Abstract
The catastrophic Sanyanyu and Luojiayu debris flows, which were induced by heavy rain, struck Zhouqu County in Gannan Prefecture, Gansu Province, at approximately midnight, 7 August 2010 (Beijing time, UTC + 8), causing 1765 fatalities and huge economic loss. The ZHQ seismic station is located approximately 170 m west of the outlet of the Sanyanyu gully, and its power system was destroyed by the Sanyanyu debris flow when its leading edge reached the vicinity of the seismic station. In this paper, seismic signals recorded approximately 10 min before its termination are collected and analyzed to study the Sanyanyu debris flow. A double-exponential model is first proposed to quantitatively characterize seismic energy distributions in the frequency domain, which reveals that the peak frequency of seismic signals is around 5 Hz. Influenced by the Doppler effect, the peak frequency of the N–S component is the highest, and the U–D component is the lowest. Time–frequency analysis is applied to the seismic signals. From the spectrogram, it is easily observed that the formation time of the Sanyanyu debris flow is around 23:33:10. The entire debris flow is divided into three phases with distinct frequency characteristics, using 23:36:20 and 23:37:35 as crucial times. The frequency energy distributions in the first two phases are relatively stable, and are constrained in 0–8.8 Hz and 0–17.8 Hz, respectively. For the third phase, the upper boundary of frequency energy increases in a nearly linear manner, reaching approximately 35 Hz at the end. We calculate synthetic seismograms of the Sanyanyu debris flow. Generally, synthetic seismograms have morphological features and characteristics in key stages similar to those of the actual seismic records, and their maximum values are of the same magnitude. Our results suggest that the seismic source of a debris flow can be represented by a single-force model, and reveal that real-time monitoring and rapid identification of potential debris flows using broadband seismic network records is possible, which can provide approximately 15 min of pre-event warning for local residents and hopefully save many lives.
Highlights
Most of western and southwest China is characterized by mountainous areas with dramatically varying terrain, resulting in extensive development of catastrophic natural geological hazards, such as landslides and debris flows, which seriously affect the lives of local residents (e.g., Cui et al 2003; Zeng et al 2014)
The ZHQ seismic station is located very close to the outlet of the Sanyanyu gully, and more than 64% of the sediments were from the Sanyanyu drainage basin, indicating that the seismic energy recorded by the station was mainly generated by the Sanyanyu debris flow
The ZHQ seismic station is located on the path of the Sanyanyu debris flow to Zhouqu County and recorded the entire debris flow process, providing a good opportunity to analyze characteristics of debris flow seismic signals recorded at a very near field
Summary
Most of western and southwest China is characterized by mountainous areas with dramatically varying terrain, resulting in extensive development of catastrophic natural geological hazards, such as landslides and debris flows, which seriously affect the lives of local residents (e.g., Cui et al 2003; Zeng et al 2014). Studies on debris flows are mainly carried out after their occurrence, since they are usually unpredictable They are very destructive, and field measurement equipment is usually destroyed; data cannot be collected. We analyze seismic signals generated by a giant debris flow and recorded on a seismic station very close to the event to extract their frequency characteristics. The ZHQ seismic station is located very close to the outlet of the Sanyanyu gully, and more than 64% of the sediments were from the Sanyanyu drainage basin, indicating that the seismic energy recorded by the station was mainly generated by the Sanyanyu debris flow. The slope is about 39° at the source area and only 5.7° in the sedimentary area, with an average of about 13.4° (Tang et al 2011)
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