Stochastic simulation of volcanic tremor from Ruapehu

The most common volcanic tremor produced by Ruapehu is a continuous signal with a dominant frequency of about 2 Hz. This signal has a sharply peaked spectrum, and an autocorrelation function with a high degree of coherence, even for lags of over 20 seconds. These characteristics strongly suggest that the cause of this tremor is a single resonator, probably a fluid-filled cavity resonating in an “organ-pipe” mode.The stochastic simulation of such a resonator uses the equation of motion of a Simple Harmonic Oscillator, which applies to an “organ-pipe” fundamental resonance, with either the characteristics of the oscillator, or the forcing function, containing a random element. A “white noise” forcing function, which would be appropriate for excitation of the cavity by a high pressure gas input, gave good agreement with the observed spectra and autocorrelation functions. Another possible model used an oscillator with a damping factor which varied randomly, and was sometimes negative, so oscillations built up, rather than decayed. This also gave a reasonable simulation of Ruapehu tremor.The third excitation model used a Poisson process, in which during each time interval there was a certain probability of applying a fixed impulse to the resonator. It was found that the impulses had to be frequent, i.e. several times a second, to match the characteristics of Ruapehu tremor.It has been suggested that tremor is composed of a succession of low-frequency (“B-type”) earthquakes. The results of this simulation show that at Ruapehu tremor could be produced by a resonator with positive feedback just sustaining oscillation, or by a resonator excited by external impulses. The most promising model for low-frequency earthquakes describes them as the result of a major external disturbance of the resonator.     

Theoretical issues in the ‘food desert’ debate and ways forward

Food is essential to life—yet the spatial and economic configuration of the conventional food system does not meet nutritional needs and exacerbates issues of food insecurity. Relevant options for policy change have been explored in light of evaluations of geographic disparities in food access, but the dominant ‘food desert’ discourse often focuses uncritically on insufficient conceptions of access. Understanding the complexity of food deserts is important for moving into meaningful policy action. We present a theoretical position to inspire future empirical research. The ecological model recognizes both endogenous and built environment factors in shaping health. Interventions in the food environment, however, often concentrate exclusively on structural determinants of health (e.g. retail-based initiatives). Yet retail-based interventions are difficult to implement due to governance systems which limit the ability of government bodies to influence private retail development. As well, recognizing the complexity of debates over the influence of structure and agency, we apply structuration theory to food deserts. Behavioral economics further informs both structural and behavioral determinants of health. This approach sidesteps the issue of victim-blaming, as all consumers are viewed as ‘predictably irrational’ in decision-making. In combining these theories, we challenge methodological and theoretical assumptions by showing the complexity of food desert interventions. Policy recommendations focus on behavioral determinants of health and the opportunities for empowerment through local food systems. These recommendations recognize the limits of translating research into policy and in devising effective food based interventions, and are sensitive to social, economic, and political constraints uncovered throughout the paper.     

Extraordinary heat during the 1930s US Dust Bowl and associated large-scale conditions

Unusually hot summer conditions occurred during the 1930s over the central United States and undoubtedly contributed to the severity of the Dust Bowl drought. We investigate local and large-scale conditions in association with the extraordinary heat and drought events, making use of novel datasets of observed climate extremes and climate reanalysis covering the past century. We show that the unprecedented summer heat during the Dust Bowl years was likely exacerbated by land-surface feedbacks associated with springtime precipitation deficits. The reanalysis results indicate that these deficits were associated with the coincidence of anomalously warm North Atlantic and Northeast Pacific surface waters and a shift in atmospheric pressure patterns leading to reduced flow of moist air into the central US. Thus, the combination of springtime ocean temperatures and atmospheric flow anomalies, leading to reduced precipitation, also holds potential for enhanced predictability of summer heat events. The results suggest that hot drought, more severe than experienced during the most recent 2011 and 2012 heat waves, is to be expected when ocean temperature anomalies like those observed in the 1930s occur in a world that has seen significant mean warming.