Nonlinear Dynamics of Volcanic Tremor Recorded at Mt. Erebus Volcano, Antarctica

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题名:	Nonlinear Dynamics of Volcanic Tremor Recorded at Mt. Erebus Volcano, Antarctica
作者:	瑪麗莎;Sudibyo,Maria R.P.
贡献者:	地球科學學系
关键词:	火山;震顫;非線性的決定性系統;地震學;埃里伯斯火山;volcano;tremor;nonlinear deterministic system;seismology;Erebus
日期:	2016-07-15
上传时间:	2016-10-13 13:22:24 (UTC+8)
出版者:	國立中央大學
摘要:	火山震顫的複雜特性被認為可代表一個非線性的決定性系統，而其動態演化可以少量的參數進行模擬。我們以此種方法分析南極洲上的埃里伯斯火山震顫的動力學。我們分析了從2002年5月17日至2002年7月31日的120筆數據。從波譜分析中我們可以得知，在埃里伯斯火山的動態震顫可分為諧波震顫、寬帶震顫和混合震顫。諧波震顫和混合震顫，可得到1到23的泛音，他們的基本頻率在五月至六月中旬期間出現擾動，但隨後穩定在1赫茲左右。諧波震顫和混合震顫的頻率都顯示出滑升和滑降，其中上揚的幅度可高達200%，而下降的幅度則為90%。為了分析這個火山震顫的動態演化現象，藉由Takens(1981)的延遲嵌入理論，我們從原始地震波形重建了描述相位空間。在給定4-12(0.04秒-0.12秒)的延遲時間範圍內，所運用的延遲時間是依據自相關函數和平交互信息(AMI)而來。使用嵌入維假近鄰法，所得到的數值在5-8之間。在埃里伯斯火山的震顫中有很大量的噪訊，所以我們在描述相空間內以流形維度3-5使用一個非線性的過濾器。此吸子的指數歧異度就是混亂狀態標誌，可以利用最大Lyapunov指數(MLE)來量化。MLEs的值由0.003-0.023。根據Julian的模型來說，低壓的狀態下會產生諧波震顫及一些泛音。在壓力提高的狀況之下，會產生更多的泛音，導致系統更加的混亂及造成更高的MLE。在埃里伯斯火山的MLE變化並不大，顯示系統內的流體壓力並不顯著。因此，此系統被歸類於簡單型(極限環)和混亂狀態之間。此震顫的來源可能是連接兩個岩漿庫的噴嘴，其中一個正好位於熔岩湖下方。岩漿從底部通過此狹窄的噴嘴噴發到上方。此流體被認為是氣態的二氧化碳，而其低黏度的特性和振盪的現象就如Julian的模型所述。;Ria Sudibyo·Friday, July 15, 2016 The complex behavior of volcanic tremor can be represented as a nonlinear deterministic system, where its dynamic evolution can be modeled mathematically with a small number of parameters. This approach is applied to analyze the dynamics of tremor at Mt. Erebus, Antarctica. We identified 120 episodes of tremor from 17 May to 31 July 2002. Based on the spectral analysis, tremor at Erebus can be categorized into harmonic, mixed episodes of harmonic and broadband, and broadband tremor. The harmonic and mixed episodes exhibit various numbers of overtones from 1 to 23, and their fundamental frequencies seem to fluctuate during May to mid-June but stabilize around 1 Hz afterwards. Both harmonic and mixed episodes reveal frequency gliding up and gliding down, where the percentage of gliding up may reach up into 200%, while the percentage of gliding down is up to 90%. To analyze the dynamical evolution of tremor, we reconstructed phase space from the original seismograms using the delay embedding theorem suggested by Takens. The delay time was selected using the autocorrelation function and Average Mutual Information (AMI), giving delay times in the range 4 to 12 (0.04 to 0.12 s). The sufficient embedding dimension was obtained using the false nearest neighbors method giving values in the range 5 to 8. As tremor at Erebus contains a high amount of noise, we applied a nonlinear projective filter in phase space using manifold dimension from 3 to 5. The exponential divergence of the attractor is the indicator of the chaotic behavior and is estimated as the maximal Lyapunov exponent (MLE). MLEs were found to vary from 0.003 to 0.023. According to Julian’s model, small pressure will create harmonic tremor with a small number of overtones. As the pressure is increasing, it will generate more overtones, leading the system to become more chaotic and resulting in higher MLE. The MLEs variation at Erebus is considered small, implying that the fluid pressure working in the system does not exhibit large variations. Therefore, the system can be categorized in the borderline between simple (limit cycle) and chaotic behavior. The source of tremor is likely a nozzle connecting two magma chambers, with one chamber located just beneath the lava lake. The nozzle works as a constricted channel that transports the fluid from the deeper to the shallower chamber. The fluid is expected to be CO2 gas which has low viscosity and can generate the kind of oscillations describe in Julian’s model.
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