Magma chamber detected beneath an arc volcano with high-resolution velocity images

Abstract

Arc volcanoes are underlain by complex systems of molten-rock reservoirs ranging from melt-poor mush zones to melt-rich magma chambers. Petrological and satellite data indicate that eruptible magma chambers form in the topmost few kilometres of the crust. However, very few chambers have ever been definitively located, suggesting that most are too short-lived or too small to be imaged, which has direct implications for hazard assessment and modelling of magma differentiation. Here we use a high-resolution technology based on inverting full seismic waveforms to image a small, high-melt-fraction magma chamber that was not detected with standard seismic tomography. The melt reservoir extends from ~2 to at least 4 km below sea level (b.s.l.) at Kolumbo – a submarine volcano near Santorini, Greece. The chamber coincides with the termination point of the recent earthquake swarms and may be a missing link between a deeper melt reservoir and the high-temperature hydrothermal system venting at the crater floor. The chamber poses a serious hazard as it could produce a highly explosive, tsunamigenic eruption in the near future. Our results suggest that similar reservoirs (relatively small but high melt-fraction) may have gone undetected at other active volcanoes, challenging the existing eruption forecasts and reactive-flow models of magma differentiation.

Heralds of Future Volcanism: Swarms of Microseismicity Beneath the Submarine Kolumbo Volcano Indicate Opening of Near‐Vertical Fractures Exploited by Ascending Melts

“…The polygons encircled by a solid gray line in panels b and c indicated the position of a shallow crustal melt reservoir imaged by Chrapkiewicz et al. (2022). Note, the vertical and horizontal scales are identical in panels b and c. Panel a is slightly enlarged.…”

“…Panels a and b show cross‐sections through the 3D Vp model of Chrapkiewicz et al. (2022) with contour lines spaced at 0.5 km/s and hypocenters of this study superposed. Dashed white lines in panels a and b indicate the depth of panel (c) Note, this 3D tomography model terminates at 4 km depth.…”

“…The tip of the cone‐shaped seismically active volume below Kolumbo coincides with a prominent anomaly of strongly reduced Vp, ∼1.5 km/s slower than the regional average (Figure 10). This anomaly, with an approximate volume of 6 km 3 , was imaged through full‐waveform inversion of active‐source seismic data acquired in 2015 by the PROTEUS experiment and is interpreted as a body of 26%–53% percent partial melt (Chrapkiewicz et al., 2022). It should be noted that the active seismic survey was conducted about nine years after our deployment and the size of the crustal melt reservoir may have changed in the meantime.…”

“…An increase in fluid pressure, intruding magma, or aseismic slip are feasible processes that locally increase the shear stress and/or reduce the effective normal stress and thereby trigger earthquake swarms (Shelly et al., 2013; Vidale & Shearer, 2006; Yukutake et al., 2011). At Kolumbo the presence of fluids is documented independently from seismicity by vigorously venting hydrothermal chimneys inside the crater (Carey et al., 2013; Rizzo et al., 2019) and the identification of a crustal melt reservoir below the crater (Figure 10; Chrapkiewicz et al., 2022). Geochemical analyzes of fluids vented inside the Kolumbo crater suggest that the root of the hydrothermal system is located at ∼1 km depth below sea level (see cartoon in Figure 11; Rizzo et al., 2019).…”

“…First arrival time tomography from a more recent active seismic experiment in 2015 found only slightly reduced velocities between 3 and 5 km depth beneath Kolumbo, consistent with little to no melt (0%–1%) (McVey et al., 2019). However, full‐waveform inversion of the same data set identified a low Vp anomaly of limited spatial extent at 2.1–4.0 km depth below sea level which is interpreted as volume of partial melts with melt fractions of 28%–44% (Chrapkiewicz et al., 2022). The findings of this study do in particular reconcile the findings of the recent full‐waveform tomography study of Chrapkiewicz et al.…”