Tomographic image of melt storage beneath Askja Volcano, Iceland using local microseismicity

摘要

We use P wave and S wave arrivals from microseismic earthquakes to construct 3-D tomographic V_p and V_s images of the magma storage region beneath Askja's central volcano in the Northern Volcanic Zone of Iceland. A distinctive ellipsoidal low-velocity anomaly, with both V_p and V_s velocities 8-12% below the background, is imaged at 6-11 km depth beneath the caldera. The presence of a shallow magma chamber is corroborated by geodetic and gravity studies. The small V_p/V _s anomaly suggests a lack of pervasive melt. We interpret this anomaly as a region of multiple sills, some frozen but hot, others containing partial melt. A second, smaller low-velocity anomaly beneath the main magma storage region may represent a magma migration pathway. This interpretation is supported by the close proximity to the anomaly of clusters of deep, magmatically induced earthquakes. However, the location and shape of this deep anomaly are poorly constrained by the current data set. Key Points We image a sub-caldera low velocity anomaly that extends from 6-11 km in depth Within the anomaly both Vp and Vs velocities are 8-12% below background values Anomaly due to hot, sub-solidus rock with melt distributed in many small sills