HHT在近斷層脈衝型地震記錄特性研究之應用

Abstract

由於脈衝型地震會使結構物需要更高的耐震需求來避免遭到破壞，因此長久以來備受學界關注。近年來許多學者發展出各自的脈衝篩選機制，其中以Shahi and Baker (2014)所提出基於小波轉換之理論為現今主流篩選機制，然而脈衝波形多樣且無明確定義，導致此方法存在限制脈衝的判斷或扭曲真實之脈衝形狀的隱憂，此外由於小波分析之特性隱含較長脈衝週之脈衝將先被提出，可能會導致忽略對結構物較有影響之週期。 為此本研究引進時頻分析工具－希爾伯特-黃轉換（Hilbert Huang Transform，HHT），HHT主要分為兩個部分進行，第一部分為經驗模態分解（Empirical Mode Decomposition，EMD）會將地震訊號拆成數條本質模態函數（Intrinsic Mode Functions, IMF）；第二部分為瞬時頻率計算會將所有IMF轉換到時頻域上呈現。本研究最終提出一套新的脈衝篩選機制，與Shahi and Baker (2014)之篩選結果相比，由於本研究所提出之方法並非使用預設脈衝函數基礎，對脈衝形狀限制較為寬鬆，被判斷為脈衝型地震之數量上升，較適合用以篩選出脈衝波數較少之脈衝型地震；新脈衝週期計算採用HHT時頻分析結果，最終能將脈衝型地震中較為主要之頻率內涵一併提出，使得一筆地震紀錄會有ㄧ個以上脈衝週期供人參考。藉由本研究之結果能更了解脈衝型地震之內涵特性。

Pulse-type ground motions produce different characteristics and higher demands on long-period structures and have drawn great attention from academic circles in the past three decades. In recent years, many scholars have developed their own methods to identify pulse-like ground motions. Among those, Shahi and Baker (2014) used wavelet transform to separate a pulse-like velocity time series into a velocity pulse and residual time series and developed index for the identification of such type of ground motion and its pulse period. Their procedure has become the most popular one. However, the procedure has its limitation since a specific type of mother wavelet is used while the shape of velocity pulse can be very different from the mother wavelet used. Furthermore, the pulse with longer period will be prior to extract because of characteristics of the wavelet analysis. The identified velocity pulse sometimes has a very long period that is not important to most buildings. In order to avoid above disadvantages, this study introduces Hilbert Huang Transform (HHT), a time-frequency analysis tool. HHT includes two parts. The first part, Empirical Mode Decomposition (EMD), is able to decompose the seismic signal into a finite and often small number of Intrinsic Mode Functions (IMF); the second part is that converting IMFs from its time domain to a representation in the frequency and time. This study finally proposes a new method to identify pulse-like ground motions, and then comparing with the method proposed by Shahi and Baker (2014). Owing to using adaptive basis the method proposed in this study has less limitation on extracted pulse shape and therefore increases the number of pulse-like ground motions. Especially, the method is suitable to identify pulse-like ground motions having lower number of pulses. Furthermore, the new method can calculate pulse periods on time-frequency domain and identify all important frequencies from the time series.