PrepTest 62, Section 4, Question 8

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. Using radiocarbon dating, they measure the quantity of the radioactive isotope carbon 14 present in wood or other organic material trapped in the sediments when they shifted. Since carbon 14 occurs naturally in organic materials and decays at a constant rate, the age of organic materials can be reconstructed from the amount of the isotope remaining in them. These data can show the location and frequency of past earthquakes and provide hints about the likelihood and location of future earthquakes.

Geologists William Bull and Mark Brandon have recently developed a new method, called lichenometry, for detecting and dating past earthquakes. Bull and Brandon developed the method based on the fact that large earthquakes generate numerous simultaneous rockfalls in mountain ranges that are sensitive to seismic shaking. Instead of dating fault-line sediments, lichenometry involves measuring the size of lichens growing on the rocks exposed by these rockfalls. Lichens�symbiotic organisms consisting of a fungus and an alga�quickly colonize newly exposed rock surfaces in the wake of rockfalls, and once established they grow radially, flat against the rocks, at a slow but constant rate for as long as 1,000 years if left undisturbed. One species of North American lichen, for example, spreads outward by about 9.5 millimeters each century. Hence, the diameter of the largest lichen on a boulder provides direct evidence of when the boulder was dislodged and repositioned. If many rockfalls over a large geographic area occurred simultaneously, that pattern would imply that there had been a strong earthquake. The location of the earthquake's epicenter can then be determined by mapping these rockfalls, since they decrease in abundance as the distance from the epicenter increases.

Lichenometry has distinct advantages over radiocarbon dating. Radiocarbon dating is accurate only to within plus or minus 40 years, because the amount of the carbon 14 isotope varies naturally in the environment depending on the intensity of the radiation striking Earth's upper atmosphere. Additionally, this intensity has fluctuated greatly during the past 300 years, causing many radiocarbon datings of events during this period to be of little value. Lichenometry, Bull and Brandon claim, can accurately date an earthquake to within ten years. They note, however, that using lichenometry requires careful site selection and accurate calibration of lichen growth rates, adding that the method is best used for earthquakes that occurred within the last 500 years. Sites must be selected to minimize the influence of snow avalanches and other disturbances that would affect normal lichen growth, and conditions like shade and wind that promote faster lichen growth must be factored in.

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. Using radiocarbon dating, they measure the quantity of the radioactive isotope carbon 14 present in wood or other organic material trapped in the sediments when they shifted. Since carbon 14 occurs naturally in organic materials and decays at a constant rate, the age of organic materials can be reconstructed from the amount of the isotope remaining in them. These data can show the location and frequency of past earthquakes and provide hints about the likelihood and location of future earthquakes.

Geologists William Bull and Mark Brandon have recently developed a new method, called lichenometry, for detecting and dating past earthquakes. Bull and Brandon developed the method based on the fact that large earthquakes generate numerous simultaneous rockfalls in mountain ranges that are sensitive to seismic shaking. Instead of dating fault-line sediments, lichenometry involves measuring the size of lichens growing on the rocks exposed by these rockfalls. Lichens�symbiotic organisms consisting of a fungus and an alga�quickly colonize newly exposed rock surfaces in the wake of rockfalls, and once established they grow radially, flat against the rocks, at a slow but constant rate for as long as 1,000 years if left undisturbed. One species of North American lichen, for example, spreads outward by about 9.5 millimeters each century. Hence, the diameter of the largest lichen on a boulder provides direct evidence of when the boulder was dislodged and repositioned. If many rockfalls over a large geographic area occurred simultaneously, that pattern would imply that there had been a strong earthquake. The location of the earthquake's epicenter can then be determined by mapping these rockfalls, since they decrease in abundance as the distance from the epicenter increases.

Lichenometry has distinct advantages over radiocarbon dating. Radiocarbon dating is accurate only to within plus or minus 40 years, because the amount of the carbon 14 isotope varies naturally in the environment depending on the intensity of the radiation striking Earth's upper atmosphere. Additionally, this intensity has fluctuated greatly during the past 300 years, causing many radiocarbon datings of events during this period to be of little value. Lichenometry, Bull and Brandon claim, can accurately date an earthquake to within ten years. They note, however, that using lichenometry requires careful site selection and accurate calibration of lichen growth rates, adding that the method is best used for earthquakes that occurred within the last 500 years. Sites must be selected to minimize the influence of snow avalanches and other disturbances that would affect normal lichen growth, and conditions like shade and wind that promote faster lichen growth must be factored in.