Dynamics of movie competition and popularity spreading in recommender systems.

We introduce a simple model to study movie competition in recommender systems. Movies of heterogeneous quality compete against each other through viewers' reviews and generate interesting dynamics at the box office. By assuming mean-field interactions between the competing movies, we show that the runaway effect of popularity spreading is triggered by defeating the average review score, leading to box-office hits: Popularity rises and peaks before fade-out. The average review score thus characterizes the critical movie quality necessary for transition from box-office bombs to blockbusters. The major factors affecting the critical review score are examined. By iterating the mean-field dynamical equations, we obtain qualitative agreements with simulations and real systems in the dynamical box-office forms, revealing the significant role of competition in understanding box-office dynamics.

P-wave teleseismic tomography : Contribution to the delineation of the upper mantle structure of Italy

In recent years seismic tomography has emerged as a powerful tool to study the crustal and upper mantle structure and to constrain kinematic models of tectonic evolution in regions of plate margins. We applied a well established and robust inversion technique ("ACH" technique) to compute a three-dimensional model of the compressional wave velocity in the lithosphere-asthenosphere system beneath Italy. We used a high quality dataset of about 4700 selected arrival times of teleseismic events digitally recorded at the National Seismic Network. The results show a substantial improvement, in terms of variance reduction, model resolution, and standard errors, with respect to similar inversions (same technique, same starting model, same station and event distribution) performed with data derived directly from bulletins. Our interpretation of the computed velocity model, mainly based on the observation of regions of positive velocity anomalies (up to about 5 percent), substantiates the hypothesis that a complex subduction system has developed beneath the Italian peninsula. The most prominent high velocity regions are the Alps, the northern Apennines, and the southern Tyrrhenian sea. Low velocity regions are found in the upper mantle of the Adriatic microplate and beneath Sicily. Some of the high velocity regions are also characterized by intermediate and deep seismicity, whereas some others are apparently aseismic (like the Alps), mainly depending on the age of subduction. In the southern Tyrrhenian sea, where the subduction is still active, a high velocity zone as deep as about 500 km is associated with the well known seismicity related to the Wadati-Benioff zone. In the northwestern portion of the Apennines, a strong high velocity anomaly is observed in the upper 200-250 km. We found that well located subcrustal earthquakes as deep as 90 km occur in the same region, gently dipping to the southwest. We propose that they delineate the upper portion of the Adriatic lithosphere sinking beneath the Tuscany-Corsica block, whose deepest part is depicted by the high velocity anomaly. The high velocity becomes weaker and deeper moving southeastward along the Apenninic chain, suggesting that the subducted slab in no continuous at depth. The complex geometry of the subducted lithosphere may be due to the stretching of the downgoing slab in response to arc migration, and/or to an irregular geometry of the two converging plates