臺灣位於歐亞板塊及菲律賓海板塊相互聚合的板塊邊界上，在臺灣東部主要由菲律賓海板塊朝西北隱沒至歐亞板塊之下，進而構成琉球隱沒帶。在臺灣東北部的高解析P波速度構造研究中，發現除了隱沒的菲律賓海板塊呈現高速的特性外，東北角正下方的地函楔中也存在著高速異常體，約由20公里深處延伸至80公里，且其P波波速可達約8.6 km/s。本研究利用臺灣陣列(Formosa Array)對臺灣東北部底下一起中深部地震事件(25.02°N, 121.86°E, 138 km, Mw 4.7)進行觀測分析，發現震央周圍的測站有著雙重P波的位移波形，包含一個較微弱的先行P波(P1)及緊跟在後的延遲P波(P2)，而在震央距約25公里外的測站僅能觀測到單一P波波相(P)。更為特別的是，P2有著逆走時的特性──在震央西側約20公里的測站反而更早接收到P2波相，且P和P2雖振幅相近，但在波相上卻有明顯地不同。透過量測P1-P2的到時差以及震源至各測站的波線，發現大多行經地函楔高速體的波線皆有著雙重P波的觀測。本研究透過自行建立模型以進行二維有限差分模擬，嘗試找出造成雙重P波觀測的生成機制。從結果顯示，震源上方的地函楔高速體為產生P1的主因──此地震事件高傾角的震源特性，使節面周圍所輻射出較弱的能量恰好進入上方高速地函楔並且向外折射，導致震央周圍的測站皆能接收到P1波相；在另外尺度較大的速度構造研究中，顯示西北臺灣底下的歐亞岩石圈也有著較高速的特性，又因琉球隱沒帶與歐亞岩石圈之側向耦合，形成了東西方向上的側向速度變化。本研究認為，P2可能是由此邊界折射自地表的波相。透過引入此高速邊界至二維速度模型中，我們得以成功模擬出P2，包含波形特徵及逆走時的觀測。因本研究僅使用簡化後的二維速度模型進行順推模擬，對於擬合波形及觀測資料尚存少許的不吻合處，但對於欲分析的異常波形，其較明顯的特徵皆可被我們的模型所重現。因此本研究能提供一個簡單的機制來解釋本起地震事件的異常觀測波形，並且對於東北部地下構造有進一步的約束。

Tectonics of northeastern Taiwan are mainly dominated by the north to northwest subduction of the Philippine Sea Plate (PSP) beneath the Eurasian Plate along the Ryukyu Trench. Recent high-resolution 3-D P wave velocity structures reveal high-velocity anomalies not only in the PSP slab but also in the mantle wedge. We investigate a deep event beneath northeastern Taiwan (12 Mar. 2019) that exhibit two P arrivals by using the data from Formosa Array and attempt to figure out the causes through waveform modeling. After constructing 2-D velocity models as a process of trial and error, the pattern of the separate two P phases can be well reproduced in our synthetics. The weaker waves radiated from the nodal plane pass through the high-velocity mantle wedge just beyond the source so that the first P arrival (P1) with small amplitude can be observed around the epicenter. The second P phases (P2) are the refracted waves impinging on the nearly vertical boundary of the Eurasian lithosphere, which is ~15 km away from the source. The consistency of the reverse travel-time curve and the phase change of P2 between synthetics and observations can also support this conception. Such a simple model cannot fully mimic the observations, but we only provide a mechanism for explaining these complicated waveform effects.

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