Abstract: In view of the clustering of earthquakes with M_s ≥5.0 in Ludong-Huanghai active block in recent years, the research on the period of medium strong earthquakes, space-time evolution, energy release and the impact of large earthquakes around the block is carried out. The period of historical earthquake activity is determined by wavelet analysis. The nowcasting method is used to evaluate the risk of moderate and strong earthquakes. Through the above research, the following understandings are obtained. By qualitative analysis and quantitative calculation of historical earthquake periods, there are active-quiescent characteristics with an active period of about 66.1-78.6 years, for M_s ≥5.0 earthquakes in the west of Ludong-Huanghai block.
Since 1997, the western part of the Ludong-Huanghai block has entered the late stage of the active period of earthquakes (weak release stage). The results of the frequency and magnitude difference of earthquakes M_s ≥5.0 indicate that the current period may transition from the end of the active period to the quiescence period. Although the active period has lasted for about 48 years (exceeded the duration of the previous active period), the energy release of M_s ≥5.0 earthquakes is insufficient. In the next few years, seismicity may be dominated by earthquakes with magnitude of 5.0-5.9. At present, the main active area in the active period is the southern depression of the Yellow Sea and its vicinity. According to the assessment results of strong earthquake risk in Nowcasting method, the potential risk score of M_s ≥5.5 earthquakes in the west of Ludong -Huanghai block is 100% indicating that there is a strong background of earthquakes with M_s ≥ 5.5 in this area. At present, M_s ≥ 5.0 earthquakes in the west of Ludong-Huanghai block and its vicinity are weakly active, while M_s ≥ 6.0 earthquakes in North China are remarkably quiet. There is a background of synchronous activity in these two regions. In recent years, the seismicity in the west of the Ludong-Huanghai block is mainly M_s4.0-5.0 earthquakes. Without significant concentration of M_s ≥ 5.0 earthquakes within 5 years in the west of Ludong-Huanghai block, there is little possibility of M_s ≥ 6.0 earthquakes at present.