| 摘要: | 地震波輻射能量(Es)與地震力矩(Mo)為地震的兩個重要參數，尺度能量(Scaled Energy)之定義為Es/Mo，其表示為單位地震力矩所釋放之輻射能量。許多因素為影響所使用近場、區域與遠場之地震記錄所估算Es和Mo，進而造成Es/Mo產生很大差異。強烈的場址效應會造成地震訊號在高頻段有振幅放大進而導致高估了震源參數；有限頻段的地震訊號則會低估了震源參數。 使用quarter wavelength approximation的方法來計算位於台灣中部地區之87個強震站場址的放大效應，根據地表下30公尺深的井測結果與由地震波研究所推求的三維速度構造，來重新建立新的速度與密度模型，進而來計算87個場址的振幅放大倍率。 另外，假設震源之頻譜為w-squared衰減模型，對於有限頻寬地震訊號在估算Es、Mo和Es/Mo的影響也進行理論研究，以減低在有限頻寬估算地震源參數所造成的影響。 本研究使用中央氣象局之近斷層強地動紀錄來估算1999年集集大地震22個餘震之Es，Mo和fc，其中fc為拐角頻率，其區域規模為5.1£ML£6.8。利用Andrew(1986)速度與位移地震記錄平方後之積分方法，以所得到的數值來計算Es，Mo和fc；消除影響估算震源參數的因子，如輻射型態、自由表面放大效應、衰減因子、場址效應以及有限頻寬限制。所估算的結果為Es=2.0´1018 – 8.9´1021 ergs 和 Mo=1.3´1023 – 1.4´1026 dyne-cm，Es/Mo=7.4´10-6 – 2.6´10-4，其平均值為 ~7.9´10-5。本研究所估算的Es/Mo對於地震規模大小呈現些許的正相關性。在地震力矩與拐角頻率之關係為Mo~fc-3.65。另外，尺度能量與震源深度有些許的正相關，其關係為Es/Mo=1.92´10-5e0.09h，其中h為震源深度。 The seismic radiated energy, Es, and seismic moment, Mo, are two fundamental parameters of an earthquake. The scaled energy, Es/Mo, is defined to be the ratio of seismic radiated energy to seismic moment, denotes the radiated energy per unit seismic moment of an earthquake. Several factors could affect the measures of Es and Mo from local, regional, and teleseismic data, thus, resulting in high divergence of Es/Mo. The strong site effect, especially at higher frequencies, can produce an over-estimates of source parameters; while finite frequency bandwidth limitation leads to an opposite effect. Based on the quarter-wavelength approximation method proposed by Boore and Joyner (1997), the site amplifications at 87 strong-motion stations in central Taiwan are evaluated from the velocity and density structures constructed from well-logging data in the topmost 30-m and 3D velocity models inferred from earthquake data. In addition, the formulas used to eliminate the effects on Es, Mo, and Es/Mo caused by finite frequency bandwidth limitation based on the w-squared source model are presented. In this study, we measure the values of Es and Mo for 22 aftershocks with 5.1£ML£6.8 of the 1999, Chi-Chi, Taiwan earthquake based on the local seismograms from the Central Weather Bureau. The method proposed by Andrew (1986) is used. After the corrections, i.e., the radiation pattern, free surface amplification, Q-factor, the site effects, and finite frequency bandwidth limitations, the results are Es=2.0´1018 – 8.9´1021 ergs and Mo=1.3´1023 – 1.4´1026 dyne-cm. This gives Es/Mo=7.4´10-6 – 2.6´10-4, with an average of ~7.9´10-5. Es/Mo slightly increase with earthquake magnitude. The Mo – fc scaling almost has the following relation: Mo~fc-3.65. In addition, Es/Mo slightly depends on the depth in the following form: Es/Mo=1.92´10-5e0.09h, where h is focal depth. |
