The hazard of the counterfeit E1′ signal in quartz to the ESR dating of fault movements

We have previously found (Toyoda and Schwarcz, in press) that γ-ray irradiation of granitic quartz produces a distinct ESR signal which overlaps the E1′ centre signal; this signal is removed by a heat treatment at 170°C for 15 min. Here we report the presence of this ‘counterfeit’ E1[′ signal in irradiated samples from fault gouge taken from the Hayward fault California. The contribution of the counterfeit E1′ signal to the total apparent E1′ centre signal intensity is significant and appears to account for most of the growth of the combined signal with irradiation. The intensity of the ‘real’ E1′ centre signal rises abruptly between irradiation doses of 300 and 1000 Gy, after removing the contribution of the counterfeit signal by heating.