Dating earthquakes through lichen growth

How can we learn about an earthquake that happened centuries ago? The opening gives us the established approach: dig along visible fault lines, find sediments that shifted, and date organic material trapped in them. Carbon 14 decays at a constant rate, so the amount remaining helps researchers work backward to the material's age.

I want to connect those steps, rather than just remember the phrase radiocarbon dating. The material is associated with shifted sediments, so dating it supplies evidence about past earthquakes. That history can also give hints about future ones. We aren't directly dating a future event; we're learning from earlier events.

Then Bull and Brandon introduce a new method: lichenometry. Large earthquakes can trigger many rockfalls at once. Instead of digging for organic material along a fault line, researchers examine lichens on the newly exposed rocks. My first guess is that the growth will tell us how long a rock has been sitting there. Let's see how that works.

Lichens quickly colonize the exposed surface and then spread slowly at a constant rate if undisturbed. They are a partnership between a fungus and an alga. The important feature for the method is their steady growth. One North American species spreads only about 9.5 millimeters per century. At that pace, the size of a lichen can preserve information about a long stretch of time.

So researchers measure the largest lichen on a boulder and use its growth rate to estimate when the surface became exposed. That connects something we can measure now with a rockfall in the past.

I still have questions. Couldn't rocks move for other reasons? Couldn't something disturb the lichens or change their growth? Those are reasons to pay attention to the method's conditions, not reasons to ignore what the account tells us. The next step helps with the first concern: researchers look for a pattern across a large area, not just one fallen rock. Many rockfalls dated to the same time suggest an earthquake. Rockfalls also become less abundant farther from the epicenter, so mapping the pattern can help locate it.

The final paragraph compares the methods. Radiocarbon dates can be off by about 40 years because environmental carbon 14 levels vary with incoming radiation. Fluctuations during the last 300 years have made many dates from that period particularly unhelpful. Bull and Brandon claim that lichenometry can date an earthquake to within ten years. If that accuracy is achieved, it's a meaningful improvement.

But the method needs care. They say it works best for events within the last 500 years, requires suitable sites, and depends on accurate growth rates. Avalanches or other disturbances can interfere, while shade and wind can accelerate growth. We have to minimize some influences and account for others; we can't assume every lichen grows under identical conditions.

That brings us back to my earlier concern about reliability. The account does recognize complications and tells us how researchers must address them. Our main-point prediction should stay balanced: lichenometry is a promising new way to date earthquakes, with advantages over radiocarbon dating and conditions that matter to its use.