OxCal: Versatile Tool for Developing Paleoearthquake Chronologies--A Primer

Abstract

Ages of paleoearthquakes (events), i.e., evidence of earthquakes inferred from the geologic record, provide a critical constraint on estimation of the seismic hazard posed by an active fault. The radiocarbon calibration program OxCal (4.0.3 and above; Bronk Ramsey 2007, 2001) provides paleoseismologists with a straightforward but rigorous means of estimating these event ages and their uncertainties. Although initially developed for the chronologic modeling of archaeological data from diverse sources ( e.g. , radiocarbon, historical knowledge, etc.), OxCal is readily adaptable to other disciplines requiring chronological modeling, such as paleoseismology (Fumal et al. 2002; Lindvall et al. 2002; Kelson et al. 2006; Noriega et al. 2006; Lienkaemper and Williams 2007; Yen et al. 2008). OxCal employs Bayesian statistics as a means of incorporating all available chronological constraints. When radiocarbon ages are calibrated to calendar ages, the results can be expressed as probability distributions, which are often irregular and multimodal as shown in Figure 1. These distributions can be tightened by including additional chronological information. Stratigraphic order, the timing of the most recent event, and historical constraints are inputs to the model. Applying the stratigraphic order as a constraint is particularly powerful where calibrated age distributions overlap, in which case the modeling calculations reweight the distributions to reflect the knowledge that overlying layers must be younger. This paper is intended as a primer for paleoseismologists or those modeling paleoseismic data. The current version of OxCal contains an option for additional output that is useful for seismic hazard modeling, such as mean and median values of age for paleoearthquakes (events); mean recurrence interval; and probability density functions for a variety of variables, including earthquake age, interval between events, and the average of all intervals. The standard deviation of the last quantity models the uncertainty …