震源研究:地震與地崩

Abstract

台灣具有密集的地震站分布與高地震活動度，提供了大量的地震波形資料，而眾多大小規模之地震活動，有助於探討區域性地地震構造。本研究論文，先是利用1999 Mw7.6集集地震、2003 Mw6.8成功地震與2006 Mw6.1台東的強震地動記錄，檢驗本研究所發展的自動化基線修正程序，基於與前人的比較結果，說明此程序可適用於恢復大、小之同震變形記錄，並有助於快速且穩定地計算同震變形量。近年來由於台灣的三維速度模型已發展成熟，且計算機的快速發展，有助於我們利用有限差分法運算三維應變格林張量之資料庫，基於震源與測站間的互換定理，能有效地增加模擬三維合成理論地震波之效率。在此，我們發展利用應變格林函數逆推震源機制解的方法，其具有下列優勢: (1)同時結合P波初動與波形擬合程度，(2)運用基因演算法來進行最佳化程序，(3)利用三維應變格林張量，提供較為精確的合成理論地震波。我們將此程序應用於探討1999嘉義地震序列之震源機制解與地震深度，結果顯示該地區主要為走向與逆衝的斷層面解，與前人P波初動解和地震矩張量反演之結果呈現一致性。本研究更進一步發展近場P波逆投影法，其是假設觀測紀錄波形上的各別P波脈衝訊號，來自於斷層面上在不同時間之單點破裂源所形成的，故在時間可逆的假設之下，分別將各測站之P波脈衝振幅疊加，逆投影至原本在斷層面上的破裂位置，即可了解地震之細部破裂過程。我們初步將此方法應用於2010甲仙地震，由結果顯示與餘震序列和前人研究之斷層面滑移分布呈現一致。綜合本研究所發展的高效率與自動化之程序，便能在災害性地震發生後，運用即時強震紀錄來演算地表同震變形量、地震斷層機制解與地震之破裂過程和滑移量之分布。 在颱風季節所伴隨之豪雨，易誘發地崩(岩石崩落、岩石崩滑、土石流與岩石崩落)的產生，往往造成嚴重的生命以及財產的損失。因此，減緩地崩災害已成為全球重視之課題。傳統地動訊號分析方法提供了研究地崩與輔助其他地崩資料的機會。本研究在2009莫拉克颱風期間，利用即時寬頻地震儀所記錄的崩塌地動訊號進行時頻分析，共判識出12個大型地崩事件，此颱風共造成675人死亡、24失蹤，約估計有3.3億美元的經濟損失。本研究更進一步發展一套自動化程序，以達到即時決定地崩位置與其崩塌面積，且利用崩塌地動訊號之參數值，來判別崩塌行為與其崩塌形態，尤其針對具有堰塞湖之崩塌事件。此程序可以提供防災部門地崩事件之相關資訊，以達到減災之作用。

Due to the density of seismograph stations in Taiwan is among the highest of the world and high seismicity, provides a lot of seismic data of small-to large-sized earthquakes. Frequent earthquakes of various magnitudes provide valuable information for understanding the regional seismotectonic environment. The first part of the dissertation focuses on the coseismic deformation which can be determined from strong-motion records of large earthquakes. The baseline corrections are often required to obtain reliable coseismic deformation because baseline offsets lead to unrealistic permanent displacement. We use strong-motion records from three large earthquakes in Taiwan (1999 Mw7.6 Chi-Chi, 2003 Mw6.8 Chengkung and 2006 Mw6.1 Taitung earthquakes) to illustrate our automatic baseline correction procedure. Based on the comparisons for coseismic deformation results from previous studies, our new procedure is suitable for quick and reliable determination of coseismic deformation from strong-motion records. According to recent 3D structural models in Taiwan are already available, and efficient numerical methods have been developed for calculating Green’s function, a database is established for strain Green’s tensors (SGTs) obtained by the finite-difference method in this study. With this SGTs database, 3D synthetic seismograms can be computed by simple retrieving the appropriate SGTs from database and applying the reciprocity theorem. Here, we develop an approach to determining the focal mechanisms of small and moderate earthquakes by combining three effective ingredients: (1) an optimization criterion including both first-motion polarity and P-waveform fitting, (2) Green’s functions in 3D velocity models, and (3) a grid search for source depth and focal mechanism based on the genetic algorithm (GA). We apply this approach to determine the focal mechanisms and centroid depths of the 1999 Chia-yi earthquake sequence. Results show dominant strike-slip and thrust mechanisms that are in good agreement with previous results based upon P-wave first-motion polarities and moment-tensor inversions. To further understand the characteristic of source rupture process, we propose an approach of imaging source slip distribution by direct back-projection of local absolute P-wave displacement in strong-motion records. This approach is applied to estimate the slip distribution of an earthquake occurred in March 4, 2010, in Jiasian in southern Taiwan. Our resulting slip images are consistent with the distribution of aftershocks and slip distributions obtained from dislocation model and finite-fault inversion. Aforementioned highly automatic and efficient approaches allow for rapid determinations of coseismic deformations, fault-plane solutions and slip distributions after earthquakes using strong-motion records, providing important, useful and timely information for seismic hazard mitigation. Landquakes (e.g. rock collapse, rock slide, debris flow and rock avalanche) induced by the excessive rainfall during typhoon season can be catastrophic, both in loss of human life and to the economy. Thus, landquake risks have become an important issue in global natural mitigation effort. Conventional seismic data analysis offers a unique approach to studying landquake that is independent of and complementary to other types data. In this study, we apply time-frequency analysis to detect 12 landquakes in Taiwan during the passage of Typhoon Morakot in 2009, which resulted in 675 death, 24 missing, and an estimated economic loss of $3.3 billion. These events were recorded by seismic stations of the Broadband Array in Taiwan for Seismology (BATS). We develop an automatic and efficient approach for rapid determination of landquake centroid location and collapse area, and for identifying dam-formation event using records from existing real-time broadband seismic network, thus providing an important alternative for landquake hazard mitigation.