Fault–Dike–Magma Interactions Inferred from Transcrustal Conical Structures under Akutan Volcano

Abstract

Abstract Volcano monitoring and eruption forecasting require accurate characterization of transcrustal magmatic structures to place volcanic unrest in context within the system where it occurs. Structural imaging using local seismicity is limited to seismogenic depths. Here, we exploit arrivals in teleseismic receiver functions that change polarity with backazimuth to image two surfaces beneath Akutan volcano in the Aleutian arc. The two surfaces delineate an upper to midcrustal inverted conical volume that deepens and thickens away from the volcanic center, with thicknesses of 3–13 km. The top of the volume is at depths of 2–3 km below sea level at distances of ∼5–15 km from the caldera center. The bottom is at depths of 7–15 km at the same distances, and the cone’s thickness increases outward from ∼5 to ∼10 km. The signal is best fit by a volume with anisotropy with fast symmetry planes that dip outward from the center and downward increases in shear velocity at both interfaces. The upper boundary coincides with the top of Akutan’s volcanotectonic (VT) seismogenic zone, with the VT seismicity exhibiting outward dipping planar features that match the anisotropic fast plane orientation within the volume. The bottom of the anisotropic volume is below the termination depth of the majority of the VT seismicity and is therefore likely associated with the brittle–ductile transition. Long-period (LP) events associated previously with magma movement are concentrated below the anisotropic VT volume. Because of the strong spatial association with VT seismicity, we interpret the volume as consisting of concentric outward dipping faults and dikes that align the seismogenic response to stress changes from magmatic processes. Our observations map this volume independent of the present-day seismicity distribution and thus provide a spatially more complete image of the magmatic system.