The application of seismometer on monitoring of low-frequency ground vibrations generated by debris flows

Abstract

The aim of this study is using the ES-1 a newly developed seismometer deployed along Aiyuzi Stream in Nantou County Taiwan to detect the low-frequency signals generated by debris flows With the low attenuation of low-frequency signals after long distances ES-1 is expected to improve the small detection area of a Geophone via its high sensitivity and capability of detecting low frequencies as low as 0 13 Hz to detect the ground vibration of debris flows in advanced Strong ground vibrations were measured in the convectional rainfall event and two typhoon events After the correlation of the CCD cameras and wire sensors the vibration signals in convectional rain and Typhoon Soulik event were identified as being caused by debris flows The lowest frequencies in the two events were 4 Hz and 2 Hz respectively which is lower than the conventional bandwidth of debris flows from 10 to 150 Hz From the precipitation data and spectral analysis the ground vibration had positive correlation with rainfall; further the leading signals of the debris flows detected by ES-1 were exponentially increasing If the 2 Hz signals were extracted for independent analysis it could be observed that the low frequency signals were detected 4 minutes earlier both by upstream and downstream ES-1 sensors before the debris flow surges approached ES-1 sensors This phenomenon is attributed to the ground vibrations caused by the debris flows formation and flowing at the upper reach; moreover the approximate location where the debris flows generated can be calculated by this characteristic Before the summer rainy season which spawns debris flows the ES-1 system sensed ground vibrations from both local earthquakes and teleseism Although the main objective of this study was detecting ground vibration from debris flows measuring ground vibration from other seismic sources helps distinguish the characteristics of debris flow ground vibrations from others In the event of local earthquakes with the spectral analysis ES-1’s spectral analysis received broad band frequency signals because of the short distance between the epicenter and the ES-1 station In contrast ES-1 only received low-frequency signals under 1 Hz from Ya-an in Sichuan Province of China Besides seismic signals from earthquakes ground vibrations from 10 to 60 Hz induced by increasing discharge and bed load transport from convectional rainfall were also detected by ES-1 From these results the ES-1 was verified as capable of measuring low-frequency ground vibration signals generated by debris flows and clarifying the spatial-temporal frequency characteristics of debris flows

Date of Award	2014 Jul 29
Original language	English
Supervisor	Ching-Jer Huang (Supervisor)