Filippos Vallianatos

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.

Use of engineering geophysics to investigate a site for a building foundation

The combination of geophysical data and geotechnical measurements may greatly improve the quality of buildings under construction in civil engineering. A case study is presented here at a vacant building site. Initially, boreholes indicated a complex geology. A dipole–dipole configuration was selected for electrical resistivity tomography (ERT) implementation and the data were processed and interpreted by applying 2D and 3D inversions. An electromagnetic survey was also carried out at a different time periods and successfully used to verify the results of the resistivity measurements. It is demonstrated that engineering geophysics is able to provide solutions for determining subsurface properties and that different prospection techniques are necessary for developing a reasonable model of the subsurface structure.

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.

Monofractal and multifractal approaches in investigating scaling properties in temporal patterns of the 1983–2000 seismicity in the western Corinth graben, Greece

Abstract Much recent work has been focused on understanding the statistical properties of time-occurrence series of earthquakes. Investigating into the patterns of seismic sequences reveals evidence of scaling features in temporal behavior. This is shown in the clustering properties of seismicity of the western Corinth graben, Greece from 1983 to 2000. Power-law behavior has been found by means of Allan factor analysis and Fourier spectra for the earthquake sequence, with consistent values for the scaling exponents, that decrease with the threshold magnitude. The analysis of the temporal variation of the scaling exponent, performed with different threshold magnitudes, reveals an enhancing of the clusterization in correspondence to larger events. The multifractal analysis of the temporal distribution of the events has shown a decrease of the intermittent character with the threshold magnitude.

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.

Non-extensivity of the isothermal depolarization relaxation currents in uniaxial compressed rocks

The isothermal depolarization current relaxation in uniaxial compressed rocks follows a generalized exponential function which explicitly introduces hierarchically constrained dynamics and macroscopic interactions. The interactions are associated with the non-extensive entropy parameter q and exhibits a behavior indicating a scaling with normalized uniaxial stress Sigma = sigma/sigma(Y), where sigma(Y) is the yield stress where deviation from the elastic region starts in a stress-strain curve. Combining ideas of Levy and Tsallis statistics we argue the remarkable result that a Levy-walk-type mechanism can organize the geometry of the heterogeneous system to criticality. The stress-dependent q-estimation leads to the conclusion that fracturing is a subextensive process with strong interaction. Copyright (C) EPLA, 2011

A non-extensive statistical physics view to the spatiotemporal properties of the June 1995, Aigion earthquake (M6.2) aftershock sequence (West Corinth rift, Greece)

In the present study, the spatiotemporal properties of the Aigion earthquake (15 June 1995) aftershock sequence are being studied using the concept of non-extensive statistical physics (NESP). The cumulative distribution functions of the inter-event times and the inter-event distances are being estimated for the data set which is assumed to be complete and the analysis yielded the thermodynamic q parameter to be qT = 1.58 and qr = 0.53 for the two distributions, respectively. The results fit rather well to the inter-event distances and times distributions, implying the complexity of the spatiotemporal properties of seismicity and the usefulness of NESP in investigating such phenomena. The temporal structure is also being discussed using the complementary to NESP approach of superstatistics, which is based on a superposition of ordinary local equilibrium statistical mechanics. The result indicates that very low degrees of freedom describe the temporal evolution of the Aigion earthquake aftershock seismicity.

Self-similarity properties of seismicity in the Southern Aegean area

We investigate the time self-similarity properties of seismicity in the Hellenic are, which is one of the most rapidly deforming and seismically active regions of the Mediterranean area. The presence of time-scale-invariance properties in the shallow and intermediate depth seismicity is revealed. In particular, 1/f(beta) fluctuations are detected from the analysis of earthquake sequences using the counting statistics methodology. The shallow seismicity is characterized by a time clustering effect and shows a beta value that is a typical fingerprint of self-similar behaviours, whereas the intermediate seismicity can be considered as a realization of a purely poissonian process. The investigations cover instrumentally recorded earthquakes occurring between 1964 and 1996, and the analyses have been carried out, selecting different magnitude thresholds. The beta value for shallow seismicity decreases from almost 0.5 to 0.2 as the threshold magnitude increases from 3.3 to 4.5

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

Seismogenic radioemissions as earthquake precursors in Greece

A network has been constructed along Crete island (Greece) for earthquake prediction using seismogenic electromagnetic emissions. The network consists of four observation points. Each observation point has: (a) loop antennas tuned to 3 and 10 kHz. (low frequencies-LF). The final stage of the antennas is an RMS to DC converter. Thus the output of the receiver is a DC voltage that is proportional to the power spectrum density ΦH of the magnetic field that excites the antenna, (b) λ2 dipoles to observe variations in 41 and 53 MHz (high frequencies-HF). In this stage the DC output is proportional to the electric field which appears in the λ2 antenna. In the present contribution recordings of the electromagnetic anomalies that precede earthquakes during the last three years are presented. The experimental results indicate the presence of radioemissions, associated to shallow and intermediate depth earthquakes in the vicinity of Crete island. It is concluded that the electromagnetic variations appear to follow an invariant time pattern (i.e. LF variations - HF variations - Earthquake event).