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This paper analyzes the seismicity in Bohai Sea, introducing a shape factor K to characterize the seismic risk distribution in sub-regions of the sea. Based on the seismic design ground motions for 46 platforms located in the Bohai Sea, a statistical analysis was performed for different peak ground acceleration (PGA) ratios at two different probability levels. In accordance with the two-stage design method, a scheme of two seismic design levels is proposed, and two seismic design objectives are established respectively for the strength level earthquake and the ductility level earthquake. By analogy with and comparison to the Chinese seismic design code for buildings, it is proposed that the probability level for the strength level earthquake and ductility level earthquake have a return period of 200 and 1000–2500 years, respectively. The validity of these proposed values is discussed. Finally, the PGAs corresponding to these two probability levels are calculated for different sub-regions of the Bohai Sea. [ABSTRACT FROM AUTHOR]

Based on the existing materials of fault segmentation, characteristic earthquakes, and their empirical relationships, we calculated the parameters of the fault segments, such as length, width, magnitudes of characteristic earthquakes, etc. Constrained by GPS velocity field, the slip rates of these fault segments in depth were inversed using the 3-D half-space elastic dislocation model. As not all of the recurrence periods and co-seismic displacements of characteristic earthquakes are known, we selected the fault segments with these two parameters known and calculated the accumulation rate of average co-seismic displacement, which shows the faults' slip rate in seismogenic layer. Then, the slip rate in depth was compared with that in seismogenic layer, the relationship between them was obtained, and this relationship was used to get the recurrence periods and co-seismic displacements of all fault segments. After the studies above, we calculated the co-seismic deformation field of all the earthquakes larger than M6.8 from AD 1700 one by one and inversed the potential displacement in the co-seismic deformation field. Then, we divided the potential displacement by the slip rate from GPS inversion to get the influences of these fault segments, added the influences into the elapsed time of the characteristic earthquakes, and obtained the earthquake hazard degree of all the segments we studied in the form of the ratio of elapsed time to recurrence period; so, we name the ratio as the Impending Earthquake Risk (IER). Historical earthquake cases show that the fault segment is in safety when the IER is less than 1 but in danger after the IER becomes larger than 1. In 2009, the IER is larger than 1 on the following segments, 1.35 on the Tagong segment of Xianshuihe fault, 1 on the Menggu-Dongchuan segment, 1.04 on the Dongchuan-Xundian segment, and 1.09 on the Yiliang-Chengjiang segment of Xiaojiang fault. [ABSTRACT FROM AUTHOR]

Coulomb stress accumulation and releasing history and its relationship with the occurrence of strong historical earthquakes could deepen our understanding of the occurrence pattern of strong earthquakes and hence its seismic potential in future. The sinistral strike-slip Xianshuihe- Xiaojiang fault zone (XXFS) is one of the most dangerous fault zones in China, extending 1 500-km-long from the central Tibetan Plateau to the Red River fault zone. There are 35 M≥6.5 historical earthquakes occurred since 1327, hence it is an ideal site for studying the Coulomb stress evolution history and its relationship with the occurrences of strong earthquakes. In this study, we evaluated the Coulomb stress change history along the XXFS by synthesizing fault geometry, GPS data and historical earthquakes. Coulomb stress change history also revealed different patterns of historical earthquakes on different segments of the XXFS, such as characteristic recurrence intervals along the Salaha-Moxi fault and super-cycles along the Xianshuihe fault. Based on the occurrence pattern of past historical earthquakes and current Coulomb stress field obtained in this study, we suggest positive ΔCFS and hence high seismic potential along the Salaha-Moxi fault and the Anninghe fault. [ABSTRACT FROM AUTHOR]