-term (Early Middle Pleistocene) cumulative effects of the Colfiorito Fault System and the short-term behaviour of the fault planes observed during this earthquake swarm, favouring the idea of a seismogenic source producing clustered moderate-size earthquakes rather than large events scattered in time.
Abstract: In the Umbria-Marche region, space and time variations concerning the b value were studied by instrumental seismicity from January 1987 to May 1999, according to the Bender method. Data were divided into two partially independent data sets. The first set, (January 1987 - December 1996), does not include the Colfiorito seismic sequence that occurred in the autumn of 1997. The second data set includes all events from January 1987 to May 1999. Using square cell dimensions of 80, 40 and 20 km, the examined area was divided respectively into three grids. The b value was estimated for each cell using the first data set, thus allowing us to reveal b value space variations and determine the resolution. To evaluate the stability of our result we estimated the b value on the basis of historical seismicity within the region. Several synthetic tests were also performed to estimate the stability of the Bender method and to verify its consistency with respect to other methods commonly used. Finally we estimated the b values using the second data set to prove the time variations. Results from the area examined show that the lowest possible spatial resolution of the b value is about 40 km and that there is a correlation between the b value pattern and the main active tectonic structures of the area. The most important time variations occur within the Colfiorito area, in which the b value drops significantly within the second data set. Results suggest two different ways of strain release: the first one produces continuous seismicity that spreads all over the examined area, while the second, which concerns stronger earthquakes, is localized.
Abstract: We introduce a new detection algorithm with improved local and regional seismic signal recognition. The method is based on the difference between seismic signals and background random noise in terms of fractal dimension D. We compare the new method extensively with standard methods currently in use at the Seismic Network of the Istituto Nazionale di Geofisica. Results from the comparisons show that the new method recognizes seismic phases detected by existing procedures, and in addition, it features a greater sensitivity to smaller signals, without an increase in the number of false alarms. The new method was tested on real continuous data and artificially simulated high-noise conditions and demonstrated a capability to recognize seismic signals in the presence of high noise. The efficiency of the method is due to a radically different approach to the topic, in that the assertion that a signal is fractal implies a relationship between the spectral amplitude of different frequencies. This relationship allows, for the fractal detector, a complete analysis of the entire frequency range under consideration.
Abstract: We develop a viscoelastic dynamic model to calculate the stress distribution in central Italy induced by different geodynamic processes likely active in the region: Africa-Eurasia convergence, Adriatic underthrusting/subduction and asthenospheric upwelling underneath Tuscany. Model results are compared with the hypocentral distribution of earthquakes and with the orientation of P and T axes available for the region. The first order characteristics of the seismotectonic observations in central Italy are: subcrustal seismicity, the pair extension-compression in Tuscany and in the outer Apennines, and the flexural behaviour evident in long wavelength orientation of T axes. These features are reproduced by the combination of two processes: underthrusting of the Adriatic plate under the Apennines and asthenospheric upwelling underneath Tuscany. The comparison with available data does not give insights about slab pull as an ongoing process.
Abstract: In this paper we investigate the forces possibly active in the area of the northern Apennines by means of two-dimensional finite element modelling assuming a viscoelastic rheology. The forces included in the models are related to continental convergence between Africa and Eurasia, to negative buoyancy of the subducted Adriatic lithosphere and to positive buoyancy of anomalously hot mantle material underneath Tuscany. Model-predicted stress distribution is compared with seismotectonic data available for the area of the northern Apennines and with earthquakes distribution. Our results indicate that continental convergence cannot be the only mechanism acting in the study area since it causes insignificant subcrustal stresses, in disagreement with intermediate seismicity observed under the Apennines, Asthenospheric upwelling under Tuscany (back-arc region) is shown to play a crucial role in the present day dynamics of the Tyrrhenian-Apennines. Positive buoyancy causes an upwards and eastwards flow that generates extensional bending stresses in Tuscany and at the top of the slab under the Apennines. Horizontal pushing of the slab is proposed as an alternative mechanism to slab pull to generate subcrustal stresses. Activation of slab pull does not result in a significant change in orientation of the principal stress axes at shallow depths with respect to the model in which only asthenospheric diapirism is active. Therefore, the existence of a continuous gravitationally sinking slab cannot be ruled out on the basis of comparison with seismotectonic observations. Both models successfully reproduce extension in Tuscany and compression in the outer margin of the Apenninic belt, but fail to reproduce extension along the inner portion of the chain. Our modelling highlights the importance of better constraining the lateral extent of the asthenospheric diapir in order to find out whether the extension and tectonic uplift in the Apennines is caused by asthenospheric upwelling as in the area of Tuscany.
Abstract: Data recorded by the Italian Telemetered Seismic Network (ITSN) of the Istituto Nazionale di Geofisica (ING) have been widely used in recent years to image slab structures and to find evidence for active processes along the Italian Peninsula. However, the use of seismic data for geostructural purposes may be affected by the well-known trade-off between earthquake location and seismic-velocity parameters. Furthermore, the confidence ellipse predicted by standard procedures may be inadequate for the representation of the probable error of a computed localization. This paper evaluates the probable errors on the hypocentre determinations of the seismic events recorded by the ITSN, using a Monte Carlo method.
We compute synthetic arrival times using a 1-D velocity model appropriate as an average for the Italian area. The hypocentres used are all those recorded by the ITSN during the period January 1992 to March 1994 (1972 events). Station locations are those of the current ITSN configuration. The synthetic arrival times are perturbed with a Gaussian distribution of errors and input to ING's standard hypocentral location procedure, but using crustal velocities differing by 10 per cent from those used to generate them. Each simulation is repeated at least 30 times. Average absolute shifts of hypocentres are assessed in grid cells of linear dimension 33 km covering the whole Italian region.
For regions within the ITSN, shifts are typically 5-10 km in location and up to 20 km in depth. However, for offshore and coastal regions, they are much greater: 50 km or more in both location and depth (far exceeding the equivalent uncertainties quoted by ING bulletins). Possible consequences of this are highlighted by producing a cross-section of subcrustal hypocentres from the Adriatic to the Tyrrhenian Sea, where the large uncertainty in depth precludes any confident interpretation of dipping tectonic features.
Abstract: This paper compares different source mechanism models that fit first onset amplitudes for a set of seismic events that occurred in the Apennines, Italy. We selected earthquakes of magnitude greater than 1.0 that match the criteria of appropriate instrumental coverage of the epicentral area, low localisation errors and high signal-to-noise ratio. We analysed also microearthquakes recorded by dense seismic arrays very close to the epicentral area. Three source models have been tested: a simple double-couple (three parameters, after amplitude normalisation), a double couple associated with compensated linear vector dipole (four parameters), a double couple associated with coplanar tensile crack (four parameters). Making use of statistical criteria, we compared source behaviour of seismic events occurring in two different areas. An F-test did not allow us to discriminate among the three models in the range of the analysed magnitudes. Anyway, the resulting mechanisms show consistent patterns in terms of tectonic stress regimes, being the T axis in the anti-Apenninic direction.
Notes: Monograph: Earthquake source mechanics; observations, models and impact on plate motion; Editor: Bonafede, M; Gudmundsson, A; Roth, F; Rybicki, K R; Stein, R S
Abstract: In this study, we used both spectral and array techniques to investigate microtremor characteristics in an urban area in Central Italy and to relate them to the properties of shallow geological structures. We made noise measurements for sites located on the various surface geological conditions that can be found in the town of Rieti. Both sediments and bedrock are present at the surface, and the transition zone between them is very narrow and well localized. Using a small area reduced the effects related to the poor knowledge about the distribution of noise sources; using spectral ratio techniques, a good image of resonance frequencies on the alluvial sediments was obtained. Moreover, the use of an array allowed to describe better the morphology of the bedrock and to study Rayleigh-wave dispersion. Starting from the obtained surface-wave dispersion curve, we calculated a velocity model producing both a theoretical dispersion relationship and resonance frequencies for sedimentary layers in good agreement with experimental data. Our work suggests that the use of array techniques can be very useful to increase the quality of microtremor data, giving new answers to microzoning problems.
Abstract: Users can interactively query, search, and download parametric information, and digital recordings collected by the Italian Telemetered Seismic Network (ITSN) through the Istituto Nazionale Di Geofisica Seismic Network Databank (ISND). The databank is completely menu-driven and easy to use.The ITSN comprises about 80 seismic stations (Figure 1) equipped with several seismometers acting at a critical damping of 70% and characterized by a period of 1 Hz. The seismometers' signals are transmitted over telephone lines or radio relay systems and then demodulated and recorded by an automatic acquisition system developed in cooperation with the U.S. Geological Survey. Digital data are stored on a magnetic disk and then processed through interactive procedures. The automatic selection of the seismic phases is checked daily and eventually corrected by seismic analysts. New data are available to users within a week.
Abstract: Automated amplitude response of the complete seismometer, telemetry and recording system is obtained trom sinusoidal inputs to the calibration coil. Custom-built software was designed to perform fully automatic calibration analyses of the digital signals. In this paper we describe the signals used for calibration and interactive and batch procedures designed to obtain calibration functions in automatic mode. By using a steady-state method we reach a high degree of accuracy in the determination of both the frequency and amplitude of the signal. The only parameters required by this procedure are the seismometer mass, the calibration-coil constant and the intensity of the current injected into the calibration coil. This procedure is applicable to telemetered seismic systems and represents an optimization of the processing time. The software was designed to require no modifications if the device used to generate the sinusoidal current should change. In particular, it is possible to change the number of monofrequency packages transmitted to the calibration coil with the only restriction that the difference between the frequency of two consecutive packages be greater than 5%; for these reasons the procedure is expected to be useful for the seismological community. The paper includes a generaI description of the designing criteria, and of the hardware and software architecture, as well as an account of the system's performance during a two year period of operation.
Abstract: The Istituto Nazionale di Geofisica (ING) Seismic Network Database (ISND) includes over 300000 arrivaI times of Italian, Mediterranean and teleseismic earthquakes from 1983 to date. This database is a useful tool for Italian and foreign seismologists (over 1000 data requests in the first 6 months of this year). Recently (1994) the ING began storing in the ISND, the digital waveforms associated with arrival times and experimentally allowed users to retrieve waveforms recorded by the ING acquisition system. In this paper we describe the types of data stored and the interactive and batch procedures available to obtain arrivaI times and/or associated waveforms. The ISND is reachable via telephone line, P.S.I., Internet and DecNet. Users can read and send to their E-mail address alI selected earthquakes locations, parameters, arrivaI times and associated digital waveforms (in SAC, SUDS or ASCII format). For medium or large amounts of data users can ask to receive data by means of magnetic media (DAT, Video 8, floppy disk).
Abstract: In questo capitolo vengono studiate le forze che possibilmente agiscono nell’area degli Appennini settentrionali tramite una modellazione bidimensionale agli elementi finiti, nell’assunzione di una reologia viscoelastica. Le forze incluse nel modello sono dovute alla convergenza continentale tra Africa e Eurasia, allo sprofondamento della litosfera adriatica subdotta e alla risalita di materiale astenosferico caldo al di sotto della Toscana. La distribuzione di stress calcolata dal modello è confrontata con i dati sismotettonici disponibili per gli Appennini settentrionali e con la distribuzione degli eventi sismici. I risultati indicano che la convergenza continentale non può essere il solo meccanismo che agisce nell’area, dal momento che esso causa un accumulo trascurabile di stress al di sotto della Moho. Tale risultato è in contraddizione con la presenza di sismicità intermedia osservata sotto gli Appennini. Si verifica che la risalita di materiale astenosferico sotto la toscana assume un ruolo cruciale per descrivere la dinamica attuale del Tirreno e degli Appennini. Tale risalita causa un flusso diretto verso l’alto e verso est, flusso che a sua volta genera uno stress estensivo in Toscana e nella parte superiore della subduzione, sotto gli Appennini. La spinta orizzontale dello slab è proposta come un meccanismo alternativo allo slab pull per generare un accumulo di stress subcrostale. L’attivazione dello slab pull non comporta una variazione significativa nell’orientazione degli assi principali dello stress a profondità crostali, rispetto al modello in cui il solo diapirismo astenosferico sia attivo. Di conseguenza, l’esistenza di uno slab continuo che sprofonda a causa della forza di gravità non può essere esclusa sulla base del solo confronto con le osservazioni sismotettoniche. Entrambi i modelli riproducono efficacemente l’estensione in Toscana e la compressione nel margine esterno della catena Appenninica, ma non generano con sufficiente accuratezza l’estensione lungo la porzione interna della catena. I modelli sviluppati mettono in evidenza l’importanza di migliorare le conoscenze sul comportamento dell’astenosfera, e sulla posizione e dimensioni della zona “calda”, allo scopo di comprendere se l’estensione e il probabile sollevamento tettonico) negli Appennini possa essere causato da una risalita astenosferica, come per l’area della Toscana. Una ipotesi alternativa di una sorgente locale di stress, non inclusa nel modello e probabilmente legata al collasso dell’orogene, viene presentata.
