# Radon gas in earthquake prediction - Why Rn?

I recently studied an article about predicting earthquakes and how correctly realizing that an increase in $\ce{Rn}$ gas is a sign for the earthquake saved a whole city. The following is derived from the wikipedia page for earthquake prediction:

There are reports of spikes in the concentrations of such gases prior to a major earthquake; this has been attributed to release due to pre-seismic stress or fracturing of the rock. One of these gases is radon, produced by radioactive decay of the trace amounts of uranium present in most rock.

Radon is useful as a potential earthquake predictor because being radioactive it is easily detected, and its short half-life (3.8 days) makes it sensitive to short-term fluctuations. A 2009 review found 125 reports of changes in radon emissions prior to 86 earthquakes since 1966.

However, wikipedia then attempted to prove that most of the assretions related to radon are false statements,

but all this leaves me with one question: Why radon? (Why isn't another chemical species used for identifying the danger of feasible earthquake?) Maybe things I studied were too technical for me to understand, so please be as simple as you can be.

(Feel free to edit tags)

• I wrote an answer with references for a similar question on Earth Sciences. (Had to delete a similar answer here). – user12839 Jan 25 '15 at 2:57

Uranium is found in low-levels in all rocks and soil. Radon is a gaseous radioactive decay product of uranium. As the uranium undergoes radioactive decay, radioactive radon is generated and trapped in the rocks that contain the uranium.

The earthquake theory involving radon suggests that prior to the actual quake, there is some subterranean movement where rocks are crushed, soil is uncompacted and the trapped radon is released producing a pre-quake spike in radon concentration.

So radon presents the following attributes:

• it is present in all rocks and soil
• if a rock is broken or if soil is disturbed, radon will be released
• it is gaseous and air currents / thermal gradients will carry it up to the earth's surface producing a detectable plume
• it is radioactive, this makes detecting small amounts or small changes in radon concentration relatively routine due to well-developed and very sensitive methods for detecting and accurately measuring radioactivity.
• radon has a very short radioactive half-life, a bit under 4 days. Being a gas and having a short half-life is very useful in terms of measuring radon emissions. If a radon emission spike occurs, the gas will dissipate quickly and after about 10 half-lives (40 days) normal background levels of radioactivity will return.
• Another important characteristic is that radon is a noble gas, so it won't react chemically with anything on the way to the surface. – thomij Jan 25 '15 at 1:45
• @thomij Excellent point! – ron Jan 25 '15 at 1:46