Abstract: In this paper, forecasting effectiveness of seismic gap was evaluated in Eastern China since 1970. The results show that accurate rate of M_L≥3.0 earthquakes gap is 0.45, inflating rate is 0.55, missing rate is 0.57, R value is 0.26. The values of M_L≥3.5 earthquakes are 0.31, 0.69, 0.75 and 0.08 respectively. M_L≥4.0 gap without corresponding to M_S≥5.5 earthquake. R value of M_L≥3.0 earthquakes gap is higher than R₀ value of 97.5% confidence level, and is has certain forecasting capability, M_L≥3.5 and M_L≥4.0 earthquakes gap have no forecasting capability. The mainshocks are usually located in the gap and near the edge, the gap will not be broken before an mainshock. There is no obvious linear relationship between the gap lasting time, gap scale, magnitude lower limit and mainshock magnitude. The gaps are mainly distributed in Jiangsu, Shandong and adjacent sea areas.

Key words: Moderate-strong earthquake, Seismic gap, Forecasting effectiveness evaluation, Eastern China