Four-Dimensional Monitoring of Volcanoes Using Ambient Seismic Noise

Four-Dimensional Monitoring of Volcanoes Using Ambient Seismic Noise (a) Map of the cumulative changes in seismic velocity that had occurred just before the September 1999 eruption of Piton de la Fournaise volcano, Réunion. White dashed line shows the limit of coverage. Solid white lines are topographic contours. Black dashed oval is a region of normally high velocity thought to be an effect of solidified dikes associated with the zone of magma injection. For this small eruption, the high-velocity regions decreased in velocity; the maximum change was about 0.1%. (b) Velocity changes before a larger eruption in July 2006 reached about 0.3% shown by the red curve. Green shaded area indicates period of eruption. (Reprinted with permission from Macmillan Publishers Ltd.: F. Brenguier, N.M. Shapiro, M. Campillo, V. Ferrazzini, Z. Duputel, O. Coutant, and A. Nercessian, 2008. Towards forecasting volcanic eruptions using seismic noise, Nature Geoscience, 1:126–130, doi:10.1038/ngeo104, ©2008.)
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Seismic noise has traditionally been viewed as a nuisance that obscures transient seismic signals, making it more difficult to detect small earthquakes or to image deep Earth structure. However, because background noise is continuously generated by physical sources, such as ocean waves interacting near the coasts, it contains seismic waves that propagate coherently across arrays of seismographs. Although ground motion complexity makes it appear random, by correlating the recordings at two stations and averaging over long time periods, a coherent signal can be extracted, yielding a seismogram equivalent to what would be produced by seismic waves propagating from one station to the other. By combining noise data from many station pairs, a 3D tomographic image of subsurface velocity structure can be constructed. If this procedure is repeated over time, a 4D representation of temporal variations in the medium can be obtained.
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This method of using seismic noise provides a tremendous opportunity for monitoring of temporal changes in structure around volcanoes. Seismic noise analysis of Piton de la Fournaise volcano on Réunion Island in the Indian Ocean demonstrated that short-term (few days) changes in velocity in the volcanic edifice on the order of 0.05% could be recognized and mapped. Before each of six monitored eruptions occurring between 1999 and 2007, decreases in velocity began a few weeks before the eruption and increased in intensity up to the time of the eruption. The total velocity change was greater for the larger eruptions. The decrease in velocity was probably caused by opening of near-surface cracks in the volcanic edifice as it was inflated by increased pressure changes show that different parts of the volcano were affected in the precursory activity leading up to different eruptions.


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