Preliminary Study on Shallow Seismic Activity Model in Taiwan Region
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摘要: 本文从中国台湾地区气象厅（CWB）网站收集到了1900—2022年ML2.0以上震源深度小于350 km的地震目录数据集，按照台湾地区更新完善后的地震目录（1900—2014）建议的方法，将该地震目录的地震震级统一为矩震级MW。以此地震目录数据集为基础分析台湾地区的地震活动特征，探讨台湾地区不同时期记录的完备性震级下限MC的历史沿革。本文采用目前国际比较流行的Gardner-Knopoff除丛方法删除了地震目录中的前震和余震，分别选用Gardner-Knopoff时空窗、Grünthal时空窗、Uhrhammer时空窗计算获得了相应的主震目录。结果表明，不同的除丛方法对地震目录影响显著。本文推荐使用Gardner-Knopoff除丛方法的Uhrhammer时空窗参数去除地震目录中的前震和余震事件，并建议在概率地震危险性分析中应该考虑由多种除丛方法引起的认知不确定性。文中采用Stepp（1971）方法对台湾地区的地震目录进行了完整性分析，根据各震级档地震平均发生率估计的方差偏离所绘制的预期回归线的时间，给出了各震级档的完整性时间表，避免了人为主观判断所引入的认知不确定性。最后，分别利用Weichert（1980）、Bender（1983）以及Kijko等（2012）的极大似然估计方法对台湾地区G-R关系中的活动性参数进行估算，获得了台湾地区的地震活动性参数值。本文的研究结果能够为后续台湾地区的概率地震危险性分析提供数据输入，为下一代区划图的编制提供地震活动参数计算方法和参考。Abstract: This paper compiles an earthquake catalogue dataset containing earthquakes with magnitudes above ML2.0 and focal depths less than 350 km, sourced from the Taiwan Meteorological Bureau (CWB) for the period from 1900 to 2022. All magnitudes in this catalogue are standardized to moment magnitudes (MW). Utilizing this dataset, we analyze the characteristics of seismic activity in Taiwan and discuss the historical evolution of the lower limit for completeness magnitude (MC) recorded during different periods. To refine the earthquake catalogue, we apply the widely used Gardner-Knopoff declustering method to remove foreshocks and aftershocks. Three time-distance window parameters—Gardner-Knopoff, Grünthal, and Uhrhammer—are employed to derive the corresponding mainshock catalogue through calculations. Our findings indicate that different declustering methods significantly impact the resulting earthquake catalogue. In this study, we recommend the Gardner-Knopoff declustering method combined with the Uhrhammer time-distance window parameters for effectively removing foreshock and aftershock events. We emphasize that epistemic uncertainty arising from various declustering approaches must be considered in probabilistic seismic hazard analyses. The completeness of the earthquake catalog in Taiwan is assessed using the Stepp (1971) method, which establishes completeness times for each magnitude bin based on the point at which the estimated variance of the average earthquake occurrence rate deviates from the expected regression line, thus minimizing subjective bias. Finally, we estimate the activity parameters of the Gutenberg-Richter (G-R) relationship in Taiwan using maximum likelihood estimation methods developed by Weichert (1980), Bender (1983), and Kijko et al., (2012). The results of this research provide valuable data inputs for future probabilistic seismic hazard analyses in Taiwan and serve as a reference for calculating seismic activity parameters for the upcoming seismic ground motion parameters zonation map of China.1)
1 2 中国地震动参数区划图编制组, 2011. 中国地震动参数区划图技术报告-地震活动性参数确定专题报告. -
图 1 台湾地区MW≥2的浅源地震分布图
Figure 1. Distribution of shallow source earthquakes with MW≥2 in Taiwan
图 2 台湾地区ML≥2的浅源震源深度频度分布图
Figure 2. Frequency distribution of focal depth of shallow source earthquakes with ML≥2 in Taiwan
图 3 台湾地区ML≥2的浅源地震震级-发震时间的密度分布图
Figure 3. Relationship between the magnitudes and time of shallow source earthquakes with ML≥2 in Taiwan
图 4 利用3种时空窗方法除丛获得的台湾地区主震空间分布
Figure 4. Spatial distribution of mainshocks in Taiwan obtained by using various declustering approach methods
图 5 地震目录中各震级档地震平均发生率的方差与观测时间关系
Figure 5. Relationship between observation time and variance of the average incidence of earthquakes at each magnitude bin
图 6 不同极大似然估计方法拟合的台湾地区地震活动性参数
Figure 6. Seismic activity parameters fitted by different maximum likelihood estimation methods
表 1 不同除丛时空窗方法计算得到的台湾地区完整性时间表
Table 1. Integrity schedule in Taiwan calculated by different declustering process
震级档 完整性起始年份 Gk时空窗 Gr时空窗 Uh时空窗 2.0～2.5 1996 2001 1991 2.5～3.0 1988 1994 1984 3.0～3.5 1981 1986 1974 3.5～4.0 1972 1976 1971 4.0～4.5 1966 1969 1969 4.5～5.0 1940 1950 1958 5.0～5.5 1926 1925 1947 
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表 2 不同极大似然估计方法计算的台湾地区地震活动性参数
Table 2. Seismic activity parameters calculated by different maximum likelihood estimation methods
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