We show that the entropy S, defined as S≡⟨χ0.2em0exln0.2em0exχ⟩-⟨χ⟩ln⟨χ⟩ [Phys. Rev. E 68, 031106 (2003)] where χ stands for the natural time [Phys. Rev. E 66, 011902 (2002)], exhibits positivity and concavity as well as stability or experimental robustness. Furthermore, the distinction between the seismic electric signal activities and ``artificial'' noises, based on the classification of their S values, is lost when studying the time-reversed signals. This reveals the profound importance of considering the (true) time arrow.
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Varotsos et al. (2005) studied this question.
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