The concentration of radon at shallow subsurface depths (~1 m) changes in response to the vertical flow of fluids past the measuring site. Thus locally produced radon serves as a tracer gas to detect the flow of subsurface fluids which may occur in association with earthquakes. Therefore it is not ecessary to invoke either changes in the local rate of radon production or the long-distance transport of radon to explain changes in the concentration of radon, although these phenomena may also occur. We have measured radon concentration at 60-cm depth by using solid state nuclear track detectors to measure weekly values and electronic detectors to measure hourly values. An automated system records the outputs of the electronic detectors of radon, as well as the values of various atmospheric parameters. The radon variations we observe at Blue Mountain Lake, New York, are not controlled by changes in atmospheric parameters. From a 3-year record of weekly radon readings we conclude that the largest seismie event in New York State during this period (M=3.9), which occurred at the beginning of our record at a distance of 14 km from our site, may be responsible for the highest radon readings, which were also recorded at this time. The seismicity and the radon concentrations have decreased jointly since then. The automated system shows a tenfold increase in radon concentration in association with an unusual burst of seismic activity. Radon was also measured in a grid of 54 sites near Thoreau, New Mexico. The largest nearby earthquake (M=4.6) occurred during the most anomalous monthly pattern of radon.