臺灣近地表震波構造的時序變化及其地質推論

Abstract

本研究利用垂直的井上地震儀與井下地震儀測站資料，結合尾波干涉法來建立兩個測站之間的體波經驗格林函數（EGF），以研究2015年至2023年間臺灣近地表（<500 m）震波結構（S波速度和非均向性）的變化。Chen et al. (2017)和Lee et al. (2023)通過規模大於4.5的地震事件，可以獲取穩定且高品質的EGF，從而初步了解臺灣近地表震波構造的特性，並根據非均向性的成因機制，將測站劃分為兩種類型：平行應力的非均向性（SAA）和平行山脈走向的非均向性（OPA）。本研究為了加入時間解析，需要大量資料，故而將地震規模調降至3.0，並進行EGF穩定度、疊加天數和疊加方法的測試，確保長期監測的可靠分析結果。從7個SAA和6個OPA測站的時序分析顯示，除了位於花東縱谷的玉里站(EYUL)在2022年期間受到強烈擾動外，SAA測站相較於OPA測站更穩定。為進一步了解其中的機制，本研究對時序結果進行(1)季節性分析、(2)降雨分析和(3)地震影響分析。結果顯示，臺灣近地表的震波構造僅在少數測站呈現顯著季節性變化，而降雨是南投國姓站(WCS)S波速度呈現季節性變化的主要因素。雖然WCS屬於OPA測站，其震波構造長期由構造主導，但在受到極端降水時，仍會因其帶來的地層應力變化而產生影響。這暗示了OPA機制較為複雜，可能同時受到構造及應力擾動的影響，也導致在長期監測結果中，震波構造比SAA更為波動。此外，發生於2022年9月18日的池上地震序列是導致EYUL測站主要變化的原因，該地震序列導致EYUL測站的S波速度和非均向性強度下降、快軸急劇變化，對稍遠的EGFH（SAA）測站影響較小，但對距離相近的EDH（OPA）測站並未有明顯影響，因此推斷震源距離與非均向性成因機制皆是地震事件是否對震波構造產生強烈影響的重要因子。然而，不同測站間的差異顯示，影響臺灣震波構造變化的原因相當複雜，並非單一因素所致，還可能受到其他未知參數的干擾。

This study utilizes data from vertical borehole and surface stations pairs, combined with coda wave interferometry, to establish empirical Green's functions (EGFs) for body waves between two stations. The aim is to investigate changes in the near-surface (<500 m) seismic structure (S-wave velocity and anisotropy) in Taiwan from 2015 to 2023. Chen et al. (2017) and Lee et al. (2023) obtained stable and high-quality EGFs from seismic events with magnitudes greater than 4.5, providing preliminary insights into the near-surface seismic structure in Taiwan. Based on the mechanisms of anisotropy, stations were categorized into two types: stress-aligned anisotropy (SAA) and orogen-parallel anisotropy (OPA). For the purpose of incorporating temporal resolution, this study required a substantial amount of data, leading to the reduction of the seismic event magnitude threshold to 3.0. Tests on EGF stability, stacking days, and stacking methods were conducted to ensure reliable long-term monitoring results. Time-series analysis of seven SAA and six OPA stations revealed that, except for the Yuli (玉里) station (EYUL) in the Longitudinal Valley, which experienced significant disturbances in 2022, SAA stations were more stable compared to OPA stations. To further understand the mechanisms behind these observations, this study conducted (1) seasonal analysis, (2) rainfall analysis, and (3) earthquake impact analysis on the time-series results. The results indicated that only a few stations showed significant seasonal variations in the near-surface seismic structure of Taiwan, with rainfall being the primary factor for the seasonal variation in S-wave velocity at the Guoxing (國姓) station (WCS) in Nantou(南投). Although WCS belongs to the OPA category and its seismic structures are primarily driven by structural factors, it can still be influenced by stress changes induced by extreme rainfall. This suggests that the OPA mechanism is more complex, potentially influenced by both structural and stress perturbations, leading to more variable seismic structures over long-term monitoring compared to SAA. Furthermore, the Chishang (池上) earthquake sequence on September 18, 2022, was identified as the main cause of changes at EYUL station, resulting in decreases in S-wave velocity and anisotropy strength, as well as rapid changes in the fast polarization direction. This earthquake sequence had a minor impact on the more distant EGFH (SAA) station but no significant effect on the nearby EDH (OPA) station. It is inferred that both the distance to the earthquake source and the anisotropy mechanism are crucial factors in determining whether a seismic event strongly impacts the seismic structure. However, variations between different stations indicate that the factors influencing changes in Taiwan's seismic structures are complex, not attributed to a single cause, and may be affected by other unknown parameters.