Abstract: Based on the statistical result of two seismicity precursors of blast earthquake and seismic quiescence, the background probability, conditional probability and probability gain of earthquake occurrence are estimated. The time history of probability of earthquake occurrence before Zhangbei earthquake is introduced. The background probability P (E) increased a t first to the conditional probability P (E|A) after appearance of intermediate-term precursor A (blast earthquake ), then to the combined conditional probability P ( E|A, B ) after appearance of the short-term precursor B ( Seismic quiescence). Within a month after December 17, 1997, the combined conditional probability of earthquake occur-rence with magnitude greater than 6 in Northchina was 38% with probability gain over 20. How to use the result of probability prediction is discussed in this pa per. It is noticed that the probability of earthquake occurrence strongly depends on the time, space and magnitude spa n to be predicted. A high probability for a short-term prediction is hard to beobtained at present, and a high probability gain is important. The hypotheses test for various precursors and prediction methods is essential for a sound development of earth-quake prediction research. For the practical prediction methods the trace test is suggested. While a prediction is made by a certain metho d, the latest result of hypo theses test for the method is suggested to be provided if possible.

Key words: Probability prediction, Conditional probability, Probability gain, Bayes method