Seismic observation of magma mixing inside the magma reservoir after the 2015 eruption of Axial Seamount

The dynamic processes of magma replenishment and eruption of an active volcano are key to understanding the magma plumbing system and the mechanism of magma movement. Myriad studies focus on the long-term processes of melt accumulation and migration before the eruption, the transient and massive outflow and influx of magma during the eruption, and the associated mixing processes of melt and crystal mush are poorly resolved so far. Capturing the transient magma movement at depth is an important yet challenging task, for it can provide direct evidence of such a magmatic process. Shear-wave velocity is sensitive to the melt content and melt connectivity, therefore the velocity variation is a good proxy for detecting the interaction between fresh melts with the existing crystal mush.

            Axial Seamount (AS), located in the intersection of the Juan de Fuca ridge and the Cobb hotspot, is an active submarine volcano and has erupted in 1998, 2011, and 2015 in the past three decades. Significant effort has been made to use the ambient noise for continuous monitoring of magmatism at Axial Seamount. Unfortunately, none of the results so far can capture the change inside the reservoir during the eruption, either due to the lack of data or the lack of spatial sensitivity to the magma reservoir.

            Since 2014, the Ocean Observatory Institute (OOI) has running 7 cabled permanent ocean bottom seismometers around the caldera and a satellite seismometer approximately 25 km to the southeast, providing a great chance to resolve the magma movement inside the magma reservoir during the 2015 eruption. In this study, to investigate the magma movement inside the reservoir during the eruption, we calculated the empirical Green’s function along the long paths between satellite station AXBA1 and the caldera array and successfully extracted Rayleigh waves at periods of 4-6 s (0.16 – 0.25 Hz), which are most sensitive to the velocity in the depth range of the major magma reservoir (MMR) from 1.5-2.5 km. Given that a considerable portion of the paths lies outside the caldera region, the predominant velocity variation originating from the caldera area could be as large as 4%. The velocity decrease, which is significant enough (> 2sigma) from the background seasonal variation, occurred in a consequential manner from the central to the southwestern magma reservoir. We propose that the rapid influx of melt after the 2015 eruption caused a strong mixing of the fresh melt with the crystal mush in a time period of a few months, presenting a very different way of replenishment than the long-term trend.