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The VAN method – named after P. Varotsos, K. Alexopoulos and K. Nomicos, authors of the 1981 papers describing it〔. and (pp. 32, 36) cite a paper – "Electric signals predicting earthquakes" – by the same authors said to be have been presented at the 1981 (4th International Conference on Basement Tectonics ), but the record shows no such paper. See also , p. 512.〕 – measures suspected "seismic electric signals" (SES), by which Varotsos and several colleagues claim to have successfully predicted earthquakes in Greece.〔 (preface to a special edition about VAN); . See also .〕 Both the method itself and the manner by which successful predictions are claimed have been severely criticized;〔; (proceedings of a conference that reviewed VAN); ; and twenty articles in a special issue of Geophysical Research Letters ((table of contents )).〕 the ''International Commission on Earthquake Forecasting for Civil Protection'' declared: "subsequent testing has failed to validate the optimistic SES prediction capability claimed by the authors".〔.〕 The VAN method was revised in 2001, and this new method has been independently reproduced.〔.〕 It awaits further independent verification. ==Description of the VAN method== Prediction of earthquakes with this method is based on the detection, recording and evaluation of seismic electric signals or SES. These electrical signals have a fundamental frequency component of 1 Hz or less and an amplitude proportional to the magnitude of the earthquake.〔; ; .〕 According to VAN proponents, SES are emitted by rocks under stresses caused by plate-tectonic forces. There are three types of reported electric signal:〔.〕 *Electric signals that occur at the same time as a major earthquake. Signals of this type were recorded 6.5 hours before the 1995 Kobe earthquake in Japan, for example.〔.〕 *Electric signals that occur some time before a major earthquake. *A gradual variation in the Earth's electric field some time before an earthquake. Several hypotheses have been proposed to explain SES: *Stress-related phenomena: Seismic electric signals are perhaps attributed to the piezoelectric behaviour of some minerals, especially quartz, or to effects related to the behavior of crystallographic defects under stress or strain. These may appear a few weeks before an earthquake, when the mechanical stress reaches a critical value.〔.〕 The generation of electric signals by minerals under high stress leading to fracture has been confirmed with laboratory experiments.〔; .〕 *Thermoelectric phenomena: Alternately, Chinese researchers proposed a mechanism which relies on the thermoelectric effect in magnetite.〔.〕 *Groundwater phenomena: Three mechanisms have been proposed relying on the presence of groundwater in generating SES. The electrokinetic effect is associated with the motion of groundwater during a change in pore pressure.〔.〕 The seismic dynamo effect is associated with the motion of ions in groundwater relative to the geomagnetic field as a seismic wave creates displacement. Circular polarization would be characteristic of the seismic dynamo effect, and this has been observed both for artificial and natural seismic events.〔.〕 A radon ionization effect, caused by radon release and then subsequent ionization of material in groundwater, may also be active. The main isotope of radon is radioactive with a half-life of 3.9 days, and the nuclear decay of radon is known to have an ionizing effect on air. Many publications have reported increased radon concentration in the vicinity of some active tectonic faults a few weeks prior to strong seismic events.〔.〕 However, a strong correlation between radon anomalies and seismic events has not been demonstrated.〔.〕 While the electrokinetic effect may be consistent with signal detection tens or hundreds of kilometers away, the other mechanisms require a second mechanism to account for propagation: *Signal transmission along faults: In one model, seismic electric signals propagate with relatively low attenuation along tectonic faults, due to the increased electrical conductivity caused either by the intrusion of ground water into the fault zone(s) or by the ionic characteristics of the minerals.〔.〕 *Rock circuit: In the defect model, the presence of charge carriers and holes can be modeled as making an extensive circuit.〔.〕 Seismic electric signals are detected at stations which consist of pairs of electrodes (oriented NS and EW) inserted into the ground, with amplifiers and filters. The signals are then transmitted to the VAN scientists in Athens where they are recorded and evaluated. Currently the VAN team operates 9 stations, while in the past (1999) they could afford up to 17.〔.〕 The VAN team claimed that they were able to predict earthquakes of magnitude larger than 5, with an uncertainty of 0.7 units of magnitude, within a radius of 100 km, and in a 2-hour to 11-day time window. Several papers confirmed this success rate, but the earthquake sample size was too small to make a statistically significant conclusion.〔.〕 For example, there were eight M ≥ 5.5 earthquakes in Greece from January 1, 1984 through September 10, 1995, and the VAN network claims to have forecast six of these.〔.〕 The VAN method has also been used in Japan,〔.〕 but in early attempts success was "difficult" to attain.〔.〕 A preliminary study of sites for investigation of seismic electric signals in France was abandoned based on criticism of the first VAN method.〔.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「VAN method」の詳細全文を読む スポンサード リンク
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