A Revised Lichenometric Method and Its Application Dating Great past Earthquakes

Abstract

An improved technique is put forward for dating rock surfaces using the sizes of lichens that grow on them. The technique has its theoretical basis in a statistical distribution model for lichen diameters. The technique differs from the conventional one in that the whole set of lichens of particular species is used in the dating. A statistical processing of the data enables one to estimate the uncertainty of age determinations and obtain results of higher accuracy than was previously possible. The use of a model for the distribution of lichen diameters allows one, in contrast to techniques that employ lichen growth curves, to determine age, not only in areas where there are objects of known age, but also over a fairly large area. This is achieved by establishing correlative relationships between the model parameters and altitude. The model parameters are determined from measurements of lichens of species Lecanora muralis (Schreb.) and Aspicilia sp. for 18 objects of known age. Specific results from applications of this version of lichenometry are the southern Tien-Shan and the Pamirs, USSR, areas that exhibit extremely active exogenous and seismic processes and which have been the key areas of lichenometric studies in the paleoseismological aspect. The ages of several earthquake-induced features have been estimated. Conclusions have been drawn about the simultaneous formation of some seismic dislocations and their association with known large earthquakes and about the recurrence of seismic events in the epicentral zones of known catastrophic earthquakes and outside these zones.