Recent seismicity and tomographic modeling of the Mount Etna plumbing system

Abstract

The monitoring of seismic activity in eastern Sicily (southern Italy) has been recently improved, in the framework of the Poseidon Project, to investigate both tectonics and volcanic processes of Mount Etna. This effort has produced a homogeneous and complete data set which we use to image the deep structure of the volcano and to define the space and time distribution of the recent seismicity, encompassing the 1995 eruption, diffuse eruptions between 1995 and 1997, a further magma intrusion started in 1997, and an increase of volcanic activity in July 1998. We inverted P wave arrival times from 307 selected local earthquakes to obtain a three‐dimensional velocity model of the volcano, with a simultaneous inversion for hypocenters and velocity parameters. The new tomographic images permit us to define the structure beneath the volcano from the surface down to 18 km depth. The main structural feature revealed by our inversion is a high‐velocity body located beneath the central craters whose lateral extent increases from ∼6 km between 18 and 9 km depth to ∼12 km between 9 and 3 km depth. Near the surface the fast anomaly branches in two separate high‐Vp regions, which are located below the summit craters and the eastern flank (Valle del Bove), respectively. The high‐velocity features are interpreted as high‐density cumulates of solidified magma that intruded the shallow crust. We hypothesize that magma ascends the crust within the relatively small high‐velocity conduit (below 9 km depth) and is stored at depth shallower than 9 km within the broad high‐velocity region, as also suggested by petrological data. A sharp increase of seismicity in 1997, with earthquakes occurring at the border of the high‐velocity body, suggests that Mount Etna sustained recent episodes of intrusions, which possibly herald future eruptions. No large low‐velocity anomaly interprétable as a melted magma chamber is imaged in the upper 18 km of the crust, while a broad low‐velocity anomaly in the uppermost mantle, revealed by regional seismic data, possibly indicates the magma source region at depth. Earthquake occurrence from the deep source to the shallow intrusive area helps to trace the magma migration and the feeding of the volcanic system.