GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 1.2 106 experiments that simulate magmatic processes at depth. Therefore, deciphering volcanic pre- cursors is key to forecasting volcanic eruptions. In this study, we used synchrotron-based X-ray tomographic microscopy to observe felsic magma vesiculation at high temperature (1100 °C) to establish the pre-eruptive conditions that favor explosive activity (Fig. 1). Specifically, we studied the increase of seismic attenuation with increasing gas content during magma vesi- culation. These results are applied to the natural range of volcanic tremors (0.2–5 Hz) to de- tect the proximity of gas-charged magma to the surface prior to volcanic eruption (Fig. 2). We observe that the accumulation of volatiles in the shallow magma reservoir (< 100 MPa = 4 km) exacerbate the low frequency range (< 10 Hz; Fig. 2) and, at equivalent volume of excess fluids, crystal-poor magmas can be detected at larger depth than crystal-rich systems. Specifically, gas-rich, crystal-free felsic magmas are detected at larger depth (40 MPa = 2 km) than gas-rich, crystal-bearing systems (10 MPa = 0.5 km) (Fig. 2). This critical evaluation of the actual depth of storage of gas-charged magma prior to unrest using seismic attenuation is key to the asses- sment of volcanic forecast and hazard to a specific well monitored active volcano and may be applied in a timely manner. Fig. 2 - Modeling of seismic attenuation of two ascending H 2 O-saturated magmas: Soufrière Hills (Montserrat) and Chaitén (Chile). Decompression-induced change of the physical properties of both magmas having nearly identical melt composition but different crystal volume fraction ( f ): A) b = bubble volume fraction, H 2 O melt = residual H 2 O dissolved in melt, B) r melt = melt density, h melt = melt viscosity (Table 1), and C–D) Q = quality factor. Figure and caption from Pistone et al. (2021), GRL 48, https://doi. org/10.1029/2020GL092315 .