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 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 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 growth, and conditions like shade and wind that promote faster lichen growth must be factored in.
What this question is testing
Topic
The author is introducing a clever new way to figure out when past earthquakes happened — using the size of lichens growing on rocks.
Framework
Highlight Noteworthy.
Main Point
The simpler version: scientists used to date past earthquakes by digging trenches and using carbon-14 dating on organic material in shifted dirt. Two geologists (Bull and Brandon) came up with a new approach: when earthquakes happen, rocks fall, and lichens — which grow slowly but steadily — start growing on the newly exposed rock. Measure the biggest lichen on a boulder, and you know roughly when the rock fell. The new method is more accurate than carbon dating (within 10 years vs. 40), though it has its own quirks.
P1: The old way
Dig along faults; carbon-date the organic material trapped in shifted sediments.
P2: The new way
Earthquakes shake rocks loose. Lichens then start growing on those rocks at a known rate. Measure the biggest lichen and you've dated the earthquake. Find lots of same-age rockfalls in one region and you've found an earthquake; map them and you find the epicenter.
P3: Why it's better — but not perfect
Carbon dating is only accurate to ±40 years, and the last 300 years are especially noisy. Lichenometry can hit ±10 years. Catch: it works best within the last 500 years, you have to pick the right sites, and you have to factor in things that affect lichen growth.
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