大規模劇烈物理現象如核爆、地震等，在現象產生過程中皆會產生次聲（即頻率低於人類聽覺下限(20Hz)的聲波）。次聲因其低頻而含有下面兩種特性：1.能量衰減速率對比於高頻波較低 2.穿透性較高頻波高。土石流之次聲波與土石流組成特性以及運動型態有關，而土層中之地聲相對於空氣中之聲波而言，其環境噪音極低，但因快速遞減使其觀測位置必須緊靠土石流流經地點。鑒於近年來全球氣候變遷，極端雨量出現機率增加，土石流發生機率因而提高，經由地聲及次聲監測系統來收集現地資料是降低坡地災害之可行方法。本研究經由空氣傳播之次聲波和土層傳播之地聲來分析瞭解土石流之音頻特性與其地聲波傳遞特性，並對比實驗及現場量測資料來分析土石流運行時之次聲特性。本研究以土石流現地之雨量分析與事件前後之地形變化來佐證土石流的發生，冀望能求取現地土石流啟動之雨量門檻值與運行形態機制。一般現地低頻噪音成因為風、雨聲、雷聲、地震等因素，實驗及現地資料顯示其特徵頻率皆在5Hz以下。2006年六月之暴雨事件苗栗火炎山土石流音波與地聲資料中，濾去5Hz以下噪音後音波與地聲資料時域與頻率域皆具有相似特性，若以HHT求取解析度更高之特徵頻率約為5-15Hz。而地聲所收集到之樣本數較多，通常其頻率特性概括於10~50Hz間。而在奧地利及火炎山所收集到之部份音波及地聲資料顯示其在事件開頭有特徵頻率下降及事件尾端特徵頻率升高之情形。火炎山現地雨量則由距離反比法推算並非為最近距離之雨量站所得資料最接近，在當地所設置雨量計所得資料顯示，2006~2007年間其土石流雨量門檻值主要為67 mm有效降雨量，之後則超過3mm/hr之強度皆有可能發生土石流。Naturally occurring infrasounds produced by large geophysical motions (e.g. nuclear explosions and earthquakes). Infrasound propagates with two characteristics: 1.Traveling in long distance in the atmosphere at the speed of sound for the low adsorption in the air. 2. Transmission of infrasound is large than the high-frequency wave. The infrasonic signals induced by debris flows are related to the magnitude and composition of the failure zone as well as the slope areas. The location of geophones should be close to the debris flow channel. The acoustic noises such as wind gusts, rains, thurders and earthquakes are below 5Hz. The infrasound and geophone data are measured with 5Hz high-pass filter make the perfect match in time domain and frequency domain during the heavy rain debris flow events on June,2006. The debris flows occurred in Houyenshan with the peak frequencies between 5-15 Hz during the surges. The frequency of seismic data are located between 10-50Hz Non-stationary process of the debris flow acoustic signals are demonstrated by using the HHT approach.Frequency rising by surge and falling by tail flowing are showed in the spectrum methods. Debris flows tend to occur in Houyenshan, Taiwan once the total rainfall exceeds 67 mm and and the rainfall intensity is higher than 3 mm/hr preceded by heavy hourly rainfall after 2006.
