Characteristics of moderate-strong seismicity in the western Ludong-Huanghai block and the influence from surrounding earthquakes

The seismicity in the western Ludong-Huanghai block, a sub block in the eastern side of the North China block, has a significant periodic characteristic. Therefore, it is of great importance to study the seismicity in this area, the characteristics of its internal moderate strong seismicity and the influence from surrounding large earthquakes. In this paper, the wavelet analysis method is used to quantitatively calculate the activity period of historical earthquakes in the block. At the same time, in combination with the data on qualitative analysis of moderate-strong earthquakes, spatio-temporal evolution, energy release and the influence from surrounding large earthquakes, the state and characteristics of seismicity in the block are discussed. The results show that: ① There is a long periodicity of 60.6−78.6 years for M≥5.0 earthquakes in the western Ludong-Huanghai block, which reflects the rhythmic characteristics of seismic active-quiescence. Based on which, M≥5.0 earthquakes can be divided into active periods Ⅰ （1844−1879）, Ⅱ （1905−1949）, and Ⅲ （1974 to present）. Since 1997, the seismicity in the western Ludong-Huanghai block has been in the weak release stage at the end of the active period; ② In the three active periods, the main seismic areas are different. In the first and second active periods, the seismicity in the sea is dominant, and the earthquakes in the sea are stronger than those in the land, while in the third active period, it is very different. In which, M≥6.0 earthquakes presented in both land and sea; ③ Since 1815, M≥5.0 earthquakes in the western Ludong-Huanghai block and those with M≥6.0 earthquakes in the North China block have roughly exhibited synchronous activity characteristics. In other words, when M≥5.0 earthquakes in the western Ludong-Huanghai block happen in a strong active period, it generally firstly broke the significant calm of M≥6.0 earthquakes in the North China block. Then, the seismicity moves towards the central and western areas of the North China block; ④ When intermediate and deep-focus earthquakes with M_W≥5.0 in the subduction zone are active and those with M_W≥7.0 happen at the same time on the western side of the Pacific plate, it may promote the occurrence of M_S≥6.0 earthquakes in the western Ludong-Huanghai block, which may reflect the strengthening of deep subduction tectonic dynamic action.