Simon Klemperer, Anthony Fraser-Smith, Paul McGill, Arman Bernardi, Jonathan Glen, Darcy Karakelian McPhee

Thomas et al. (2009) (hereafter TLJ09) attempt to cast doubt upon the report by FraserSmith et al. (1990) (hereafter FS90) of unusual large-amplitude ultra-low frequency (ULF; f < 10 Hz) magnetic fields preceding the M ~7 Loma Prieta earthquake. TLJ09 suggest there are instrumental problems with FS90’s data that we, the designers, builders and long-term operators of the equipment, are presumed to have been unable to identify. We refute these claims and demonstrate that TLJ09’s conclusion, that the large-amplitude magnetic fields were an artifact of faulty amplifiers, is false. In contrast, we believe the most straightforward interpretation of the limited available data (FS90), including the single most compelling feature – 100 times above background levels, appearing just three hours before the earthquake, but barely mentioned by TLJ09 – is as a true earthquake precursor.