A. Tzanis

Electric current generation associated with the deformation rate of a solid: Preseismic and coseismic signals

Abstract We present a model for the generation of electric current in rocks under stress, involving the strain rate, (e) which is influenced by the motion of charge bearing dislocations. The relationship between current density and strain rate is demonstrated. On the basis of laboratory data, we estimate the deformation rate necessary to generate an electric signal observable at distances far enough from the source, as to qualify it as an electric earthquake precursor. Using this mechanism and the geometrical characteristics of such a type of source we simulate the propagation of the electric signal and its ‘received’ characteristics as a function of the source-receiver separation. We conclude that the expected signal waveforms at long distances from such a kind of source are similar to a class of signals (bay like waveforms), independently observed prior to earthquakes by several investigators.

Identification and discrimination of transient electrical earthquake precursors: fact, fiction and some possibilities

Abstract The possibility of electrical earthquake precursors (EEPs) has long been appreciated, but to date there still exists neither a solid theory to describe their generation and expected characteristics, nor proven techniques to identify and discriminate true precursors from noise. Experimental studies have produced a prolific variety of signal shape, complexity and duration, but no explanation for the apparently indefinite diversity. Statistical analyses on the basis of such poorly constrained data were inconclusive, leading to scepticism and intense debate. The most objective means of EEP identification would be to construct generic models of their source(s) and compare the model predictions with field observations. We attempt to show the merits of this approach with two studies. The first study expands on the phenomena of spontaneous electric field generation during crack propagation (microfracturing), demonstrated by laboratory experiments. Large-scale microfracturing may appear at the terminal stage of earthquake preparation. We apply a generic, qualitative model, based on a kinetic theory of crack interaction and propagation. The model suggests that EEP signals from such a type of source may have a limited class of permissible waveforms, with characteristic bay- or bell-shaped curves of variable width and duration. We provide two examples consistent with this model: the VAN claims of precursors on 15/1/1983 and 18/1/1983. The magnetic field that may accompany an anomalous electric signal is the subject of the second study. This has been a grossly overlooked quantity, although valuable for identification and discrimination, because it is considerably less sensitive to distortion than the electric field, less sensitive to inhomogeneities along the propagation path, insensitive to the local geoelectric structure and sometimes, telltale of the source (for instance, external magnetic fields can only be generated by (sub)horizontal current configurations). We investigate the 18/4/1995 and 19/4/1995 electric and magnetic signals observed at Ioannina, Greece, used for the prediction of the 13/5/1995 M6.6 Kozani event by the VAN group. The electric and magnetic waveforms are inconsistent with the crack propagation model. By their observed characteristics, the magnetic signals preclude any (sub)vertical electrokinetic current. Using analytic formulations, we investigate whether they might have been generated by an electrokinetic source across a lateral interface, either at the focal area or locally, at Ioannina. We conclude that the magnetic field properties are also inconsistent with such a type of source. Conversely, we cannot rule out their local industrial origin. The examples presented herein indicate that the successful identification and discrimination of EEP and noise may be possible by working out plausible theories of the source.

Identification and analysis of electromagnetic signals in Greece: The case of the Kozani Earthquake VAN Prediction

An electric station was installed in July 1993, 4.5 km away from the VAN station of Ioannina, and recorded in the following two years a number of anomalous signals, including those of April 18 and 19, 1995, interpreted by the VAN group as Seismic Electric Signals precursors to the May 13 Kozani earthquake (West-Macedonia). A magnetic station was also installed and clearly recorded the magnetic components of the same events. The amplitude, shape, characteristic pattern and duration, magnetic characteristics and polarisation of the anomalous signals suggest that they are generated by artificial (industrial) sources.

On possible scaling laws between electric earthquake precursors (EEP) and earthquake magnitude

We assume, without reference to any particular electrification mechanism, that a pre-seismic, time dependent polarization appears in a number of spherical volumes distributed in some earthquake preparation zone embedded in a half space of constant resistivity. We estimate the resulting transient electric and magnetic fields in the quasi-static approximation. On assuming that the number of polarized spheres N is scaling with their radii l as N∝1/lD, we show that at some distance r from the zone, the electric field and the magnitude of the earthquake are related as LogE=αM+C, where α=(3-D)/2 and similarly for the magnetic field. Fragmentation experiments and theoretical simulations indicate that 2.2 ≤ D ≤2.6, yielding 0.4>α>0.2. The lower fractal dimensions correspond to the case of dynamic crack propagation. Letting D≈2.3, yields α≈0.35 which is comparable to the experimental value of 0.35 given by Varotsos and Alexopoulos, (1984) on the basis of a few earthquake sequences in Greece. This indicates that electric and magnetic earthquake precursors may obey scaling laws that are direct consequence of the fractal distribution of their generators and also implies that transient precursors may result from microfracturing and fragmentation processes in the earthquake preparation zone.

Seismic and electrical precursors to the 17-1-1983, M7 Kefallinia earthquake, Greece: Signatures of a SOC system

Prior to the 17-1-1983 event, the seismicity of the broader area of die Ionian islands and western Greece exhibited several phenomena interpretable in the context of a self-organised critical system with long range interactions. The regional seismic energy release exhibited power law acceleration towards the time of rupture, the numerical modelling of which yields a time-to-failure of 1983.1+/-0.2. Time dependent changes were also observed in the b-values, assuming the form of monotonic increase that promptly reversed after the earthquake. This indicates the induction of instability to the region due to the earthquake preparation process, which is consistent with the critical point earthquake model. The critical point model predicts that failure is a co-operative effect occurring at small scale, and cascading from the microscopic to the macroscopic scale. This involves a crack propagation avalanche at the terminal phase of the seismic cycle, the time function of which has been modelled with a limited class of characteristic transient bay-like shapes, featuring a corner frequency and inverse power energy distribution law. Electrification processes due to crack propagation may generate an electrical precursor with similar characteristics. Such a potential precursor has been observed independently on 15-1-1983, approx. 120km from the epicentre. In consequence of our observations, we discuss a model relating seismicity and electrical precursors

Seismic and electrical anisotropy in the Mornos Delta, Gulf of Corinth, Greece, and its relationship with GPS strain measurements

Shear-wave splitting studies on seismo-grams from local earthquakes at the site of the Mornos delta (Gulf of Corinth, Greece) has revealed a clear seismic anisotropy, with a fast S polarization striking N55 + 25 o. Magneto-telluric soundings in the frequency range 0.01 to 100 Hz on the delta showed a clear electric anisotropy, with a N55 o + 10 o direction for the highest frequency. This anisotropy spans through the whole layer of sediments, about 1 km thick. Comparison of the 1966-1972 triangulations and the 1991-1995 GPS positions of geodetic points in and around the delta showed rapid extension strain (2x10-S/year) in the direction N340o + 30 o, perpendicular to the fast S and the highest conductivity directions, suggestive of a causal relationship between these observations. We thus propose that this strain controls the two reported anisotropies, by the formation and maintaining of fluid filled, steeply dipping antithetic faults and fractures, and fluid filled vertical cracks, all striking N55 o 4-20 o. The source of strain is likely to be active normal faulting near and under the delta, as independently evidenced by the recent discovery of nearby offshore faults, striking about N60 o, significantly different from the dominant E-W strike of the major faults of the Gulf.

Comparative Study of Microtremor Analysis Methods

During a multidisciplinary microzonation pilot project in the city of Heraklion (Crete, Greece), microtremor data were collected at the top of exploratory boreholes specifically designed for the purposes of the project, over a period of 5 days, for 4 h/day at 125 Hz (continuous recordings). The data were analysed with the SSR and H/V Ratio techniques, using the standard FFT (applied to long data series) and a Multi-variate Maximum Entropy (MV-MAXENT) spectral analysis method. Both techniques, implemented with both spectral analysis methods, identify the same major resonance frequency band, albeit with different amplification levels. The MV-MAXENT however is effective in handling short data lengths while yielding high resolution spectra and addressing several shortcomings of the conventional FFT (windowing, zero padding etc.). Thus, it yields competitively similar results, with only a fraction (a few minutes) of the data required by the lower resolution (FFT) method and appears to be a powerful tool for site effect investigations. Moreover, the results of both microtremor-based techniques are consistent and remarkably similar to the results of microzonation methods that require (expensive) borehole data.

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 cavitys 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.

Magnetotellurics and seismotectonics in the analysis of active domains: An essential combination?

Abstract The geoelectric structural trends ( GST ) derived from the spatial analysis (rotation/decomposition) of MT and GDS data in the vicinity of active deformation zones, exhibit remarkable correlation with seismotectonic trends and nodal planes of earthquake mechanism solutions, in practically all the cases we have examined in Greece and abroad. In the upper crust of an active domain, the GST may be explained in terms of conductors formed by water infiltrating through fault planes and related discontinuities, aligned microcracks and anastomosing shear zones. If this interpretation is correct, the GST should be good indicators of the corresponding trends and location of active faulting. In this context, the spatial analysis of geoelectromagnetic data is important in constraining the geometry of active faulting and hence, the modes of deformation. Two characteristic examples are presented and discussed from the island of Milos, Greece, located in the Hellenic Volcanic Arc and experiencing extensional tectonics and the island of Terceira, the Azores, simultaneously experiencing normal and strike-slip faulting.

On the Problem of Identification and Discrimination of Electrical Earthquake Precursors

The possibility of electrical precursors to earth- quakes has long been appreciated, but to date there still exists neither a solid theory to describe the expected pre- cursory waveforms, nor proven techniques to identify and discriminate precursors from noise. In addressing the latter problem, the only published approach and criteria involve simultaneous observations on elaborate arrays of short and long dipoles (e.g. Varotsos & I.,a78ridou, Tectonophysics, 188, 321, 1991). It is shown tlmt these techniques are in- effective and can easily be deceived by local noise into identifying it as a distant signal. As an alternative ap- proach, we discuss that some problems in identifying local, anthropogenic noise can be addressed with simultaneous measurement in a number of simple and inexpensive dis- tributed stations operating as a network. In support of this point we present an example from an experiment in Ioan- nina : we have been able to successfully identify local noise with a single station recording the horizontal components of the telluric field in a two point configuration.