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Comment on the Ages in "Paleoseismology of the Johnson Valley, Kickapoo, and Homestead Valley Faults: Clustering of Earthquakes in the Eastern California Shear Zone" by T. K. Rockwell, S. Lindvall, M. Herzberg, D. Murbach, T. Dawson, and G. Berger

Physics Department
Simon Fraser University
Burnaby, British Columbia
V5A 1S6, Canada
(D.J.H.)

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The chronology given in this comment depends primarily on thermoluminescence and optical ages obtained for several samples. I present, in this comment, evidence that the samples dated are considerably older than the ages presented. The optical ages quoted in the article were obtained from data from my laboratory, but the presentation was incomplete. The authors have chosen not to include the information that severe anomalous fading was observed for the two samples for which measurements were made, and the optical ages should consequently have been presented as lower limits.

In either thermoluminescence dating or optical dating, one measures the luminescence from selected mineral grains of the sample, compares it with the luminescence arising from laboratory irradiation, and thus determines the past radiation dose and hence age. Ideally, the intensity of the luminescence arising from the laboratory irradiation, after the required heat treatment, should not depend on the length of time elapsed between irradiation and measurement. If this is not the case, but the intensity decreases as a function of this time, there is said to occur anomalous fading. It is called anomalous because it was, and to some extent is still, not understood. This phenomenon does not seem to occur in quartz, but it does occur in feldspars. It . . . [Full Text of this Article]