Stress-induced piezoelectric fields produce freely propagating electromagnetic radiation when microscopic rock fractures occur in quartz-bearing rocks. A laboratory study is presented which shows that electromagnetic emissions are produced when microfractures occur in Westerly granite. A detailed model is presented for the emission process which allows an estimate of the total radiated power from an individual microfracture. On the basis of these results a case is presented that an unusual radio emission seen on several widely separated radio astronomy receivers in the northern hemisphere on May 16, 1960, was due to a stress-induced microfracture along the Chilean fault. This radio event occurred 6 days prior to the great Chilean earthquake of May 22, 1960, and may have been a precursor to one of the largest earthquakes of this century.