台灣位於歐亞大陸板塊和菲律賓海板塊交界處，由於板塊的隱沒與碰撞造成台灣本島複雜的地體構造以及頻繁的地震活動。而台灣的強震網(TSMN)為全球最密集的區域，此豐富的波形資料恰好提供一個機會使構造研究有良好的解析。台灣地體構造的分析自1986年以來，經由地震資料分析得出細部的三維速度構造(Roecker et al., 1986；Ma et al., 1996；Rau and Wu., 1995 )。但此三維速度構造在對於構造的瞭解，仍無法有較深入的分析。由於某些地體構造的岩石特性對於Q的影響遠勝於對速度構造的影響(Eberhart-Phillips and Chadwick, 2002；Chen et al., 1996)，而S波對於孔隙液體的塑性也較P波有更高的敏感度。因此，若能求出Qs，對於速度構造所無法給予的訊息，可以有更進一步的參考，進而加強對於台灣地體構造的解釋。 為避免震源複雜性的影響，本研究利用1993~1998年，ML4.5~5.5的強震資料。經資料篩選及處理後，共用173個地震，超過2000筆的資料，在ω2-source model以及Qs與頻率無關的假設下(Rietbrock, 1999)，以非線性逆推擬合1~20Hz的頻率範圍求得t*。進而利用已知的三維S波速度構造，以影像層析法(Tomography)逆推求得台灣三維Qs模型。 逆推結果顯示本研究所得的Qs分布大致反應出台灣地體構造現況。在台灣西部沿岸以北緯23.5∘為界，淺於十公里的西北部和西南部Qs值差異正好反應出台灣西部沿岸南北基盤深度不同所造成岩性的差異，以北較淺的基盤構造比南部的沉積物有較小的衰減值。臺東縱谷東西部Qs的差異則恰好反應出其西側為大陸邊緣山脈，東側為由島弧改造而成的山脈在構造特性上的變化。在27公里至38公里深處，由北緯24.0∘向北延伸至北緯25.0∘的高Qs構造可能與連結臺灣北部陸上火山帶和琉球島弧的隱沒構造有關。中央山脈地質區下為一低Qs構造，深度至少達30~40公里深，且其高的衰減值和地熱流的分布有很大的關係。而在北緯23.7∘~24.05∘，有一個自東經120.55∘下5公里深的位置以20∘向東傾斜的高Qs構造，此構造可能與集集地震的機制有很大的關聯。在地震和Qs的分布關係上，地震主要分布在較高Qs的區域，低Qs的區域，其地震分布相當少，可能原因為地熱流的影響，導致該處地殼的塑性增加，降低地震的發生率。最後，台灣北部和中部的Qs深度分布較初始模型有較大的變化。北部的Qs值大致隨深度而增加，中南部則在20~30公里處有突然變小的情況。 The S-wave attenuation structures (Qs) of Taiwan are imaged using the S-wave data from the dense deployed strong motion network of Taiwan. The image of 3-D Qs is essential in Taiwan because Qs is sensitivity to localized permeability and heterogeneity, which are important characters for interpreting active tectonic regions as Taiwan. The attenuation images of S-wave are obtained using t* values measured from S-wave spectra from Taiwan strong motion network for moderate size earthquakes (ML4.5-5.5) to avoid the source complexity. The time period used in this study is from 1993-1998. Over 2000 velocity spectra of S wave arrivals from 173 earthquakes were analyzed. A non-linear least square technique is applied to the spectra for t* by assuming a ?-2 model for the frequency band of 1-20Hz. A frequency-independent Qs was assumed in this study. Combined the existed three-dimensional S-wave velocity model, the three-dimensional Qs images were obtained. The Qs-tomography shows general consistent feature to geological structure. At shallow depth (4~9km), the Qs tomography separated the western Taiwan into high Qs and low Qs images to the north and south, respectively, along latitude of 23.5∘N. The low Qs image within southwestern Taiwan is related to its thicker sediment; while the high Qs image within northwestern Taiwan is related to the formation of foreland sequence. The image also reveals the variation across Longitudinal Valley, which is the suture zone of the arc-continent collision. At deeper depth, 27-38 km, a high Qs images extended from latitude of 24∘N to 25∘N, which is related to the subduting slab. A relative low Qs zone beneath the Central Range was found in the depths of 30-40 km. This low Qs zone is consistent with the low velocity zone obtained in Vp and Vs velocity images. The profile across the Chi-Chi hypocenter shows clear variation on Qs image across the ruptured Chelungpu fault. Our results show that the Qs images provide significant information for further understanding the tectonic structure of Taiwan.
