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We present some comments on two issues dealt with in Johnston et al. (2006), which reported that there were no electric and magnetic (EM) precursors for the 28 September 2004 M 6.0 Parkfield earthquake (EQ). The first comment is on their introduction, which, while raising critical comments on seismic electric signals (SESs) in Greece and Japan, concluded that useful prediction of EQs seems unlikely using electromagnetic data. We show point by point that their criticism is unfounded. The second comment is on their assertion of the nonexistence of SESs at Parkfield. We point out that this assertion may be premature as their efforts to identify signals appear insufficient. They show, in a timescale too coarse to recognize SESs, only band-passed magnetic and electric data for the five day period before the Parkfield EQ, which is much too short. Even if the claimed nonexistence of an electric precursor is demonstrated by further data analysis, the probability will remain that their single observation site was insensitive to SESs. They apparently have made no search effort for SES sensitive sites. Our experiences show that SES insensitive sites are not uncommon even near epicenters. On these grounds, their overall negative attitude on electromagnetic methods seems much too premature and inappropriate. Even if no EM precursors were generated for the Parkfield EQ, we suggest it would be more scientifically sound to seek the physical reason for it before negating SESs in general, which have been well observed elsewhere. Johnston et al. (2006) (J2006, hereafter), in their introduction, list four reasons for doubting the observations of SESs in Greece and Japan. Although this portion seems to have rather little to do with their own work, it demonstrates their basic negative stance against the VAN method (which is a method for short term earthquake prediction by monitoring … |
