Seismic Detection of Debris Flow in the Himalayas and Their Spatiotemporal Characteristics

During the 2003 summer monsoon, the Hi-CLIMB seismological stations deployed at the front of the High Himalayan range have detected bursts of high-frequency seismic noise (up to 30 dB increase in the 2-22 Hz frequency band) that last for several hours. The largest transient event on August 15th 2003 is located by cross-correlating the envelope of the signal observed at 10 stations, nearby a village partially destroyed the same day by a devastating debris flow. Detailed analyses of the seismic signal help to depict the time sequence of the debris flow generation as well as the evacuation of the sediment supply. Moreover, a systematic scan of one year of seismic noise, focusing on the detection of similar events, brings information on the spatial and temporal occurrences of slope failures at the front of the High Himalayan range. A total of 19 debris flows are detected by the seismic stations, most of them being generated in a region of steep slopes, large gullies and loose soils during storms in the 2003 summer monsoon period. These seismic events are further compared to local meteorological data to determine some rainfall thresholds for hillslope processes, including the cumulative rainfall needed to bring the soils to moisture at which they reach their failure capacity. The inferred threshold value is consistent with previous estimates deduced from soil studies as well as sediment supply investigations in the area. These results point out the potential of using seismic noise as a dedicated tool for monitoring the spatiotemporal occurrence of landslides and debris flow at regional scale.