Evidence of a Shallow Magma Reservoir Beneath Askja Caldera, Iceland, From Body Wave Tomography.

In August 2021, Askja caldera switched to reinflation following ∼40 years of continuous deflation that was first measured some 20 years after its last eruption in 1961. Various lines of evidence, including from geodetic modeling, suggest that both the deflation and reinflation events are related to a shallow magma body. To better understand the subsurface plumbing system, we derive P‐wave velocity (Vp), S‐wave velocity (Vs), and Vp/Vs models of the mid‐upper crust by leveraging a new local earthquake traveltime data set. A cylindrical low‐velocity zone, ∼3 km wide and extending to ∼8 km below sea level (bsl), was imaged beneath the caldera. Within it, two distinct lower velocity and higher Vp/Vs anomalies are illuminated, one centered at ∼0.5 km and the other at ∼6 km bsl. The shallower anomaly lies directly beneath the zone of uplift and is likely associated with the current reinflation event. Plain Language Summary: The uplift at Askja, which started in August 2021, has drawn attention not only from volcanologists but also from the general public because it followed ∼40 years of subsidence first measured in 1981, 20 years after its last eruption in 1961. Several studies have related the subsidence to the deflation of a shallow magma body beneath the volcano, but this has not previously been detected by body wave tomography, which is a technique to image 3‐D variations in the subsurface, like a medical CT scan. It uses earthquakes that occur below and surrounding the volcano. In this study, we take advantage of a new earthquake data set to image the crust immediately beneath the volcano. We discover a shallow velocity anomaly that directly underlies the locus of surface uplift. This anomaly is likely caused by the presence of partial melt and higher temperatures, which is consistent with the existence of a shallow magma body that is causing the uplift. Understanding the nature of the current volcanic unrest is crucial, particularly in light of the large explosive eruption of 1875, which dispersed ash over large parts of Iceland. Key Points: A cylindrical low‐velocity zone, ∼3 km wide and extending to ∼8 km bsl, has been imaged beneath Askja calderaWithin this zone, two distinct lower velocity and higher Vp/Vs anomalies are detected at ∼0.5 km and ∼6 km bslThe shallower anomaly underlies an area of recent surface uplift, which is consistent with shallow melt accumulation [ABSTRACT FROM AUTHOR]