Stability analysis of cosmological models through Liapunov's method

We investigate the general asymptotic behaviour of Friedman-Robertson-Walker (FRW) models with an inflaton field, scalar-tensor FRW cosmological models and diagonal Bianchi-IX models by means of Liapunov's method. This method provides information not only about the asymptotic stability of a given equilibrium point but also about its basin of attraction. This cannot be obtained by the usual methods found in the literature, such as linear stability analysis or first order perturbation techniques. Moreover, Liapunov's method is also applicable to non-autonomous systems. We use this advantadge to investigate the mechanism of reheating for the inflaton field in FRW models.Comment: Latex file, 8 pages, no figures, accepted for publication in Class. & Quant. Gra

The folding of knotted proteins: insights from lattice simulations

We carry out systematic Monte Carlo simulations of Go lattice proteins to investigate and compare the folding processes of two model proteins whose native structures differ from each other due to the presence of a trefoil knot located near the terminus of one of the protein chains. We show that the folding time of the knotted fold is larger than that of the unknotted protein and that this difference in folding time is particularly striking in the temperature region below the optimal folding temperature. Both proteins display similar folding transition temperatures, which is indicative of similar thermal stabilities. By using the folding probability reaction coordinate as an estimator of folding progression we have found out that the formation of the knot is mainly a late folding event in our shallow knot system

Scaling solutions from interacting fluids

We examine the dynamical implications of an interaction between some of the fluid components of the universe. We consider the combination of three matter components, one of which is a perfect fluid and the other two are interacting. The interaction term generalizes the cases found in scalar field cosmologies with an exponential potential. We find that attracting scaling solutions are obtained in several regions of parameter space, that oscillating behaviour is possible, and that new curvature scaling solutions exist. We also discuss the inflationary behaviour of the solutions and present some of the constraints on the strength of the coupling, namely those arising from nucleosynthesis.Comment: RevTeX, 21 pages, 8 figure

An approach for the estimation of the magnitude of historical earthquakes: a sensitivity study of the 1980 and 1998 earthquakes in Azores

In regions with low-to-moderate seismicity, the return-period of seismic events with large magnitudes is relatively high. Nevertheless, historical seismic events are relevant for the evaluation of seismic hazard in those regions. Thus, seismologists study the records of the effects of historical earthquakes to map the distribution intensity points, using an Intensity Scale. Afterwards, the maximum intensity point is identified as well as the probable epicentral location and magnitude. Another method, introduced by earthquake engineers, incorporates the knowledge of the behaviour of structures into posterior distributions of magnitude using fragility functions and the damage reported in historical documents. The method uses the total probability theorem to combine the uncertainty in the structural behaviour, ground motion intensity, site-to-source distance. Then, the Bayes’s theorem is employed to update a prior magnitude model into a posterior magnitude distribution. Thus, the reduction of the uncertainty in the final estimates requires the preliminary application of the method to instrumental events in order to validate the appropriate framework to address historical seismicity, namely ground motion and structural response. This paper investigates the earthquakes of January 1st 1980 with Mw=6.8-7.2 and of July 9th 1998 with Mw=5.9-6.2 in Azores Islands (Portugal) as study cases to test the sensitivity to different attenuation models Ambraseys et al. (2005) and Akkar et al. (2014). A single set of fragility functions, derived from a detailed vulnerability assessment in Faial, is assumed to model the structural response in both events. The results show that, for both events, the attenuation model from Akkar et al. (2014) and the fault source model presented results closer to those of detailed methods. Discrepancies can also be explained by differences in the prior distance model resulting from source models assumptions. The intervals Mw=5.96±0.53 and Mw=6.91±0.42 have been estimated for the 1998 and the 1980 earthquake, respectively.info:eu-repo/semantics/publishedVersio

Implementation and validation of an approach for the estimation of the magnitude of historical earthquakes in Azores Islands

In many regions of the globe, the seismic data associated with higher magnitude seismic events usable in the characterization of the seismic hazard is scarce. Therefore, seismologists have been estimating the magnitude and probable epicentral location of historical earthquakes using the historical records and intensity scales to map the observed damage. Another method, employed by earthquake engineers, uses fragility functions, a ground motion model and the Bayes’ theorem to estimate the probable magnitude of historical seismic events. Nevertheless, this method requires assembling an approach consistent with the local seismicity and building stock of the period of reference. This paper implements and validates an approach for the estimation of the magnitude of historical seismic events in Azores Islands, Portugal, that uses fragility functions. In order to carry out this study, data from the damage surveys, a vulnerability distribution and approximate epicentral positions of the 1998 Faial earthquake Mw = 6.0–6.2, as well as a specific attenuation model, are utilized in a calibration phase. The vulnerability index methodology is employed to derive fragility functions from a detailed vulnerability assessment. Afterwards, the approach is validated using analogous data corresponding to the 1980 Terceira earthquake Mw = 6.8–7.2. The different assumptions and results are discussed and compared. The results show that this approach can model expected magnitude values with accuracy: Mw = 6.01–5.76 for the 1998 Faial and Mw = 6.90–6.55 for the 1980 Terceira earthquakes. Additionally, the real PGAs may have been in several locations lower than those predicted by the selected ground motion equation.info:eu-repo/semantics/publishedVersio