以臺灣陣列探討南琉球隱沒帶中深層地震頻散特徵與二維地震波場模擬

Abstract

隱沒帶地震產生低頻初達波（小於2赫茲）接續高頻（大於3赫茲）尾波的導波特徵在琉球隱沒帶南北兩端以及日本、阿留申、南美洲都曾被發現，過去在台灣地區則有學者認為觀測到的高頻且大振幅尾波是由地震波在具有小尺度異質體板塊內的多重散射產生，也有研究透過一維波形模擬認為導波之波形特徵為主要是由板塊中低速層狀構造配合小尺度非均質體引起。本研究使用於北台灣地區架設的密集地震觀測網—台灣陣列，以2019年間三起規模大於四且深度大於200公里的琉球隱沒帶南段中深層地震事件，探究導波在台灣北部的空間分佈和該現象相關聯的二維速度構造。 首先，本研究利用高斯時頻分析辨識地震波形是否具有P波頻散特徵並透過時頻圖輔助來挑選高頻訊號到時。發現除東北角沿岸地區以及大屯山區域的測站外，台灣陣列大多數都能觀測到頻散現象，指示出該特徵可能與方位角無關而是一定程度受到震央距及深部構造的影響。藉由三起事件的垂直分量波形記錄，可以歸納出三個主要的特徵，分別是(一)、低頻初達和高頻(3赫茲)訊號的時間差會由震央距50公里處的3秒減少至震央距100公里處的0.5秒；(二)、延遲高頻訊號相對於初達的低頻訊號具有較大的振幅，震央距在70公里和100公里左右特別顯著；(三)、隨震央距縮短(70至50公里)會發現高頻訊號振幅削弱的情形。 接著，運用二維有限差分法探尋南琉球隱沒帶中和導波相關聯的速度構造形貌。本研究成功透過250公里深的中深部地震波傳行經隱沒板塊最上方約8公里厚、速度變化負7%的低速薄層，擬合出低頻初達波接續高頻波包與相對振幅的特徵。然而，由於這些高頻波包開始出現的位置為震央距120公里之後，表示前述構造無法解釋在台灣陣列(震央距120公里以內)觀測到的頻率變化，仍須將不同速度構造納入考量。根據測試結果，在隱沒板塊150公里深上方加入一個與背景相差5%的低速條帶即可在震央距60至100公里處產生初達訊號後的一高頻波包，與前述的特徵(一)相似。 本研究首次偵測出隱沒帶弧前區域的P波頻散現象，同時印證菲律賓海板塊最上層存在低速薄層是解釋台灣地區波形特徵的一項必要條件。而雪山山脈最北端地下35公里至150公里深處存在的低速條帶配合隱沒板塊上覆的低速薄層，為現階段推論最有可能產生近震央距處(120公里以內)頻散現象的最適構造。

Subduction zone guided wave, often characterized by low frequency (<2 Hz) first P arrival followed by high frequency (> 3Hz) coda, had been discovered around the Pacific including Taiwan. Such phenomenon of delayed high frequency energy is often associated with multiple scattering from small scale heterogeneities inside the subducting slab, and/or the dispersive nature of P wave created by low-velocity structure near the slab top. In this study, we use dense Formosa array (FMarray) to investigate the waveform dispersions, propagation of guided wave from three intermediate-depth earthquakes (with magnitudes above 4 and depth greater than 200 km) in 2019 in the Ryukyu subduction zone, aiming to understand the plausible 2D velocity structures beneath northeast Taiwan. We first calculate the spectrogram to identify dispersion property of P wave and mark up the arrivals of high-frequency packets in the waveforms. Data from FMarray generally show the property of low-frequency initial wave and the delayed high frequency (>3 Hz) packet with clear move-out, except for those stations nearest to the northeastern coast and Tatun Volcanic region. The consistencies among three events infer that the systematic variation in waveforms consisting guided waves is insensitive to azimuth and is more dependent of epicentral distances as well as deep structures beneath. Seismic profiles reveal three key features: (1) relative time between high- and low-frequency energy reduces gradually from 2.0–2.5 s to 0.5–1.0 s as epicentral distance increases from 50 km to 100 km toward the trench, (2) relative amplitude shows systematic changes with the strongest high-frequency arrivals at the distance range between 70 and 95 km, and (3) high-frequency arrival diminishes very quickly within 20 km distance from ~70 km toward the source, and is completely undetectable (no dispersion) at distance of 50 km. To pinpoint the structures responsible for the waveform properties observed, we further use 2D finite difference method to simulate seismic waveforms. We successfully produce the delayed high-frequency wavelet and its relative amplitude in simulated waveform using a model for a 250 km deep earthquake located inside a low-velocity layer (LVL) with VP of about -7% (relative to slab mantle) and thickness of 8 km in the top portion of the plate subducting at 60˚. However, the predicted high-frequency signals appear at epicentral distance beyond 120 km, which fail to explain the large energy and delay time observed at shorter distances unless another slow-anomaly in the wedge is implemented. According to our synthetic tests, there should be a low-velocity channel (with VP of about -5% relative to surrounding mantle, or equivalently -7% to slab mantle) on top of the dipping slab, in order to produce high-frequency wavelet in epicentral distance 60-100 km as the feature (1) mentioned above. To conclude, our study gives the first sight of P-wave dispersion in the forearc region, and verifies that a LVL in the upper part of subducting Philippine sea plate is required in explaining the overall features of guided waves detected in northern Taiwan. To further reconcile the dispersion observed at near-side stations of FMarray (within epicentral distance of 120 km), we propose the existence of another low-velocity channel underneath northern Hsuehshan Range.