東北日本下方的地震衰減構造及其頻率相依性

Abstract

本研究計算位於千島－日本島弧交界上的東北日本隱沒帶之二維非頻率相依與一維頻率相依地震波衰減模型以了解此區域之地幔動力學與流體移遷移行為。我們蒐集陸域(Hi-net和F-net)與近年來新架設的海域(S-net)地震觀測網之地震波資料，並校正電纜式海底地震儀(S-net)波形方向以利後續準確量測P、S波振幅頻譜。我們將波形資料使用小波轉換(wavelet transform)得到平滑的振幅頻譜，並計算合成理論地震波形以扣除已知的初始模型路徑效應、幾何擴散與部分震源效應。我們以傳統單站頻譜擬合測量t*得到二維非頻率相依衰減模型，以獲取區域構造上之初步解析。另一方面，我們同時將觀測與合成振幅頻譜之殘差進行反演，得到一維頻率相依衰減模型(QP, QS, QP/QS)並計算其頻率相依性。根據非頻率相依衰減模型結果顯示，東北日本之弧前和隱沒板塊分布區域大致上呈現較弱的衰減，在弧後、火山地區與弧弧碰撞帶則呈現較強的衰減。本研究將一維頻率相依之衰減模型分為三個區塊討論：位於島弧交界隱沒帶之北區、位於典型隱沒帶之西區與靠近海溝涵括大部分弧前地區與隱沒板塊之東區。北區相較西區有較強的地震波衰減與較低的QP/QS，可能因弧弧碰撞之一系列作用導致在北區存在較大量的流體。此外，在東區較弱的地震波衰減反映海洋地殼高密度與弧前地區低溫的性質；較強的頻率相依性則可能反映東區有較強的異質性。東區淺部構造的低QP/QS異常則可能由高水含量的弧前地區與充填了含水礦物的海洋板塊正斷層所致。

We investigate the 2-D frequency-independent and 1-D frequency-dependent seismic attenuation characteristics beneath the northeastern Japan to better understand the detailed mantle dynamics and fluid migrations. We collect seismic data from inland (Hi-net and F-net) and recently deployed offshore (S-net) observatories. For the offshore data, we correct the rotation angles of S-net sensors to ensure precise spectra amplitudes of P and S waves. We measure the spectra amplitudes in multiple frequencies using wavelet transforms. All measurements are referencing to the 1-D synthetics to eliminate the path effect of reference model, geometrical spreading, and part of the source effect. We fit the spectrum from each event-station pair individually to obtain a 2-D frequency-independent attenuation model. Furthermore, we invert the residuals of observed and synthetic spectra amplitudes for 1-D frequency-dependent attenuation models (QP, QS, QP/QS). The 2-D frequency-independent models suggest stronger attenuation in the volcanic area and the collision region of the Kuril and Japan arcs. On the other hand, the raypaths passing through the subducting slab are characterized by low attenuation. To analyze the results of our 1-D model more specifically, we divide our research region into 3 zones which include arc-arc junction in the subduction zone (north zone), typical subduction zone (west zone), fore-arc region and oceanic plate (east zone). The north zone is characterized by stronger attenuation and lower QP/QS compares to the west zone. This observation may be caused by the anomalously abundant fluid generated by a series of processes owing to the arc-arc collision. By contrast, the weak attenuation of east zone reflects the dense oceanic crust and the low-temperature fore-arc region; and the strong frequency dependency suggests high heterogeneity in the east zone. Low-QP/QS anomaly is observed at the shallow depths of the east zone, suggesting rich fluid content, which may be contributed by the hydrous fore-arc with the aqueous mineral filled in the oceanic plate.