以臺灣北部為例探討地表地形效應對地震波到時與震幅的影響

經由地震波走時成像得到高解析度和高精確度的地殼速度模型，不管是在防災、震後救災或是地震災害的評估中，對於估計地震所產生的強地面運動是非常重要的。變化劇烈的地形對於地震波的傳遞會有顯著的影響，同時地表不規則的幾何形狀對入射地震波所產生的散射不僅僅與地表地形有關，還會受到許多其他因素的影響，如地震波所經過的路徑和地震波在地表的入射角，而這些條件又取決於測站所在的方向、位置和高程。這些地表地形所帶來的種種影響都會改變地震波的到時以及。本研究以台灣北部地區地形與地震測站分佈為例，通過使用有限差分數值模擬的方式計算在真實地表地形以及三維速度結構模型中精確的理論地震圖，並且利用在考慮地表地形和忽略地表地形兩種情形下計算的地震波之間的互相關，來精確測量地表地形對於地震波的到時和振幅所產生的影響。測量結果顯示，在不同頻率下地表地形對於P波與S波到時所造成的偏差量可以分別達0.4秒和0.6秒，而對S波振幅所引起的變化可以超過一倍。通過與利用TAIGER計畫下人工炸測得到的P波初動到時以及人工炸測數據反演得到的臺灣北部二維模型比較，可以發現忽略地表地形對P波到時的影響，將會在反演得到的P波速度結構模型中帶來±10%的誤差，因此，地表地形對利用P波到時來做地殼速度成像所產生的影響是不容小覷的。此外，對S波振幅的測量結果顯示在給定的地表地形條件下，地表地形對來自不同方向的地震波振幅的影響也是不一樣的，因此，在估計地表強地面運動以達到地震災害評估的研究中，僅僅使用少數的情景地震所得到的結果可能是片面的，可靠的地震災害評估需要考慮所有方向可能發生的地震，並對地形影響地面運動的結果做出統計的分析。

關鍵字

有限差分方法；地表地形效應；地震波成像；北部臺灣

並列摘要

The resolution and accuracy of the crustal models obtained from seismic tomography is very important in making reliable estimations of strong ground motions of earthquakes for both post-earthquake disaster relief and assessing future seismic risk purposes. Drastic topography changes have significant effects on the propagation of seismic waves, which leads to perturbations in waveforms and therefore the arrival times and amplitudes of seismic phases. Various factors contribute to the topography-induced waveform changes, including not only the fixed geometrical shape of the surface itself, but also the path-related incidence angles and azimuths of the seismic waves which are dependent on the location of the receiver site as well as the back azimuth, epicentral distance and depth of the earthquake. In order to assess the potential bias in crustal models obtained by traveltime tomography when the effect of surface topography is neglected, we carry out a systematic analysis of the surface topography effect on the traveltimes and amplitudes of seismic waves using accurate synthetics calculated by the finite-difference method in three-dimensional structure with realistic surface topography and quantify the topography-induced delay times and amplitude anomalies by cross-correlations of waveforms obtained with realistic topography and those with topography ignored. The quantitative measurements of topography effect is used to assess the potential biases in seismic tomography due to the omission of topography in modeling regional seismic waveforms. The results show that for the topography relief of northern Taiwan, these biases can be up to 0.4 s for P- and 0.6 s for S-wave delay times and more than 100% for S-wave amplitudes, which we demonstrate to be significant in tomography inversions for crustal structure. Our results also suggest that in PGV and PGA predictions for hazard purposes, results from a few scenario earthquakes are insufficient in producing the complete picture.

並列關鍵字

finite-difference method ； topography effect ； seismic tomography ； northern Taiwan

參考文獻

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以臺灣北部為例探討地表地形效應對地震波到時與震幅的影響

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