Abstract: Based on the analysis on the characteristics of spatial image variation of medium and small seismicity prior to M_S 6 earthquakes occurred in North China area since 1970, the space-time characteristics and the background for the occurrence of “signal shocks” are studied. According to the characteristics of “field-source” relation, a strict selection and recognition is made for the earthquakes and the relevant prediction index for the signal shocks are obtained. Generally speaking, the signal shocks occur in 2 years prior to the strong earthquake and most of them occur within 15 months; the distance between the signal shock and the strong earthquakes is no larger than 200 km and most of them are within 100km; the magnitude of the signal shocks is usually about M_L4.0~5.3. The signal shocks generally occur in the M_L4.0 seismically quiet area of a certain region, which occur in or near the medium and small seismic stripes with the seismicity gap of medium and small earthquakes nearby. The testing results indicate that in 9 months after the signal shock, its predictive probability is 0.5 and its R value is 0.27; when the radius of the predicted region is within 100 km to the epicenter of signal shocks, the probability for the occurrence of the predicted earthquakes is 0.73 and the predicted magnitude is generally M_S 6.0. The study in the paper indicates that the environmental stress value τ0 of the signal shock is obviously higher than that of other earthquakes, which shows the significant abnormal background of high stress. Its seismic waves might probably contain a great amount of essential information of the seismogenic zone for the coming strong earthquakes.

Key words: Signal shock, Prediction, Environmental stress value, North China area