Abstract
Reduced near-infrared reflectance observed in September 1973 in Skylab images of the western flank of Mt. Etna has been interpreted as an eruption precursor of the January 1974 eruption. Until now, it has been unclear when this signal started, whether it was sustained and which process(es) could have caused it. By analyzing tree-ring width time-series, we show that the reduced near-infrared precursory signal cannot be linked to a reduction in annual tree growth in the area. However, comparing the tree-ring width time-series with both remote sensing observations and volcano-seismic activity enables us to discuss the starting date of the pre-eruptive period of the 1974 eruption.
Highlights
Detection of precursors to volcanic eruptions is important in reducing risks for populations and damage to infrastructures[1,2,3,4,5,6,7,8]
MFn trees are located very close to the 1974 eruptive craters and include 10 trees, whereas the 16 trees of MFs are farther away along the lava flow that originated from Monte de Fiore (Fig. 1)
The NIR signal may have been caused by the soil moisture due to gas condensation near the surface, which reduced the NIR reflectance. Such a positive change of the ecological settings would have had a positive influence on tree growth if it had been sustained during the growth season
Summary
Detection of precursors to volcanic eruptions is important in reducing risks for populations and damage to infrastructures[1,2,3,4,5,6,7,8]. We know too little about how the magma moves under the surface during pre-eruptive periods (months to days before the magma reaches the surface) It is sometimes unclear if magma input from depths is steady[18], or, for how long magma could be stored at shallow depths (and eventually degassed) without being detected by seismic and geodetic networks. It has been shown that pre-eruptive volcanic activity occurring during the vegetation period can influence the photosynthesis of trees, as shown by Normalized Difference in Vegetation Index (NDVI) derived from satellite imagery months before the beginning of the 2002–2003 eruption of Mt. Etna and the 2002 eruption of Mt. Niyragongo[20]. In order to test whether trees were impacted by volcanic activity and with the aim of better understanding the dynamics of the early stages of the 1974 eruptive event, we built ring-width time-series of trees located near Monte de Fiore. The temporal coincidence of the October 1973 seismic crisis with both renewed activity at the Voragine and the NIR signal, raises the question of the presence of an early intrusion into the western flank in the vicinity of Monte de Fiore
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