利用寬頻地震儀分析颱風期間的低頻訊號來源;Analysis Of Low Frequency Signals From Broadband Seismometers Records During Typhoons

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Title:	利用寬頻地震儀分析颱風期間的低頻訊號來源;Analysis Of Low Frequency Signals From Broadband Seismometers Records During Typhoons
Authors:	許博欽;Hsu,Po-Chin
Contributors:	地球科學學系
Keywords:	背景噪訊;環境噪訊;颱風;寬頻地震儀;時頻圖;ambient noise;spectrogram
Date:	2016-07-28
Issue Date:	2016-10-13 13:23:24 (UTC+8)
Publisher:	國立中央大學
Abstract:	寬頻地震測站所記錄到1 Hz至10 Hz的高頻背景噪訊通常與人為活動，如汽車、火車和工廠活動有關；而低於1Hz的低頻背景噪訊之主要來源有地表微地動、大氣現象和海浪與風之影響等。為了瞭解這些低頻噪訊來源，本研究選取了幾個具有不同路徑的颱風（2012年杜蘇芮、2013年潭美跟康瑞、2014年哈吉貝跟麥德姆）靠近台灣時的寬頻地震測站記錄，藉由與海象浮標、雨量及氣象觀測資料做交叉比對，來了解各項氣象因素對於寬頻地震測站中週期2秒以上之噪訊記錄的影響。結果發現，在台灣西南外海測站通常有較高的背景噪訊值，而在台灣內陸測站則具有較低的噪訊強度，此現象顯示著噪訊來源應和海洋波浪或海流有關，因此噪訊能量才會隨著地震測站跟海岸線的距離而降低。另外，噪訊能量大致會隨著颱風與測站距離縮短而增強，主要能量增強頻段為週期2秒至10秒之間，與前人研究中由海浪交互干涉作用所得噪訊影響頻段一致，證明颱風靠近的確會增強海浪有效的交互作用而影響地震測站記錄，而這樣的結論也經由有效波高和地震測站資料能量比對結果而得到證實。但研究中也發現，噪訊能量最強時並不一定是在颱風距離地震測站最近的時候，這樣的現象似乎與颱風路徑有關，當颱風路徑與海岸線平行時的能量變化通常會較與海岸線垂直時的變化不明顯，可能表示當路徑與海岸線正交時，有效波高會有較大的交互作用而產生較大的能量。另外，在颱風並未靠近時，我們有時候發現低頻噪訊增強的情況。在與不同氣象與降雨資料交叉比對後，發現氣壓變化和連續降雨可能為主要影響因素。本研究結果顯示，寬頻地震測站所記錄到低頻噪訊的確會受到氣象和海洋環境變化影響，但在颱風靠近和平常主要的影響因素可能不儘相同，但主要還是和海浪有效波高及氣壓有關，次要為降雨量。 ;Broadband seismometers record signals over a wide frequency band, in which the high-frequency background noise is usually associated with human activities, such as cars, trains and factory-related activities. Meanwhile, the low-frequency signals are generally linked to microseisms, atmospheric phenomena and weather conditions. By comparing the broadband seismic data recorded during the pass of the typhoons with different moving paths, such as Doksuri in 2012, Trami and Kong-Rey in 2013, Hagibis and Matmo in 2014, the meteorological information, and the marine conditions, we attempt to understand the effect of the meteorological conditions on the low-frequency noise. The result shows that the broadband station located along the southwestern coast of Taiwan usually have relatively higher background noise value, while the inland stations were characterized by lower noise energy. This rapid decay of the noise energy with distance from the coastline suggests that the low frequency noise could be correlated with the oceanic waves. In addition, the noise energy level increases when the distance from the typhoon and the station decreases. The enhanced frequency range is between 0.1-0.5 Hz, which is consistent with the effect caused by the interference of oceanic waves as suggested by the previous studies. This observation indicates that the pass of typhoon may reinforce the interaction of oceanic waves and caused some influence on the seismic records. The positive correlation between the significant wave height and the noise energy could also give evidence to this observation. However, we found that the noise energy is not necessarily the strongest when the distance from typhoon and the station is the shortest. This phenomenon seems to be related to the typhoon path. When the typhoon track is perpendicular to the coastline, the change of noise energy is generally more significantly; whereas less energy changes are observed when the typhoon path is subparallel to the coastline. On the other hand, sometimes the energy of low frequency signal could increase without the inference of typhoons. After comparing the noise energy density with different meteorological and precipitation data, we found that the pressure change and the consecutive rainfall could be the main factors which affect the energy distribution. In summary, our result confirm that the broadband seismic data could indeed be influenced by the metrological conditions and marine environments. The contribution of each effect may vary. However, the main factors which control the low frequency noise energy should still be the wave height and the pressure.
Appears in Collections:	[Graduate Institute of Geophysics] Electronic Thesis & Dissertation

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