To study centuries-old earthquakes and the geologic faults that caused them, seismologists usually dig trenches along visible fault lines, looking for sediments that show evidence of having shifted.
Topic
Lichenometry — a new method for detecting and dating past earthquakes — and how it compares to radiocarbon dating.
Framework
Highlight Noteworthy. The author introduces the new method, explains how it works, and notes its advantages over radiocarbon dating along with its limitations.
Main Point
Lichenometry, developed by Bull and Brandon, is a new method for dating past earthquakes that has distinct advantages over radiocarbon dating, though its use requires careful site selection and accurate calibration.
P1: The traditional method — radiocarbon dating
Seismologists dig trenches along fault lines, looking for shifted sediments. They use radiocarbon dating on organic materials trapped in those sediments to reconstruct the age of past earthquakes — useful for predicting future ones.
P2: The new method — lichenometry
Bull and Brandon developed lichenometry: large earthquakes generate simultaneous rockfalls; lichens colonize newly exposed rock surfaces and grow at a slow but constant rate (e.g., 9.5 mm/century). The diameter of the largest lichen on a boulder dates the rockfall. Many simultaneous rockfalls over a wide area imply a strong earthquake. Mapping rockfall density gives the epicenter.
P3: Lichenometry's advantages — and caveats
Radiocarbon is accurate only to ±40 years (because atmospheric carbon-14 varies); past 300-year fluctuations make many radiocarbon datings of recent events of little value. Lichenometry can date earthquakes to within 10 years. Caveats: requires careful site selection and accurate calibration; best for earthquakes within the last 500 years; sites must minimize avalanche/disturbance; growth-affecting conditions (shade, wind) must be factored in.