Potential of thermoluminescence method to estimate the time-temperature condition of fault activity

To date the last fault activity by a radiometric dating method, the resetting of dating system, that is a function of time-temperature condition, is inevitable. To see whether a particular dating system was reset by fault activities, we often estimate a temperature rise by frictional heating under the geophysical and geological observations of stress field and displacement length. When the last fault activity occurred beyond the observation era, such attempt is difficult to apply with little knowledge on what happened in the past. Luminescence dating method (TL and OSL datings) has a potential to date the last event of active fault in Quaternary (e.g., Spencer et al., 2012, Ganzawa et al., 2013), for it is easily reset compared to other dating methods with higher closure temperatures over geological time scale. However, if a sample experienced only a partial resetting in luminescence dating system, the obtained ages do not correspond to any events. We propose the potential of thermoluminescence method to estimate the time-temperature condition of the fault-related sample. Thermoluminescence glow curve consists of signals from several traps (e.g., Spooner, et al., 2001). Lifetime (τ) of each trap is calculated from the following equation (Aitken, 1985). τ=s-1exp(E/kT), where s is the frequency factor (/sec), E is trap depth (eV), k is Boltzmann constant (eV/K), and T is temperature (K). When luminescence signal is decreased by the event from I0 to Im, the time (t) necessary for this decrease is estimated by the equation t=τln(I0/Im). If we have two trap sites in a sample, and I0 can be estimated somehow (e.g., from the signal intensity of unaffected higher trap), two unknowns (t and T) can be determined from two sets of equations. In general, signals will be regained after the event owing to annual dose rate and time passed since the event. Therefore present signal intensity (Ip) is described as equation Ip=Ix+Im ,where Ix is the signal built after the event. When we have three traps, we may be able to assess three unkwowns Ix, t, and T.