Burtin2008JGR - Click Here JOURNAL OF GEOPHYSICAL RESEARCH...

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Spectral analysis of seismic noise induced by rivers: A new tool to monitor spatiotemporal changes in stream hydrodynamics A. Burtin, 1,2 L. Bollinger, 2 J. Vergne, 1 R. Cattin, 1 and J. L. Na ´be ˇlek 3 Received 8 March 2007; revised 19 September 2007; accepted 28 January 2008; published 2 May 2008. [ 1 ] Analysis of continuous seismic data recorded by a dense passive seismological network (Hi-CLIMB) installed across the Himalayas reveals strong spatial and temporal variations in the ambient seismic energy produced at high frequencies (>1 Hz). From June to September 2003, the high-frequency seismic noise is observed to increase up to 20 dB (relative to (m/s) 2 /Hz) for all the stations located along a steep 30-km-long narrow and deeply incised channel of the Trisuli River, a major trans-Himalayan river. The early summer increase in high-frequency energy is modulated by a 24-h periodicity where the minimum of seismic noise level is reached around noon and the maximum is reached late in the evening. A detailed study of seismic noise amplitude reveals a clear correlation with both regional meteorological and hydrological data along the Trisuli River. Seasonal increase in ambient noise coincides with the strong monsoon rainfall and a period of rapid melting of snow and ice in the high elevations. The observed 24-h cyclicity is consistent with the daily fluctuation of the precipitation and river discharge in the region. River-induced seismic noise is partly generated by stream turbulence, but this mechanism fails to explain the observed clockwise hysteresis of seismic noise amplitude versus water level. This pattern is better explained if a significant part of the observed seismic noise is caused by ground vibrations generated by bed load transport. This points out the potential of using background seismic noise to quantify in continuous river bed load and monitor its spatial variations, which remain difficult with classical approaches. Citation: Burtin, A., L. Bollinger, J. Vergne, R. Cattin, and J. L. Na ´be ˇlek (2008), Spectral analysis of seismic noise induced by rivers: A new tool to monitor spatiotemporal changes in stream hydrodynamics, J. Geophys. Res. , 113 , B05301, doi:10.1029/2007JB005034. 1. Introduction [ 2 ] Assessing the level and the periodic variations of background seismic noise at seismological stations is of first importance to determine threshold limits for the detec- tion and discrimination of seismic signals. As a result, there has been considerable interest in determining the nature of the seismic noise environment [e.g., Brune and Oliver , 1959] and the factors controlling it through large permanent seismological arrays [e.g., Peterson , 1993; Stutzmann et al. , 2000; McNamara and Buland , 2004; Stehly et al. , 2006] and temporary seismic networks [e.g., Wilson et al. , 2002; de la Torre and Sheehan , 2005]. It appears that the ambient seismic noise affects the entire frequency band studied in seismology from 10 ± 3 to 10 2 Hz. Here, we focus this study on mechanisms involved in seismic noise generation, which exhibit strong temporal variations at various periods.
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