| 摘要: | 馬尼拉海溝(Manila trench)南北兩端於不同時期因弧陸碰撞(arc-continent collision)而終止隱沒系統，於北端之台灣有充足的地震資料來分析複雜之雙隱沒系統，然而，位於馬尼拉海溝的南端卻相當缺乏地震觀測資料，因此，本研究於馬尼拉海溝南端之終點—民多洛島西北(NW Mindoro)陸續佈置寬頻地震站，加上中央研究院提供之測 站，共有五個寬頻地震站，紀錄2014年至現今之地震資料。藉由分析剪力波通過非均向性介質會分離出快波以及慢波之特性，並利用遠震波相SKS、SKKS以及近震波相、ScS、PcS與S來探究上部地函至接收端下之非均向性構造，進而了解馬尼拉海溝南端之地體動力過程。本研究之方法主要利用Bowman and Ando (1987)提出之波形交相關法(Waveform Cross-correlation Method, RC)以及Silver and Chan (1991)提出之特徵值法(Eigenvalue Method, EV)來求得剪力波分離之參數解(∅,δt)。於研究結果中，RC法與EV法所得之剪力波分離參數差異不大，其平均分離時間差約為1.69秒；而快波極化方向之分佈座落於西北—東南至北北西—南南東方向，此趨勢平行於馬尼拉海溝南端。經由震波層析影像以及地震事件分佈之關係，可依據震波是否通過隱沒板塊與地函契來推論其非均向性之貢獻來源，若是沒通過隱沒帶，其快波極化方向反映之非均向性的貢獻可能來自馬尼拉海溝邊緣之海溝遷移(trench migration)所導致平行於海溝之地函流(mantle flow)；若是波線主要通過隱沒板塊內部，則推測此非均向性的貢獻可能源自於漸新世(33 Ma)南中國海海脊之南北向及西北東南向之張裂過程中(Sibuet et al., 2016)，將古非均向性紀錄於岩石圈內部(frozen-in anisotropy)，隨著板塊隱沒至民多洛島西北底下而被觀測到。;Both the ends of the Manila trench are terminated by arc-continent collision at different inception times. In the north of the Manila trench, there are dense seismic stations around Taiwan, which can provide sufficient waveform data to analyze the complex subduction system beneath NE and SW Taiwan. In contrast, the lack of the seismic data on the southern end of the Manila trench makes us have strong motivation to deploy seismic stations on NW Mindoro where is located on the southern end of the Manila trench to collect more seismic data. As a result, there are total five broadband seismic stations deployed on the NW Mindoro including one station deployed by IES, recording the seismograms since 2014. By analyzing the measurements of shear wave splitting of teleseismic phases such as SKS and SKKS and regional phases such as ScS, PcS and S, we can investigate the anisotropy from the upper mantle up to crust beneath receiver sides, which could provide us with more information about the geodynamic process beneath this area. In our study, we use Waveform Cross-correlation Method (RC) proposed by Bowman and Ando (1987) and Eigenvalue Method (EV) proposed by Silver and Chan (1991) to obtain the shear wave splitting measurements (∅,δt) including the fast wave polarization and splitting time between fast wave and slow wave. In our results, the average splitting time obtained from RC and EV method is around 1.69 s, which could roughly correspond to 170 km thickness of the anisotropic layer. The fast wave polarizations estimating by both methods are oriented between NNW-SSE and NW-SE directions which is roughly parallel to the southern end of Manila trench. From the P wave tomography and distribution of seismic events, we can judge whether the ray paths passing through the subducted slab and mantle wedge or not, and thus infer the contributions of anisotropy beneath NW Mindoro. The ray paths sampling most mantle wedge related to the asthenospheric flow induced by trench migration or little subducted slab may show the NW-SE trend of the fast directions. On the other hand, we suggest that the main contribution of anisotropy may come from the fossil spreading ridge of South China Sea (SCS) since Early Oligocene (33 Ma) (Sibuet et al., 2016) if the fast directions show NNW-SSE trend. |
