南琉球隱沒帶中深層地震的波形特徵和分類

Abstract

具導波特徵的訊號已在南琉球隱沒帶深度小於150公里的中深層地震被前人觀測到，其P波呈現低頻(<2 Hz)初達波並接著持續時間長的高頻(3-10 Hz)尾波。由於近期發生許多介於150至300公里之間的地震，讓我們有機會針對1997至2016年間、規模大於4的中深層地震做系統性地分析，並根據其波形和頻率特徵進行分類，更好地量化行經於隱沒帶板塊的各種震波特性。 首先本研究使用三分量的P波資料計算移動視窗相關係數，以偵測具有相似波形特性的地震事件，接著採用階層式分群法進行分類，被歸為同一家族的地震可能具有相同的震源破裂區域或相似的傳播路徑，結果顯示在0.5-10 Hz範圍些許地震存在著相似特徵(cc≥0.6)，但大部分地震的波形相似度偏低，並沒有找到極相似(cc≥0.9)的地震，暗示此區域的中深層地震尚未發生重複破裂的行為。 接著利用時頻圖分析高頻信號的延遲時間，不同頻率之震波的到時特徵可用以推得震波行經的速度構造。本研究發現穿越特定路徑範圍內的一組地震具有清楚的P波連續頻散現象，延遲時間在頻率0.5 Hz到6 Hz間逐漸地隨頻率增加而拉長，差距達2秒。還有另一種P波頻散則呈現兩個先後分開抵達的波至，通常高頻信號相較於低頻信號晚到將近1秒的時間。第二種頻散與前人研究在隱沒帶較淺地震的觀察相符，此現象可能是由隱沒板塊中的低速層或小尺度非均質體所造成，而第一種連續的頻散特徵則是本研究在台灣的新發現，由波形模擬結果推論，欲產生此現象，震波所通過的低速層波導構造(約5-7 %之P波異常)需具有較薄的厚度(<10公里)或較長的傳遞距離(≥100公里)。

Using intraslab earthquakes shallower than 150 km in the southernmost Ryukyu subduction zone, previous studies in Taiwan found the waveguide effect that typically shows a low-frequency (<2Hz) first P arrival followed by sustained high-frequency (3–10 Hz) wave trains. Recently occurred deeper events at depth 150-300 km allow us to better quantify the properties of those seismic waves traveling in the subduction zone. In this study, we aim to systematically scan through the local broadband waveforms of the intermediate depth earthquakes with M>4 between 1997 and 2016. Events are classified based on the waveform characteristics and their frequency contents.

To detect events with similar properties, we applied sliding-window cross-correlation (SCC) using three components of P waveform data simultaneously for a set of stations. We then sorted the events into families based on correlation coefficients (cc) and inter-distances between waveform pairs. The events within a family likely came from the same source region with similar rupture, as such their paths to a particular receiver should produce similar waveforms. In our results, although several events present enough similarities (cc≥0.6 threshold) and can be grouped as a family, we did not observe closely resembled pairs (cc≥0.9), implying no “repeating” behavior for those intermediate intraslab events. One important property is the frequency content of the arrivals that may be related to the speed of structure traveled. We have developed a work scheme to determine the delayed time of higher-frequency energy using spectrogram and spectral ratios. A set of 6 events show clear continuous dispersion at frequency range between 0.5 and 6 Hz where arrival time smoothly increases by about 2 s in total. Another type of dispersive waveforms appeared as two distinct arrivals: low frequency and then high-frequency energy, separated by around 1 s. The latter case has been reported in the previous study for shallower event and it was interpreted as effect from low-velocity layer or heterogeneity of the subducted slab. On the other hand, the continuous dispersion is a new feature observed by our study, which may infer a thinner layer and/or longer propagation for some kind of reflecting waves to develop such interference. The simulation indicates that the low-velocity waveguide should be thinner than 10 km and the propagation distances within the layer should be longer than 100 km, assuming a P-wave drop of about 5-7%.