Investigations on the properties and estimation of earth response operators from EM sounding data

Incl. 3 reprints at backAvailable from British Library Document Supply Centre- DSC:D82993 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

On the nature, scaling and spectral properties of pre-seismic ULF signals

International audienceEarlier work by the authors (Vallianatos and Tzanis, 1999b), has proposed a model for the propagation and scaling of electric earthquake precursors, according to which the pre-seismic electric field emission is due to some time dependent polarisation appearing in an ensemble of electrified crustal volumes within the seismogenic source, which are distributed according to a fractal power law. Herein, we extend this formulation to the analysis of ULF magnetic precursors. We calculate the resulting transient magnetic field, which turns out to be mainly vertical and observable only if the seismogenic process generates a source with polarization rate perpendicular to the vertical plane through the source and the receiver. Furthermore, a scaling law between the vertical magnetic field and the magnitude of the associated earthquake is provided. We also investigate the spectral distribution law expected from such a set of emitters. To this effect, we assume that the evolution of the precursory polarisation process is quasi-incoherent over the exited ensemble, i.e. there is no unique relaxation time, but rather a spectrum of these with energy dependence expressed by an Arrhenius law with uniformly distributed energies. We show that the macroscopic ULF field resulting from the superposition of such an ensemble of sources has a power density spectrum distributed proportionally to 1/f . The above theoretical prediction appears to be consistent with independent observations by other investigators

High resolution spectral characteristics of the Earth-ionosphere cavity resonances

The natural resonances of the Earth-ionosphere cavity at frequencies between 5 and 100 Hz have been studied since the fundamental paper by Schumann. While the gross features of the phenomena are now well understood, considerable work remains to be done on their detailed behaviour. In the present study a high resolution, data adaptive spectral technique is applied to digital electromagnetic data obtained at a moderate latitude. A particular feature of the method employed is that spectral properties become available on the same time scale as many ELF events, thus both time local and time averaged resonance features can be readily established. The technique can thus be applied to both dynamic and steady-state descriptions of the cavity's properties. For the data set considered, the technique adequately resolves the first six resonance modes on a time scale of 0.75 s. The presence of higher order modes is also indicated. The time averaged frequencies obtained are in accord with those of previous experimental determinations. When the time local properties of individual transient waveforms are examined, however, we observe a number of detailed effects which are predicted by theory. The precise spectral structure of the resonance modes appears influenced by the differing locations of the sources of transient excitation. In the case of the first order resonance mode, the properties of the cavity consistently support both singlet and doublet resonance behaviour

Distributed power-law seismicity changes and crustal deformation in the SW Hellenic ARC

A region of definite accelerating seismic release rates has been identified at the SW Hellenic Arc and Trench system, of Peloponnesus, and to the south-west of the island of Kythera (Greece). The identification was made after detailed, parametric time-to-failure modelling on a 0.1° square grid over the area 20° E – 27° E and 34° N–38° N. The observations are strongly suggestive of terminal-stage critical point behaviour (critical exponent of the order of 0.25), leading to a large earthquake with magnitude 7.1 ± 0.4, to occur at time 2003.6 ± 0.6. In addition to the region of accelerating seismic release rates, an adjacent region of decelerating seismicity was also observed. The acceleration/deceleration pattern appears in such a well structured and organised manner, which is strongly suggestive of a causal relationship. An explanation may be that the observed characteristics of distributed power-law seismicity changes may be produced by stress transfer from a fault, to a region already subjected to stress inhomogeneities, i.e. a region defined by the stress field required to rupture a fault with a specified size, orientation and rake. Around a fault that is going to rupture, there are bright spots (regions of increasing stress) and stress shadows (regions relaxing stress); whereas acceleration may be observed in bright spots, deceleration may be expected in the shadows. We concluded that the observed seismic release patterns can possibly be explained with a family of NE-SW oriented, left-lateral, strike-slip to oblique-slip faults, located to the SW of Kythera and Antikythera and capable of producing earthquakes with magnitudes MS ~ 7. Time-to-failure modelling and empirical analysis of earthquakes in the stress bright spots yield a critical exponent of the order 0.25 as expected from theory, and a predicted magnitude and critical time perfectly consistent with the figures given above. Although we have determined an approximate location, time and magnitude, it is as yet difficult to assert a prediction for reasons discussed in the text. However, our results, as well as similar independent observations by another research team, indicate that a strong earthquake may occur at the SW Hellenic Arc, in the next few years

Beyond Axisymmetry in Tokamak Plasmas

H-mode tokamak plasmas are characterised by quasi-periodic instabilities, called edge localised modes (ELMs), driven by unstable peeling-ballooning modes inside the pedestal region. For large scale tokamaks, like ITER, the resulting particle and heat fluxes are predicted to be unacceptable and ELM control methods are required. One promising method relies on the application of 3D resonant magnetic perturbations (MPs), and ELM mitigation or even complete suppression is observed. A computational framework is presented that aims to understand the effect of MPs on both plasma equilibria and stability. The ELITE stability code is used to find the linearised plasma response, i.e. the 3D part of the equilibrium, and compute the axisymmetric peeling-ballooning eigenmodes. This information is used to calculate the 3D stability under a perturbative and a variational formulation of the MHD energy principle. In practice, the axisymmetric peeling-ballooning modes are used as trial functions for the minimisation of the 3D energy functional. The symmetry breaking of the toroidal geometry leads to the coupling of toroidal modes which has a direct impact on the linear growth rates of unstable peeling-ballooning modes. This mechanism results in the modification of the plasma stability above a critical value of the applied MP field and field-line localisation of the peeling-ballooning eigenmode. It is observed that intermediate to high n ballooning modes are in general destabilised by the applied MP field, while external peeling-ballooning modes reorganise to an internal ballooning structure. In addition, extrema in the growth rate spectrum, due to low n kink modes, are observed to be strongly destabilised as predicted by perturbation theory. This work provides proof of principle examination of the 3D peeling-ballooning instability as well as a framework for the optimisation of MP coil configuration