GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.2 GNGTS 2023 resolving a single high-absorption area at 16 Hz at the northern end of the NNW-SSE trend. The high-absorption anomaly that covers the bay comprises NNW-SSE and NE-SW trends, visible especially at 2 km grid spacing (Figure 1a, white lines). The highest absorption anomaly within the bay is the crossing point between trends in the northern area of Port Foster (Figure 1a, dashed inset), also dominant in the maps of Prudencio et al. (2013) (Figure 1b). At 1 km node spacing, this anomaly appears elongated NNW-SSE, cross-cutting the eastern portion of the bay (Figure 1a). There are two low-absorption anomalies at both frequencies corresponding to ice-capped deformed regions comprised within Mount Pond (Ba1) and near Mount Kirkwood (Ba2). High absorption anomalies occur at the border of the caldera rim and are located at Pendulum Cove (Aa1), Fumarole Bay (Aa2), and Entrance Point (Aa3). Aa1 is near the Chilean station, destroyed during the eruption of 1969, while Aa2 is between the active Argentinean and Spanish research bases. The NNW-SSE high-absorption trend developing primarily through the bay also crosses the northern island. At 6 Hz, corresponding to larger heterogeneities and greater depths, its absorption maximum coincides with Telefon Bay and Lago Escondido, that is, the areas of the 1967 and 1970 eruptions. Images of Tectonic-Controlled Magma and Fluid Sources There is extensive geophysical evidence of the NNW-SSE-striking tectonics effect on the magmatic and hydrothermal activity on the island (Alvarez-Valero et al., 2020; Zandomeneghi et al., 2009). The wider absorption pattern largely corresponds to the area of uplift inside the basin (Cooper et al., 1998). Submarine cones, craters and mounds aligned along a NNW-SSE axis have been highlighted since the early 1990s by seismic refraction studies (Marti et al., 1996; Rey et al., 1995). Most of the mounds are located within a radius of 3 km from the anomaly Aa2 or Aa3. The mounds close to the coastline, in front of Whalers Bay, are located at about 1 km of the Aa3 anomaly and are interpreted as the consequence of hydrothermal processes supporting the underwater volcanic structures. The Aa3 anomaly overlaps a possible, although uncertain, shallow intrusion or pyroclastic agglomerate overlayed by a submarine cone, as evidenced by seismic lines (Kowalewski et al., 1990; Marti et al., 1996). Besides the seismic refraction studies, geodetic anomalies show reactivation of the volcanic system following the seismic crises of the 90s Berrocoso et al. (2006) interpret this reactivation as caused by a magmatic intrusion at around 500 m b.s.l., located in the Fumarole Bay area, coinciding with the Aa2 anomaly. Berrocoso et al. (2008) prove that NW-SE extensional and a NE-SW compressional patterns affected the island from 1991 to 2000 using the geodetic network. These patterns match the two high-absorption trends inside the bay (Figure 1a). The NNW-SSE anomaly in the inner bay was detected in the past as a broad low-velocity (e.g., Figure 1c) and attenuation (Figure 1b, 6 Hz) feature encompassing the entire bay (Luzon et al., 2011; Prudencio et al., 2015; Vila et al., 1995; Zandomeneghi et al., 2009). Our study shows higher resolution on these