Positive Feedback, Memory and the Predictability of Earthquakes

We review the "critical point" concept for large earthquakes and enlarge it in the framework of so-called "finite-time singularities". The singular behavior associated with accelerated seismic release is shown to result from a positive feedback of the seismic activity on its release rate. The most important mechanisms for such positive feedback are presented. We introduce and solve analytically a novel simple model of geometrical positive feedback in which the stress shadow cast by the last large earthquake is progressively fragmented by the increasing tectonic stress. Finally, we present a somewhat speculative figure that tends to support a mechanism based on the decay of stress shadows. This figure suggests that a large earthquake in Southern California of size similar to the 1812 great event is maturing.Comment: PostScript document of 18 pages + 2 eps figure

The effect of heavy element opacity on pre-main sequence Li depletion

Recent 3-D analysis of the solar spectrum data suggests a significant change of the solar chemical composition. This may affect the temporal evolution of the surface abundance of light elements since the extension of the convective envelope is largely affected by the internal opacity value. We analyse the influence of the adopted solar mixture on the opacity in the convective envelope of pre-main sequence (PMS) stars and thus on PMS lithium depletion. The surface Li abundance depends on the relative efficiency of several processes, some of them still not known with the required precision; this paper thus analyses one of the aspects of this ``puzzle''. Focusing on PMS evolution, where the largest amount of Li burning occurs, we computed stellar models for three selected masses (0.8, 1.0 and 1.2 Msun, with Z=0.013, Y=0.27, alpha=1.9) by varying the chemical mixture, that is the internal element distribution in Z. We analysed the contribution of the single elements to the opacity at the temperatures and densities of interest for Li depletion. Several mixtures were obtained by varying the abundance of the most important elements one at a time; we then calculated the corresponding PMS Li abundance evolution. We found that a mixture variation does change the Li abundance: at fixed total metallicity, the Li depletion increases when increasing the fraction of elements heavier than O.Comment: A&A accepted, 11 pages, 18 eps figure

Energy and XmaxX_\mathrm{max} Reconstruction for Cosmic-Ray Events Recorded by a Prototype Station of the IceCube Surface Enhancement

The IceTop array, located at the surface of the IceCube Neutrino Observatory at the South Pole, is currently used as a veto for the in-ice neutrino detection as well as a cosmic-ray detector. Over the years, additional snow has accumulated on the IceTop detectors leading to a reduction in sensitivity and resolution. In order to mitigate this issue as well as further increase the accuracy of cosmic-ray measurements, a detector enhancement is planned in the next few years. The enhanced array will consist of 32 stations, each comprising 8 scintillation detectors and 3 radio antennas, and will span an area of approximately 1km$^2$. Specifically, upgrading IceTop with radio antennas will provide precise $X_{max}$ measurements, a variable widely used to reconstruct the composition of cosmic rays. In January 2020, a complete prototype station was deployed at the South Pole. Following the measurement of cosmic-ray events with the antennas, we developed the tools necessary to use a template-matching method for energy and $X_{max}$ reconstruction. This template method uses Monte-Carlo simulations and compares them to recorded data. Thus, a set of simulated air showers is created using air shower parameters reconstructed by IceTop as input to the CORSIKA/CoREAS simulation software for each of the measured events. In this work, the method is applied to measured events, and we will show that it works for a third of the events in the sample. Subsequently, the technique is modified into a log-likelihood minimization, and the results obtained with a simulations-only study of different parameters are discussed. This work concludes with a confirmation that the template method works for air showers recorded with a frequency band of 80 to 300MHz, and shows that the log-likelihood method can potentially increase the accuracy of $X_{max}$ reconstruction for the complete planned array

Using the seismology of non-magnetic chemically peculiar stars as a probe of dynamical processes in stellar interiors

Chemical composition is a good tracer of hydrodynamical processes that occur in stars as they often lead to mixing and particle transport. By comparing abundances predicted by models and those observed in stars we can infer some constraints on those mixing processes. As pulsations in stars are often very sensitive to chemical composition, we can use asteroseismology to probe the internal chemical composition of stars where no direct observations are possible. In this paper I focus on main sequence stars Am, lambda bootis, and HgMn stars and discuss what we can learn of mixing processes in those stars from seismology.Comment: 10 pages,6 figures. accepted in Journal of astrophysics and astronomy. proceedings of aries conferemce on asteroseismology. december 200

Zipf's law in Multifragmentation

We discuss the meaning of Zipf's law in nuclear multifragmentation. We remark that Zipf's law is a consequence of a power law fragment size distribution with exponent $\tau \simeq 2$. We also recall why the presence of such distribution is not a reliable signal of a liquid-gas phase transition

Scaling Analysis and Evolution Equation of the North Atlantic Oscillation Index Fluctuations

The North Atlantic Oscillation (NAO) monthly index is studied from 1825 till 2002 in order to identify the scaling ranges of its fluctuations upon different delay times and to find out whether or not it can be regarded as a Markov process. A Hurst rescaled range analysis and a detrended fluctuation analysis both indicate the existence of weakly persistent long range time correlations for the whole scaling range and time span hereby studied. Such correlations are similar to Brownian fluctuations. The Fokker-Planck equation is derived and Kramers-Moyal coefficients estimated from the data. They are interpreted in terms of a drift and a diffusion coefficient as in fluid mechanics. All partial distribution functions of the NAO monthly index fluctuations have a form close to a Gaussian, for all time lags, in agreement with the findings of the scaling analyses. This indicates the lack of predictive power of the present NAO monthly index. Yet there are some deviations for large (and thus rare) events. Whence suggestions for other measurements are made if some improved predictability of the weather/climate in the North Atlantic is of interest. The subsequent Langevin equation of the NAO signal fluctuations is explicitly written in terms of the diffusion and drift parameters, and a characteristic time scale for these is given in appendix.Comment: 6 figures, 54 refs., 16 pages; submitted to Int. J. Mod. Phys. C: Comput. Phy