Abstract:

Ocean earthquakes pose a serious threat to the security of the economic construction in coastal areas and the marine resource exploitation of China, so building proper seismicity models for China seas and adjacent regions is one of the focuses of the next generation seismic zoning map of China. Different from mainland China, there is lack of multidisciplinary data for the sea areas, such as seismic geology and geophysical exploration, thus the earthquake catalogue is the most important basic data for analyzing its seismic activity characteristics. Recently, a unified catalogue was compiled by researchers for China seas and neighboring regions, which provides a basis for further work.
The Smoothed Seismicity Model (SSM) is a classic model based on earthquake catalogue and has become a basic model for the United States National Seismic Hazard Map. The Adaptively Smoothed Seismicity Model (ASSM) is an improved version of SSM. Compared with SSM, ASSM has optimized the value algorithm of the kernel function, thereby improving the model’s capability on mid- and long-term earthquake forecast. Due to the outstanding performance of ASSM in the CSEP project, it has become a research hotspot for seismologists. Based on a further improved ASSM algorithm and the newly compiled catalogue, this study established for the first time an adaptively smoothed seismicity model for China seas and adjacent areas.
First, similar to most studies on mid-to-long-term seismicity models, we removed the foreshocks and aftershocks from the catalogue. Then we used seismic zones as the unit to estimate the start and end time of completely recorded earthquakes in different magnitude intervals. Furthermore, the maximum likelihood method was used to estimate the seismic activity parameters such as a-value and b-value for every seismic zone. On this basis, an improved adaptively smoothed algorithm was used to build the model. The function of probability gain per earthquake was applied to evaluate the performance of models under different parameter settings. Finally, our model was compared with the traditional SSM, and the advantages and limitations of our model were analyzed.
Results show that: Compared with SSM, our model has a better performance on the probability gain function, and this advantage is not affected by the minimum magnitude(M_min)of the input earthquakes. When M_min is set as M4.0, our model achieves its best performance. The performance of the model does not necessarily improve as M_min decreases or the number of earthquakes increases. This indicates that we need to comprehensively consider the distribution of earthquakes in the study area and the integrity of the earthquake catalogue when selecting parameters. The algorithm used in this study can make full use of seismic data with varying record level over time and space, and has a certain application value in seismic hazard analysis and mid- and long-term earthquake forecast. Meanwhile, our model can be used as one of the basic models to analyze the seismic hazard for China’s maritime areas, and provide technical support for the compilation of seismic zoning maps in China’s sea areas. In addition, the completeness analysis results and seismic activity parameters obtained by this study can also provide references for other peer research.
Since ASSM is only based on historical and instrumental earthquake catalogues, it has certain limitations. For example, it cannot calculate the upper limit of the magnitude and reflect the distribution of faults, nor can it consider the time-dependence of the recurrence of large earthquakes. Therefore, this model can be used alone to describe the occurrence probability of small to moderately strong earthquakes, and it can also be used as one of the important factors to determine the spatial distribution functions for potential seismic source zones. However, when conducting seismic hazard analysis, it is recommended to combine the results of other disciplines such as geology and geodesy to form a hybrid model, so as to further improve the applicability and effectiveness of the model.

Key words: China seas and adjacent regions, earthquake catalogue, completeness analysis, adaptively smoothed seismicity model, the function of probability gain per earthquake

摘要：

海域地震对中国沿海地区的经济建设和海洋资源开发构成了严重威胁, 研究中国海域及邻区的地震活动模型是中国下一代地震区划图的重点工作之一。文中基于最新编制的中国海域地震目录, 首次建立了中国海域及邻区的自适应空间平滑地震活动模型。首先对地震目录进行除丛, 以地震带为单位评估震级间隔为0.5的震级档完整记录的起止时间, 进而采用极大似然法求出各个地震带的b值等地震活动参数; 在此基础上, 使用改进的自适应空间平滑算法建立模型, 并采用概率增益函数评估不同参数设置下模型的优劣, 最后分析了本模型的优点和局限性。结果显示, 本模型与传统的固定平滑半径的模型相比具有更优的概率增益函数表现; 将输入模型地震的最小震级设为M4.0时, 模型的表现达到最优; 模型的表现不一定随着输入地震数的增多而提高, 因此选择参数时需要综合考虑研究区内地震的分布和记录情况; 考虑到本模型只基于历史和仪器地震目录, 存在一定的局限性, 建议使用时结合断层模型等形成混合模型以完善模型体系。文中采用的自适应空间平滑算法可以充分利用完整性随时间和空间变化的地震数据, 在地震危险性分析和中长期地震预测上有一定的应用价值。同时, 文中建立的模型可以作为分析中国海域地震危险性的基础模型之一, 为编制中国海域地震区划图提供技术支撑。

关键词: 中国海域及邻区, 地震目录, 完整性分析, 自适应空间平滑地震活动模型, 概率增益函数