利用最近鄰居去叢集方法探索台灣地區地震序列特性

Abstract

地震序列的時空分佈可以提供斷層構造的三維空間型態、應力移轉和可能引發機制等資訊。台灣位於年輕的造山帶上，擁有高度地殼變形率和活躍的地震發生率，因此探討不同地質區中的地震引發機制和活動構造特性對於台灣地區的防災是非常重要的議題。為了探討台灣地區地震序列的活動特性，本研究選用了1990年一月到2018年六月中央氣象局的地震事件，並且利用最近鄰居去叢集方法（Nearest Neighbor Approach）來將所有地震事件區分為背景地震和地震序列。和其他常見的地震去叢集方法相比，最近鄰居去叢集方法保留較多地震事件以符合時間分佈的隨機性，是較適用於台灣地區的去叢集方法。而根據地震序列中的前震活動特性，本研究進一步將地震序列區分為主餘震序列和群震。主餘震序列大多分佈在西部麓山帶附近，而群震則多分布在中央山脈南北段和花蓮北部。從液壓擴散模型、金礦礦脈和溫泉分佈來看，中央山脈南北段的某些群震可能為液壓擴散所引發。此外，中央山脈北段的群震的水力擴散係數大多低於中央山脈南段，可能是和台灣由北而南的造山階段有關，或是受到地殼伸張率影響。

Spatiotemporal characteristics of earthquake clusters can give insights into 3-D geometry and stress/strain state of fault structures, and potential interaction with fluid and heat. Taiwan is one of the most active orogenic belts with high deformation and seismicity rates. It is expected to observe seismicity driven by varying mechanisms among different geological units. To investigate the tectonic complexity, a high-quality and robust catalog of earthquake clusters is critical. This study collected a long-term-effort earthquake catalog from the Central Weather Bureau from 1990/01 to 2018/06. Based on a statistics-based Nearest Neighbor Approach (NNA), we declustered the entire relocated catalog and produced independent (declustered) and earthquake cluster catalogs. In comparison with other declustering methods, the NNA-derived catalog shows better Poisson process characteristics and remains more events. Based on the foreshock activities, the earthquake clusters can be separated into the typical mainshock-aftershock sequences and the swarms. Most of the mainshock-aftershock sequences are distributed near the Western Foothill. As for the swarms, they are usually distributed in the northern and southern Central Range and northern Hualien regions and some of them may be induced by fluid diffusion. In addition, the hydraulic diffusivity of swarms in the southern Central Range is larger than that in the northern Central Range, which may be resulted from the different orogeny stages from north to south in Taiwan regions.