Abstract: In the paper, a comprehensive investigation has been made on the precursory variations in the loading/unloading response ratio (Called Y value) prior to the earthquakes M≥5 in Northern China during the period of 1970～1999. It is based on a comparison of the results of the ratio's temporal-spatial scanning with different values of related parameters such as the stress field, and the least sample amount. As aresult, the following medium-term and short-term evolution characteristics have been revealed. When the least sample amount is 15, an a noma lous area of high value with Y₃≥2 appeared in the vicinity of the epicenter about 2 years before the earthquake. It contracted or even disappeared one year before the earthquake and a small anomalous area with high value usually about 6 months before the earthquake at the epicenter. The R-value of the prediction efficiency assessment is 0.3～0.5 and that of P is 0.4～0.6. The study has also shown that anomalous area of Y₃ high value appeared before moderate earthquakes with magnitude about 4 too.

Key words: Loading/unloading response ratio, Temporal-spatial scanning, Seismogenic information, time-space evolution