Volume 5 Issue 4 - August 1, 2008
Ground vibrations produced by rock motions and debris flows
Ching-Jer Huang

Department of Hydraulic and Ocean Engineering, National Cheng Kung University

J. Geophys. Res., 112, F02014, doi:10.1029/2005JF000437, 1-20 (2007)

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Debris flows are rapid, gravity-induced flows of mixtures of rocks, mud and water.  The gradual increase in over-cultivation in mountainous regions worldwide has led to disasters caused by debris flows that threaten inhabitants living near those mountains.  Debris flows have the following characteristics: (1) the forefront looks like a bore and the largest stones accumulate at the forefront; (2) behind the flow front, the flow appears as a mud flow with a gradually decreasing discharge; and (3) the flow is accompanied by a loud noise and the ground vibrates violently.  These ground vibrations are also called underground sounds, and are speculated to be generated by the collision of large boulders with the channel bed, especially near the front of the debris flow.  Recently, measurements of ground vibrations have been utilized to detect the occurrence of debris flows.

Ground vibrations are typically caused by earthquakes, volcanic eruptions, debris flows, debris avalanches and impact of rocks on the ground.  The properties of ground vibrations resulting from debris flows can be summarized as follows.  1. The frequency ranges between 10 and 100 Hz, and only occasionally exceeds 100 Hz.  2. The peak frequencies range from 10-30 Hz at the surge front to 60-80 Hz at the flow tail, presumably because the largest stones accumulate at the forefront.  The peak frequency is the frequency of ground vibrations occurring at the peak velocity amplitude.  3. The amplitude of ground vibration is proportional to the discharge of debris flows.  It was also noted that the passage of the main front of debris flows near the sensors yields an abrupt increase in the ground vibration amplitude that can be employed to detect the presence of the debris flow front, or can be utilized to estimate the average front velocity when two or more detectors are installed along the banks of the channel.  Accordingly, debris flows should be detectable whenever the amplitude of the ground vibration signals exceeds a threshold over a certain range of frequencies for more than a given period.

The purpose of this study is to examine and compare ground vibrations produced by rock motions and debris flows in order to explore the main sources of ground vibrations caused by debris flows.  Ground vibrations were detected using three geophones installed in the channel bed surface.  Rocks of different weights fall freely onto the channel bed, causing the ground to vibrate.  Field tests were performed at Ai-Yu-Zi Creek (Nan-Tou County), where an automated Debris Flow Monitoring System was established by the Soil and Water Conservation Bureau, Council of Agriculture, Taiwan.  Background noise was measured to distinguish it from ground vibrations generated during experiments.  Time-domain signals were converted into frequency-domain signals using the Fast-Fourier Transform (FFT) and into the time-frequency domain signals using the Gabor transform.

Figure 1 displays the ground vibrations produced by a 29.4 kg rock released from a height of 1.2 m hitting the channel bed of the Ai-Yu-Zi Creek, as detected by the first geophone along the Y-axis (across the channel).  In Fig. 1, part (a) plots the time-series data, part (b) plots the signals in the frequency domain obtained using FFT and part (c) plots the Gabor coefficients of the time-domain signal.  The signals in the frequency and the time-frequency domains suggest that the features of signals along the three axes are very similar: the frequencies ranged between 10 and 150 Hz, with a peak frequency of 80 Hz.  Measurement results indicate that as the rock weight varied, the ground vibration frequency range remained constant.  However, the peak frequency shifted to 100 Hz for the 10 kg rock and to 50 Hz for the 50 kg rock.  These experimental results reveal that the peak ground vibration frequency decreases as the rock weight increases.  The ground vibration velocity amplitude generally increased with rock weight.
The phase speed of the ground vibration in the Ai-Yu-Zi Creek channel bed ranged from 333.3 m/s to 400 m/s.  The theoretical values for the propagation speed based on the sphere-packing model obtained is 241 m/s, when the sand is assumed to be quartz and the ground vibration phase speed is assumed to be the same as that for shear waves.  The sphere-packing model underestimates ground vibration phase speed, perhaps because the porous material was assumed to be composed of a regular array of identical spheres, whereas real earth materials comprise stones and sand particles of various sizes.
Figure 1.  Ground vibration along the Y-axis produced by a 29.4 kg rock released from a height of 1.2 m above the channel bed of Ai-Yu-Zi Creek; (a) time-domain, (b) signals in the frequency domain obtained using FFT, and (c) Gabor coefficients of the time-domain signal.

A debris flow occurred at the Ai-Yu-Zi Creek on July 2, 2004.  Heavy rainfall during the period of the Mindulle typhoon caused this flow.  This granular debris flow washed away the Shen-Mu roadbed downriver and severely damaged the piers of the Ai-Yu-Zi Bridge.  Figure 2 displays the ground vibration signals along the Z axis (perpendicular to the ground) recorded by the upstream geophone between 16:41 and 16:43 on July 2, 2004.  Figure 2 (c) plots the Gabor coefficients of the time-domain signal from 16:41:40 to 16:41:50, which is the 10-s period during which the main debris flow front passed the geophone.  When the main front of debris flow passes a sensor, the ground vibration velocity increases sharply.  Therefore, the experimental data (part (a) in Fig. 2) indicate that the main front of the debris flow at Ai-Yu-Zi Creek passed the upstream geophone at 16:41:44.  Notably, after the main front passed, the time-domain signal in part (a) did not appear in a normal form; likely because the width of the Ai-Yu-Zi Creek was widened from 36 m to 80 m and the upstream geophone in the bank revetment was washed away following the debris flow on July 2, 2004.  The characteristics of the ground vibrations resulting from this debris flow can be drawn from the monitored data as follows: (1) the main front reached the upstream geophone at 16:41:44; (2) the ground vibration was three-dimensional with velocity amplitudes that are roughly the same (40 cm/s) along the three axes; (3) the frequencies of ground vibrations were 10-100 Hz; and, (4) at the initial stage of the main front, the frequency is lower than 50 Hz, whereas after the main front has passed, the frequency was 50-100 Hz.  However, when the main front was closest to the sensor, the frequency spectrum covered a wide range, from 10 to 250 Hz.
Figure 2.  Ground vibration signals along the Z-axis recorded by the upstream geophone from 16:41 to 16:42 on July 2, 2004; (a) the time-series data, (b) the signals in the frequency domain, and (c) the Gabor coefficients of the time-domain signal from 16:41:40 to 16:41:50.

The geophone recorded many time-series data, when the sampling rate was 500 Hz.  These data are simplified by defining a new variable, which is the cumulative energy per second (Esec), given by the following formula
whereVxVyandVz denote the velocity amplitude along the and Z axes. Figures 3 (a) and (b) illustrate the evolution of the cumulative energy per second recorded by the upstream and downstream geophones, respectively.  The peak energy associated with the main front of the debris flow occurred at 16:41:44 at the upstream geophone and at 16:41:57 at the downstream geophone, indicating that the main front flowed from the upstream geophone to the downstream geophone in 13 s.  The distance between these geophones was 173 m, thus the mean velocity of the main front of the debris flow was 13.3 m/s.  This value is confirmed by estimating the velocity of the front flow from the images captured by a video camera.
Figure 3.  Temporal evolution of the cumulative energy per second of the ground vibration caused by the debris flow on July 2, 2004; (a) upstream geophone and (b) downstream geophone.

Comparing the ground vibration signals in Fig. 1 with those in Fig. 2 reveals that at the same channel 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.  A comparison with the ground vibrations associated with earthquakes shows that the ground tremors caused by debris flows are significantly smaller with a higher frequency range.  The attenuation of the seismic waves depends on the frequency, thus high frequencies correspond to a high decay rate.  Therefore, debris flow tremors can only be detected within a relatively short distance, as shown by the field test data and Fig. 2, which indicate that the surge front was first detected when it was near the sensors.  This finding is discouraging, because an early warning signal depends on the advance detection of tremors.  This shortcoming can be compensated for by installing sensors upstream of, or close to, the origin of debris flows.
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