This stimulated a number of experiments in other
parts of the world to monitor underground radon with time and to
look for radon changes associated with earthquakes. Since radon is
a radioactive gas, it is easy and relatively inexpensive to monitor instrumentally,
and its short half-life (3.8 days) means that short-term
changes in the radon concentrations in the earth can be monitored
with very good time resolution.While other gases have also been looked
at as possible earthquake precursors, the bulk of the experiments reported
in the scientific literature have focused on radon.