Le Sorgenti sismiche: teoria ed osservazioni

Scopo di questo lavoro è quello di fornire al lettore (più o meno esperto) uno strumento per la comprensione (concettuale ed analitica) di alcuni aspetti della teoria dinamica dell’elasticità, passando poi alla rappresentazione delle sorgenti sismiche, alla loro radiazione ed alla determinazione del loro meccanismo. Il terremoto infatti, non è altro che l’effetto delle onde sismiche, cioè il processo di oscillazione della Terra e, in particolare, della superficie terrestre, dove tale oscillazione è osservabile. La sorgente del terremoto (o sorgente sismica) è la sorgente delle onde sismiche. Nel capitolo 1 si fa riferimento in particolare al più semplice problema dinamico per un mezzo isotropo; la sorgente sismica infatti è forza impulsiva applicata ad un solo punto e orientata come uno degli assi del sistema di riferimento. E’ stata descritta la relazione che rappresenta la radiazione emessa da tali sorgenti in un mezzo elastico illimitato, omogeneo ed isotropo fino ad arrivare alla soluzione rappresentata dal Tensore di Green. Successivamente, (capitolo 2), viene fornita una descrizione sulla rappresentazione delle sorgenti sismiche sia in modo analitico che concettuale. Anche in questo caso è fatta una trattazione completa del problema, per poi giungere alla definizione di tensore momento sismico. Il terzo capitolo riguarda invece la trattazione sulla radiazione della sorgente sismica, sul campo degli spostamenti e sulle funzioni di irraggiamento, partendo dai risultati del primo e del secondo capitolo. Infine nella quarta ed ultima parte si è descritto il meccanismo focale di frattura, funzioni di irraggiamento per una frattura (orientata secondo gli assi o arbitrariamente) descrivendo la sfera focale e le sue proiezioni ed alcuni metodi per la determinazione del meccanismo focale

A rate-state model for aftershocks triggered by dislocation on a rectangular fault: a review and new insights

We compute the static displacement, stress, strain and the Coulomb failure stress produced in an elastic medium by a finite size rectangular fault after its dislocation with uniform stress drop but a non uniform dislocation on the source. The time-dependent rate of triggered earthquakes is estimated by a rate-state model applied to a uniformly distributed population of faults whose equilibrium is perturbated by a stress change caused only by the first dislocation. The rate of triggered events in our simulations is exponentially proportional to the shear stress change, but the time at which the maximum rate begins to decrease is variable from fractions of hour for positive stress changes of the order of some MPa, up to more than a year for smaller stress changes. As a consequence, the final number of triggered events is proportional to the shear stress change. The model predicts that the total number of events triggered on a plane containing the fault is proportional to the 2/3 power of the seismic moment. Indeed, the total number of aftershocks produced on the fault plane scales in magnitude, M, as 10M. Including the negative contribution of the stress drop inside the source, we observe that the number of events inhibited on the fault is, at long term, nearly identical to the number of those induced outside, representing a sort of conservative natural rule. Considering its behavior in time, our model does not completely match the popular Omori law; in fact it has been shown that the seismicity induced closely to the fault edges is intense but of short duration, while that expected at large distances (up to some tens times the fault dimensions) exhibits a much slower decay

Renewal models of seismic recurrence applied to paleoseismological data

Because paleoseismology can extend the record of earthquakes back in time up to several millennia, it represents a great opportunity to study how earthquakes recur through time and thus provide innovative contributions to seismic hazard assessment. A worldwide compilation of a database of recurrence from paleoseismology was developed in the frame of the ILP project “Earthquake Recurrence Through Time”, from which we were able to extract five sequences with 6 and up to 9 dated events on a single fault. By using the age of the paleoearthquakes with their associated uncertainty we have tested the null hypothesis that the observed inter-event times come from a uniform random distribution (Poisson model). We have made use of the concept of likelihood for a specific sequence of observed events under a given occurrence model. The difference dlnL of the likelihoods estimated under two hypotheses gives an indication of which between the two hypotheses fits better the observations. To take into account the uncertainties associated to paleoseismological data, we used a Monte Carlo procedure, computing the average and the standard deviation of dlnL for 1000 inter-event sets randomly obtained by choosing the occurrence time of each event within the limits of uncertainty provided by the observations. Still applying a Monte Carlo procedure, we have estimated the probability that a value equal to or larger than each of the observed dlnLs comes by chance from a Poisson distribution of inter-event times. These tests have been carried out for a set of the most popular statistical models applied in seismic hazard assessment, i.e. the Log-normal, Gamma, Weibull and Brownian Passage Time (BPT) distributions. In the particular case of the BPT distribution, we have also shown that the limited number of dated events creates a trend to reducing both the observed mean recurrence time and the coefficient of variation for the studied sequence which can possibly bias the results. Our results show that a renewal model, associated with a time dependent hazard, and some kind of predictability of the next large earthquake on a fault, only for the Fucino site, out of the five sites examined in this study, is significantly better than a plain time independent Poisson model. The lack of regularity in the earthquake occurrence for three of the examined faults can be explained either by the large uncertainties in the estimate of paleoseismological occurrence times or by physical interaction between neighbouring faults

Probabilistic approach to earthquake prediction.

The evaluation of any earthquake forecast hypothesis requires the application of rigorous statistical methods. It implies a univocal definition of the model characterising the concerned anomaly or precursor, so as it can be objectively recognised in any circumstance and by any observer.A valid forecast hypothesis is expected to maximise successes and minimise false alarms. The probability gain associated to a precursor is also a popular way to estimate the quality of the predictions based on such precursor. Some scientists make use of a statistical approach based on the computation of the likelihood of an observed realisation of seismic events, and on the comparison of the likelihood obtained under different hypotheses. This method can be extended to algorithms that allow the computation of the density distribution of the conditional probability of earthquake occurrence in space, time and magnitude. Whatever method is chosen for building up a new hypothesis, the final assessment of its validity should be carried out by a test on a new and independent set of observations. The implementation of this test could, however, be problematic for seismicity characterised by long-term recurrence intervals. Even using the historical record, that may span time windows extremely variable between a few centuries to a few millennia, we have a low probability to catch more than one or two events on the same fault. Extending the record of earthquakes of the past back in time up to several millennia, paleoseismology represents a great opportunity to study how earthquakes recur through time and thus provide innovative contributions to time-dependent seismic hazard assessment. Sets of paleoseimologically dated earthquakes have been established for some faults in the Mediterranean area: the Irpinia fault in Southern Italy, the Fucino fault in Central Italy, the El Asnam fault in Algeria and the Skinos fault in Central Greece. By using the age of the paleoearthquakes with their associated uncertainty we have computed, through a Montecarlo procedure, the probability that the observed inter-event times come from a uniform random distribution (null hypothesis). This probability is estimated approximately equal to 8.4% for the Irpinia fault, 0.5% for the Fucino fault, 49% for the El Asnam fault and 42% for the Skinos fault. So, the null Poisson hypothesis can be rejected with a confidence level of 99.5% for the Fucino fault, but it can be rejected only with a confidence level between 90% and 95% for the Irpinia fault, while it cannot be rejected for the other two cases. As discussed in the last section of this paper, whatever the scientific value of any prediction hypothesis, it should be considered effective only after evaluation of the balance between the costs and benefits introduced by its practical implementation

Daily earthquake forecasts during the May-June 2012 Emilia earthquake sequence (northern Italy)

The paper has no abstrac

Assessing Vegetation Decline Due to Pollution from Solid Waste Management by a Multitemporal Remote Sensing Approach

Nowadays, the huge production of Municipal Solid Waste (MSW) is one of the most strongly felt environmental issues. Consequently, the European Union (EU) delivers laws and regulations for better waste management, identifying the essential requirements for waste disposal operations and the characteristics that make waste hazardous to human health and the envi-ronment. In Italy, environmental regulations define, among other things, the characteristics of sites to be classified as “potentially contaminated”. From this perspective, the Basilicata region is cur-rently one of the Italian regions with the highest number of potentially polluted sites in proportion to the number of inhabitants. This research aimed to identify the possible effects of potentially toxic element (PTE) pollution due to waste disposal activities in three “potentially contaminated” sites in southern Italy. The area was affected by a release of inorganic pollutants with values over the thresholds ruled by national/European legislation. Potential physiological efficiency variations of vegetation were analyzed through the multitemporal processing of satellite images. Landsat 5 Thematic Mapper (TM) and Landsat 8 Operational Land Imager (OLI) images were used to calcu-late the trend in the Normalized Difference Vegetation Index (NDVI) over the years. The mul-titemporal trends were analyzed using the median of the non-parametric Theil–Sen estimator. Fi-nally, the Mann–Kendall test was applied to evaluate trend significance featuring areas according to the contamination effects on investigated vegetation. The applied procedure led to the exclu-sion of significant effects on vegetation due to PTEs. Thus, waste disposal activities during previ-ous years do not seem to have significantly affected vegetation around targeted sites