Acoustic signals and geophone response of rainfall-induced debris flows

Abstract

The infrasound and geophone response of rainfall-induced debris flows are explored based on field studies. Both the stony-type debris flow which occurred in Houyenshan, Miaoli County, Taiwan, on 10 June 2006 and the muddy viscous debris flow which occurred in Jiangjia Gully, Yunnan, China, on 11 August 2007 were examined. Infrasound propagates a long distance in the atmosphere at the speed of sound due to its low adsorption in the air and the high reflectivity of the ground. The infrasonic signals induced by debris flows are related to the magnitude and composition of the debris flows. Our findings show that the peak acoustic frequencies for the Houyenshan stony debris flows are within the range 15–40 Hz, while the corresponding geophone signals exhibit peak frequencies within 15–50 Hz. The peak frequencies of the viscous debris flows in Jiangjia Gully fell within the range 5–13 Hz. During the propagation of debris flows, the amplitude of air pressure in Houyenshan was generally greater than 1.0 Pa, and the ground velocity greater than 0.2 mm/s. The Hilbert–Huang transform approach is adopted to analyze the non-stationary acoustic signals of the debris flows. Stony-type debris flows tend to occur in Houyenshan, Taiwan, once the total rainfall exceeds 67 mm preceded by heavy hourly rainfall.