考慮時間相依之發震模型與長周期盆地效應特徵之地震危害評估

A Framework to Include Time Dependency Earthquake Probability Model and Long-Period Characterization of Basin Topography in Seismic-Hazard Assessment

張毓文(Yu-Wen Chang)

指導教授 ： 羅俊雄

國立臺灣大學/工學院/土木工程學研究所/博士(2017年)

https://doi.org/10.6342/NTU201703728

摘要

傳統耐震設計需求，即為考慮在最具潛勢的地震威脅下，建物尚保有一定的基本防護功能，確保生命財產安全。其耐震需求為一地震動反應譜。常用的評估方法多為定值法與機率法。定值法 (Deterministic approach)為配合地震動評估模型，直接推估對工址最具威脅之震源造成之地震動；而機率法則在於能完全考慮所有型態震源所引致之地震動，對工址可能造成危害之機率統計，此方法則稱為機率式地震危害度分析(probability seismic hazard analysis, PSHA)。在1997年後，重大的公共工程建設，如核能電廠，美國核能法規建議採用地震危害度分析進行地震危害評估。然而，受限於地震源與地震動特徵分析模型，部分特殊的工址所需之設計地震反應譜則需另外進行地盤反應分析，以能得到場址相依的地震動反應譜，如位於盆地地形上的工址。
進行地震危害度分析以求得耐震設計反應譜時，地震源與地震動模型為分析的兩個主要參數。隨著時間演進、資料蒐集與知識的精進，對於地震源與地震動之模擬能接近真實，將會減低分析時的不確定性。在地震源特徵方面，以適當的地震發生模型模擬地震發生在時、空分布的特徵與其不確定性，正是需要資料進行比對以反映此一要求。在地震危害度分析過程中，一般包含區域震源與活動斷層震源。常用之地震發生率的模型，包括截斷指數機率密度函數(Truncated Exponential Model)與Youngs and Coppersmith(1985)之特徵地震模型。PSHA常見的模型假設中，區域震源包含因未知斷層所引致之地震，多應用截斷指數機率密度函數，並在模型中考量一上限規模，限制不合理的地震發生。但此做法則衍伸截斷指數機率密度函數會低估大規模區域地震的發生率。本研究針對此差異將提出複合式模型，以修正區域震源模型對大規模地震發生率的低估。再者，由於台灣因地體構造環境複雜，擠壓作用導致斷層引發大規模地震，如車籠埔斷層等。斷層古地震調查資料顯示，斷層的發震行為屬於時間相依，為再現期可預測模型(time-predictable model)的特徵地震，此現象與應力回彈理論相近，說明斷層地震的發生在短期間內，並不完全符合傳統的Poisson機率分布。針對此部分，本研究以時間相依的機率分布函數，配合工程可靠度分析的理論，計算斷層在未來特定時間內發生錯動之條件機率，提出等值Poisson比，以能套用至傳統地震危害度分析程序所採用Poisson機率分布之用。本研究並提出台灣陸地上25條活動斷層在未來結構壽命(50年)可能之地震規模與對應之發生機率分佈圖，以做為結構設計或防災規劃的參考。
傳統上，地震危害度分析所考量之地震動特徵，即為地震動評估模型，是屬於規模、距離的函數。近年來對於地震動的評估，亦朝向配合震源特徵，精進並多方考慮影響震幅特徵的模型項，如近斷層效應、深度效應與地盤特性等。美國太平洋地震工程研究中心所提出之新一代衰減模型(Next Generation Attenuation)計畫即是朝此方向發展。但受限於資料量，部分模型乃採用模擬方式提出，仍有待資料的驗證。因此，對於特殊地盤上的地震動特徵，由實際資料進行分析是直接的方式。本研究針對台北盆地過去所收集的地震動資料庫為參考，以複合式奇異譜分析的數值分析方法 (Multivariate Singular Spectrum Analysis，MSSA)，將因地震所引致的盆地地形效應特徵，於一群地震動資料中取出。針對過去對台北盆地造成重大災情之三個地震(1999集集地震、2002年331花蓮外海地震及1994年南澳地震)，由位於盆地內密集之強震測站所收錄的地震紀錄進行分析，得到台北盆地在受三個地震作用下，因盆地地形效應產生共同的主要週期約為2~3秒的長周期波。
因此，在盆地內之地震危害評估，可以地震危害度分析所得之未來特定時間內之岩盤設計地震反應譜，結合盆地地形效應之長周期特徵，則針對台北盆地內工址，考慮時間相依模型與長周期盆地地形效應特徵的設計地震需求可被發展，且直接反應地形效應。MSSA分析的成果也將地震資料內與盆地內軟弱土層所引致之地盤效應分離出來，並與土層深度有關，未來將可作為地盤放大因子之考量。

關鍵字

地震危害度分析 ； 地震發生率模型 ； 時間相依 ； 條件機率 ； 複合式奇異譜分析 ； 盆地地形效應 ； 地盤放大效應

並列摘要

The objective of a seismic design of building, power plant components and structures is to ensure safety of the plant and the people around in the event of an earthquake. The design basis ground motion is generally specified by a response spectra for various values of damping. Both probabilistic and deterministic methods have a role in seismic design performed for decision-making purposes in over 30 years. Prior to the acceptance and incorporation of probabilistic approach into standard hazard assessment methodologies, most seismic-hazard assessments were completed using scenario-based, “deterministic” analyses. They are proposed to provide design earthquakes for site-specific studies such as the designing of critical structures. The deterministic approach considers the scenarios that have the small occurrence number. Relatively, the all of the possible scenarios will have considered in the probabilistic approach. PSHA provides a key element for engineering seismic design. It is concerned with evaluating the various natural effects of earthquakes at specific levels, which may cause safety or design consequences to critical structures at particular sites such as dams, public structures, and nuclear power plants.
Most of the developments in the PSHA have been primarily concerned with introducing different probabilistic models to describe randomness, such as earthquake magnitude, and recurrence rate, in order to achieve more realistic descriptions for a specific site. The analysis requires characterization of all known earthquake sources that could affect the site, including faults and areal sources. For a particular site, the hazard contributions are integrated over all magnitudes and distances for all source zones, according to the calculation of total probability theory. Extensive advances in seismic knowledge in recent years by a large and active community of researchers around the world, the gaps in the understanding of the mechanisms that cause earthquakes and the recurrence behavior of earthquake are reduced. These gaps in understanding mean that, when a PSHA is performed, the significant uncertainties in the numerical results will be decreased due to the correct models used. Up-to-date knowledge of the source data along with accurate models can provide valuable information in reducing such uncertainties and in facilitating a correct understanding of PSHA.
For the earthquake recurrence model, the seismic data fitting well will promote the reliabilities for the hazard calculation. The subjective judgment and interpretation of the limited data affect the result of PSHA. The earthquake recurrence model that is used to describe the earthquake recurrence rate commonly is defined in epistemic uncertainty due to our lack of knowledge which will be reduced in time. The objectives of this study, one is to discuss the aspects of current model setting that need improvement in the recurrence rate model in the PSHA. The Gutenberg–Richter (1994) exponential frequency–magnitude relation uses to describe the earthquake recurrence rate for a regional source. It is a reference for developing a composite procedure modelled the occurrence rate for the large earthquake of a fault when the activity information is shortage.
Given the probability distribution function relating to the recurrence interval and the occurrence time of the previous occurrence of a fault, a time-dependent probability model of a particular fault for seismic hazard assessment was developed that takes into account the active fault rupture cyclic characteristics during a particular lifetime up to the present time. The time-dependent model was used to describe the fault characteristic behavior. The effects of time-dependent and time-independent models of fault (e.g., Brownian passage time (BPT) and Poisson, respectively) in hazard calculations are also discussed. The proposed fault model result shows that the seismic demands of near fault areas are lower than the current hazard estimation where the time-dependent model was used on those faults, particularly, the elapsed time since the last event of the faults (such as the Chelungpu fault) are short. One of the aims of this study is to understand the effect of seismic source recurrence probability models on the hazard results by quantifying the differences in the design ground motions. Additionally, the difference of hazard contribution between the ranges of source models in the hazard analysis provide to contemplate for source or fault setting in the PSHA.
The analysis also considers the attenuation of seismic energy as it emanates from the earthquake hypocenter to the site of interest, which is evaluated through the use of empirical GMPE. In Addaction, the effects of earthquake-induced basin amplification are caused by the interaction of wave-fields with a basin boundary, which depend on complex source to site distances, basin geometry (topography), and sediment distribution within the basin. In this study the identification of long-period waves induced by strong earthquake motions through basin topography together with local site effects of the basin is investigated. Through multivariate singular spectrum analysis (MSSA), a unique analysis tool for extracting tendencies and harmonic components in geophysical time series is used to extract the long-period wave of basin response caused by the seismic events. Seismic response data of Taipei Basin were used to examine the existing of the lowest dominant frequency of the basin caused by basin topography. Finally, the seismic-induced ground motion data can be separated to the basin motion caused by the effect of topography. A framework to include time dependency earthquake probability model and long-period characterization of basin topography in seismic-hazard assessment will be discussed in this study

並列關鍵字

Probabilistic Seismic Hazard Analysis ； Earthquake Recurrence Model ； Time-dependent Probability Model ； Conditional Probability ； Multivariate Singular Spectrum Analysis ； Taipei Basin ； Basin topography characteristic ； Local site effect

參考文獻

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