臺灣西南海域三維P波地殼速度構造與2006年屏東地震發震構造之研究

Three-Dimensional Crustal Vp Structure off SW Taiwan and the Seismogenic Structure linked to the 2006 Pingtung earthquake

廖彦喆(Yen-Che Liao)

指導教授 ： 許樹坤

國立中央大學/地球科學學院/地球物理研究所/博士(2012年)

摘要

2006年12月26號，台灣屏東西南海域發生兩個地震規模約7.0的強震，其中第一起地震其主震震源機制為正斷層破裂機制，而第二起主震則是走向滑移斷層破裂機制。由於該地區以往並非預期會發生大地震的區域，因此本研究於震後搜集了11顆短周期海底地震儀收集為期一周的餘震資料波形，並結合氣象局部分短周期地震網的陸上測站資料。藉由這筆統合的地震資料將能聯合逆推西南海域發震區域的三維速度構造並重定餘震震源位置。逆推結果中發現主要特徵為該地區速度構造存在西南往東北下傾的高p波速度擾動區，推測該高速區應為隱沒至台灣南端歐亞大陸板塊的上部地函部分，而三維速度重定位後的餘震深度便大致落在該高速區的上部地函頂部或其上相對低速的下部地殼底部。震央分布則呈西北-東南走向與板塊隱沒邊界走向一致，同時也與低布蓋重力異常區走向吻合符合，其原因可能與台灣造山帶的荷重加諸於向東隱沒的歐亞大陸板塊相關。
其次，地震學上常見的震源定位方法均非常仰賴從波形中判讀正確的波相以及準確的初動走時時間，然而在部分餘震資料中，由於時空間隔相當短暫，來自不同地震事件的波形便會在測站資料中相互影響，因此提高了判讀波相及初動走時時間的難度，尤其是要在不同測站中的指認來自相同地震的波相時。於是本研究提出了一個改良式震源掃描演算法(Improved Source-Scanning Algorithm, ISSA)，其可用於解析複雜餘震的時空分布，並同時避免了費力耗時的波相判讀處理流程。該演算法改進的部分包括以下幾個部分: 增加地表振動分析程序用以分離P波和S波; 依據掃描模型的解析度，自動判識”亮度”方程式所需的時間窗長度; 以及改良了原有的”亮度”方程式定義，藉此達成利用多重波相來共同控制最佳解的目的。在合成波波形的測試實驗中，我們設計了一個相當有挑戰性的測試，想藉由人工判識的方式來判讀波相幾乎不太可行，透過解決該範例來證明ISSA的可行性以及可信度。最後本研究將該演算法運用到一組台灣西南海域所收集到的海底地震儀資料中，縱使透過目視亦很難辨識其正確波相，但最後ISSA仍決定出兩個地震事件其波相分布最能解釋所收集到的資料。
本研究將ISSA應用到2006年屏東地震的海底地震儀波形資料中來進行分析，來探求更多小規模的餘震事件並進行定位。此外，我們也將ISSA定位結果與其他兩個地震學上常用的重定位結果來做確認，以確保定位結果的信心程度。結果指出三種定位方法皆顯示屏東地震餘震序列大致分為兩群不同走向分布，其與第一個主震的東傾破裂面與第二個主震的西傾破裂面位態最為符合。最終本文依據該結果提出了合適的構造模型來解釋屏東地震其可能的孕震構造以及所觀測到的餘震分布。

關鍵字

2006屏東地震 ； 三維地殼速度 ； 台灣

並列摘要

To understand the more detailed three-dimensional (3-D) crustal velocity structure in the offshore region SW of Taiwan, we deployed 11 short period ocean-bottom-seismometers (OBSs) over the source zone of the 2006 Pingtung earthquake sequence for one week and recorded a series of aftershocks which were also recorded on land at the Central Weather Bureau (CWB) network stations. The joint dataset made it possible for us to perform a 3-D velocity tomography and earth-quake relocation in this region, where the velocity structures were not well known and location of earthquakes with only land data was uncertain.
The tomographic results show a prominent high Vp perturbation zone (HVPZ) that we consider as the uppermost mantle of the subducted plate dipping toward the NE beneath southern Taiwan. Most of the relocated earthquakes are distributed just above the HVPZ or along the bottom of a relatively low velocity subducted crust. Our results show that the subducted and bent Eurasian plate off SW Taiwan could have been unbent to form an upwards concave geometry for the upper 30 km. The main shock is near the bottom of the inflected surface. The distribution of the earthquake sequence is generally in the NW-SE direction, coinciding with the relative plate motion between the Philippine Sea Plate and Eurasian Plate. This orientation also follows a relatively low Bouguer gravity anomaly stripe that is interpreted as the consequence of the heavy loading of the Taiwan orogen on the east-dipping Eurasian Plate. Considering that the hypocenter of the first main-shock is near the bottom of the aftershocks, we interpret that the first normal faulting earthquake was caused by an unbending effect in the subducting crust and this event triggered the release of accumulated energy between the Philippine Sea Plate and Eurasian Plate. Thus, we suggest that the rupture surface of the Pingtung earthquake sequence had propagated upwards and northwestward in the direction of plate convergence.
Conventional earthquake location methods depend critically on the correct identification of seismic phases and their arrival times from seismograms. Accurate phase-picking is particularly difficult for aftershocks that occur closely in time and space, mostly due to the ambiguity of correlating the same phase at different stations. In this study, we introduce an improved Source-Scanning Algorithm (ISSA) for the purpose of delineating the complex distribution of aftershocks without time-consuming and labour-intensive phase picking procedures. The improvements include the application of a ground motion analyzer to separate P and S waves, the automatic adjustment of time windows for “brightness” calculation based on the scanning resolution, and a modified brightness function to combine constraints from multiple phases. Synthetic experiments simulating a challenging scenario are con-ducted to demonstrate the robustness of the ISSA. The method is applied to a field dataset selected from the OBS records of an aftershock sequence of the 2006 Pingtung earthquake. While visual inspection of the seismograms is ambiguous, our ISSA analysis clearly delineates two events that can best explain the observed waveform pattern.
Finally, this study applies the ISSA to the entire OBS data set of the 2006 Pingtung earthquake to search for events that were not detected or accurately located previously. We compare the ISSA results to the results of two conventional relocation methods to ensure the highest location confidence. All three location results consistently indicate that the seismicity can be grouped into two clusters with different orientations. Their distributions are consistent with the east-dipping fault plane of the first event and with the west-dipping fault plane of the second one, respectively. At last, a tectonic model is proposed to properly explain the seismotectonic structure of the Pingtung earthquakes and the observed seismicity distribution.

並列關鍵字

2006Pingtung EQ ； 3-D Crustal Velocity ； Taiwan

參考文獻

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