Electromagnetic variations
Various attempts have been made to identify possible pre-seismic indications in electrical, electric-resistive, or magnetic phenomena.[49] The most touted, and most criticized, is the VAN method of professors P. Varotsos, K. Alexopoulos and K. Nomicos – "VAN" – of the National and Capodistrian University of Athens. In a 1981 paper[50] they claimed that by measuring geoelectric voltages – what they called "seismic electric signals" (SES) – they could predict earthquakes of magnitude larger than 2.8 within all of Greece up to 7 hours beforehand. Later the claim changed to being able to predict earthquakes larger than magnitude 5, within 100 km of the epicentral location, within 0.7 units of magnitude, and in a 2-hour to 11-day time window.[51] Subsequent papers claimed a series of successful predictions.[52] However, the VAN group generated intense public criticism in the 1980s by issuing telegram warnings, a large number of which were false alarms.
Objections have been raised that the physics of the VAN method is not possible. None of the earthquakes which VAN claimed were preceded by SES generated SES themselves, as would have been expected. Analysis of the wave propagation properties of SES in the Earth’s crust showed that it would have been impossible for signals with the amplitude reported by VAN to have been transmitted over the several hundred kilometers distances from the epicenter to the monitoring station.[53] In addition, VAN’s publications do not account for (i.e. identify and eliminate) possible sources of electromagnetic interference (EMI). Taken as a whole, the VAN method has been criticized as lacking consistency in the statistical testing of the validity of their hypotheses.[54] In particular, there has been some contention over which catalog of seismic events to use in vetting predictions. This catalog switching can be used to conclude that, for example, of 22 claims of successful prediction by VAN[55] 74% were false, 9% correlated at random and for 14% the correlation was uncertain.[56]
In 1996 the journal Geophysical Research Letters presented a debate on the statistical significance of the VAN method;[57] the majority of reviewers found the methods of VAN to be flawed, and the claims of successful predictions statistically insignificant.[58] In 2001, the VAN method was modified to include time series analysis, and Springer published an overview in 2011.[59]
Further information: VAN method
Further information: VAN method
After the 1989 Loma Prieta earthquake occurred, a group led by Antony C. Fraser-Smith of Stanford University reported that the event was preceded by disturbances in background magnetic field noise as measured by a sensor placed in Corralitos, California, about 4.5 miles (7 km) from the epicenter.[60] From 5 October, they reported a substantial increase in noise in the frequency range 0.01–10 Hz. The measurement instrument was a single-axis search-coil magnetometer that was being used for low frequency research. Precursory increases of noise apparently started a few days before the earthquake, with noise in the range .01–.5 Hz rising to exceptionally high levels about three hours before the earthquake. Though this pattern gave scientists new ideas for research into potential precursors to earthquakes, and the Fraser-Smith et al. report remains one of the most frequently cited examples of a specific earthquake precursor, more recent studies have cast doubt on the connection, attributing the Corralitos signals to either unrelated magnetic disturbance[61] or, even more simply, to sensor-system malfunction.[62]