In this study we compare ground vibrations generated by various rocks and debris flows at the same channel to explore the origin of ground vibrations caused by debris flows. In field experiments we identified the ground vibrations caused by motions of individual rocks. Ground vibrations were detected by geophones and analyzed in the time-frequency domains. We found that ground vibrations caused by individual rocks were 10-150 Hz and that larger stones generate ground vibrations with lower peak frequencies. A sphere-packing model of granular rocks was used to estimate the propagation speed of ground vibrations. This was found to be underestimated when compared with our experimental data. The signal decay rate was obtained under the assumption that ground vibrations propagate as exponentially attenuating cylindrical waves. Ground vibrations produced by a debris flow at Ai-Yu-Zi Creek, Nan-Tou, Taiwan, on 2 July 2004 revealed that the debris flow forefront produced vibrations lower than 50 Hz and ranged between 50 and 100 Hz after the front had passed. When the main front was closest to the sensor, the frequency spectrum covered a wide range, from 10 to 250 Hz. The mean velocity of the debris flow front was 13.3 m/s. Frequencies of ground vibrations caused by individual rocks are within the frequency range of ground vibrations captured during the debris flow, confirming that one of the main sources of ground vibration caused by debris flows is the interaction of rocks or boulders with the channel bed.
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