VAN method

The VAN method – named after P. Varotsos, K. Alexopoulos and K. Nomicos, authors of the 1981 papers describing it – measures low frequency electric signals, termed "seismic electric signals" (SES), by which Varotsos and several colleagues claimed to have successfully predicted earthquakes in Greece. Both the method itself and the manner by which successful predictions were claimed have been severely criticized and defended. The VAN method was advanced in 2001 by improving the accuracy of the estimation of the time window of the forthcoming earthquake through a new analysis, termed "natural time", and this new method has been independently reproduced. Research related to the VAN method is currently carried out at the Solid Earth Physics Institute, University of Athens, Greece.

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 the logarithm of which scales with 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 shortly before 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. Series of SES, termed SES activities (which are recorded before major earthquakes), may appear a few weeks to a few months 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 (1989) 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 time window ranging from several hours to a few weeks. Several papers confirmed this success rate, leading to 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 forecast six of these.

The VAN method has also been used in Japan, but in early attempts success comparable to that achieved in Greece was "difficult" to attain. A preliminary investigation of seismic electric signals in France led to encouraging results.

Earthquake prediction using "natural time" analysis

Since 2001 the VAN team has tried to improve the accuracy of the estimation of the time of the forthcoming earthquake. They introduced the concept of natural time, a time series analysis technique which puts weight on a process based on the ordering of events. They define a variance term κ in the time series analysis. Their current method detects SES as valid when κ = 0.070. That is the first part of their protocol. Once the SES are deemed valid, a second analysis is started. All the subsequent seismic (not electric) events are now noted, and the region is divided up as a Venn diagram with at least two seismic events per overlapping rectangle. When the distribution of κ for the rectangular regions has its maximum at κ = 0.070, the critical seismic event is imminent, i.e. it will occur in a few days to one week or so, and a prediction is issued.

Here is a brief introduction to the analysis. Two terms, χ and Q, characterize each event. χ is defined as k/N, where k is an integer (the k-th event) and N is the total number of events in the time sequence of data. Q is the energy released for each event. A related term, p, is the ratio Q/Q(total), and describes the fractional energy released. They introduce a critical term κ, defined as κ = [ Σ p(k) (k/N)^2 ] − [ Σ (k/N) p(k) ]^2, where κ is the variance in natural time, k is the event index, N is the number of events, Σ signifies summation, for k = 1 to N, and p(k) is the fractional quantity of energy for the k-th event. This variance puts extra weight on the energy term, p(k).

The VAN team published to have successfully predicted twenty five of the 28 major earthquakes from 2001 through 2010 in the region of latitude N 36° to N 41° and longitude E 19° to E 27° with this new analysis. Predictions are part of papers housed in arXiv, and new predictions are currently being made and uploaded there. A description of the updated VAN method was collected in a book published by Springer in 2011, titled "Natural Time Analysis: The New View of Time."

Results

Natural time analysis revealed an additional fact showing the physical connection of SES activities with earthquakes as follows: Taking the view that the earthquake occurrence is a phase-change (critical phenomenon) where the new phase is the mainshock occurrence, the above mentioned variance term κ is the corresponding order parameter. The κ value calculated for a window comprising a number of seismic events comparable to the average number of earthquakes occurring within a few months, fluctuates when the window is sliding through a seismic catalogue. It was found that these κ fluctuations exhibit a minimum a few months before a mainshock occurrence and in addition this minimum occurs simultaneously with the initiation of the corresponding SES activity. Such a simultaneous appearance of two precursory phenomena in independent datasets of different geophysical observables (electrical measurements, seismicity) has been observed for the first time in the literature. Furthermore the natural time analysis of the seismic catalogue of Japan during the period from January 1, 1984 until the occurrence of the magnitude 9.0 Tohoku earthquake on March 11, 2011, revealed that such clear minima of the κ fluctuations appeared before all major earthquakes with magnitude 7.6 or larger. The deepest of these minima was found to occur on January 5, 2011, i.e., almost two months before the Tohoku earthquake occurrence. Finally, by dividing the Japanese region into small areas, it has been found that some small areas show minimum of the κ fluctuations almost simultaneously with the large area covering the whole Japan and such small areas clustered within a few hundred kilometers from the actual epicenter of the impending major earthquake.

Criticisms of VAN

Historically, the usefulness of the VAN method for prediction of earthquakes had been a matter of debate. Both positive and negative criticism on the early conception of the VAN method is summarized in the 1996 book "A Critical Review of VAN", edited by Sir James Lighthill and in the debate issue of Geophysical Research Letters in 1996. This criticism predates the time series analysis methods introduced by the VAN group in 2001. The main points of the criticism were:

• Predictive success: The first article in 1981 by VAN reporting the observation of the correlation between SESs and earthquakes was later criticized by claiming that the list of earthquakes used for the correlation was inaccurate. VAN refuted this criticism by explaining that the data came from the preliminary bulletin of the National Observatory of Athens (NOA), which is expected to differ somewhat from the final NOA bulletin and does not affect the observed correlation between SES and earthquakes. An additional criticism was raised when examining the probability of chance correlation of 22 claims of successful predictions by VAN from January 1, 1987 through November 30, 1989. This criticism was also refuted by VAN showing that the methodology on which the criticism was based cannot be correct since it rejects even an ideal prediction method (i.e., all earthquakes above a threshold are successfully predicted, no false alarm).
• Proposed SES propagation mechanism: An analysis of the propagation properties of SES in the Earth’s crust showed that it is impossible that signals with the amplitude reported by VAN could have been generated by small earthquakes and transmitted over the several hundred kilometers between the epicenter and the receiving station. In effect, if the mechanism is based on piezoelectricity or electrical charging of crystal deformations with the signal traveling along faults, then none of the earthquakes which VAN claimed were preceded by SES generated an SES themselves. There is also some doubt that the phenomena of rock physics seen in certain laboratory conditions can be assumed to take place in the Earth’s seismogenic crust.
• Electromagnetic compatibility issues: VAN’s publications are further weakened by failure to address the problem of eliminating the many and strong sources of change in the magneto-electric field measured by them, such as telluric currents from weather, and electromagnetic interference (EMI) from man-made signals. One critical paper clearly correlates an SES used by the VAN group with digital radio transmissions made from a military base.
• Scientific reporting: Most importantly, the method is hindered by a lack of statistical testing of the validity of the VAN hypothesis because the researchers keep changing the parameters of the hypothesis (the moving goalpost technique).

• Public policy: Finally, one requirement for any earthquake prediction method is that, in order for any prediction to be useful, it must predict a forthcoming earthquake within a reasonable time-frame, epicenter and magnitude. If the prediction is too vague, no feasible decision (such as to evacuate the population of a certain area for a given period of time) can be made. In practice, the VAN group issued a series of telegrams in the 1980s, warning of impending earthquakes that did not occur, or did not occur within the parameters listed in the telegrams. During the same time frame, the technique also missed major earthquakes. These inaccurate predictions from the early VAN method led to public criticism and the cost associated with false alarms generated ill will. Major opponents of VAN were the Greek seismologists Vassilis Papazachos and G. Stavrakakis. The debate between Papazachos and the VAN team has repeatedly caused public attention in their home country Greece and has been extensively discussed in the Greek media.

See also

• Earthquake prediction
• Seismology
• Seismo-electromagnetics

References

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External links

• The News and Analysis section of the April 1998 issue of Scientific American discussed Varotsos and the VAN method ("On shaky ground").
• Nature debates. Is the reliable prediction of individual earthquakes a realistic scientific goal?
• VAN earthquake prediction method.
• P. Varotsos' bibliography (the principal source of "VAN" papers).