Source Mechanism of Long Period events recorded by a high density seismic network during the 2008 eruption on Mt Etna

TL;DR

This study analyzes long period seismic events during the 2008 Mt. Etna eruption using moment tensor inversions to determine source mechanisms, revealing crack orientations aligned with local tectonics and linked to lava fountain activity.

Contribution

It introduces a method for accurately determining source mechanisms of LP events using constrained moment tensor inversions with nearby seismic stations.

Findings

LP events have mechanisms with strong volumetric components.

Cracks strike SW-NE and dip SE or NW, consistent with local tectonics.

Events are linked to lava fountain activity, not flank lava flow.

Abstract

129 Long Period (LP) events, divided into two families of similar events, were recorded by the 50 stations deployed on Mount Etna in the second half of June 2008. During this period lava was flowing from a lateral fracture after a summit strombolian eruption. In order to understand the mechanisms of these events, we perform moment tensor inversions. Inversions are initially kept unconstrained to estimate the most likely mechanism. Numerical tests show that unconstrained inversion leads to reliable moment tensor solutions because of the close proximity of numerous stations to the source positions. However, single forces cannot be accurately determined as they are very sensitive to uncertainties in the velocity model. Constrained inversions for a crack, a pipe or an explosion then allow us to accurately determine the structural orientations of the source mechanisms. Both numerical tests and LP event inversions emphasise the importance of using stations located as close as possible to the source. Inversions for both families show mechanisms with a strong volumetric component. These events are most likely generated by cracks striking SW-NE for both families and dipping 70\deg SE (Family 1) and 50\deg NW (Family 2). For Family 1 events, the crack geometry is nearly orthogonal to the dike-like structure along which events are located, while for Family 2 the location gave two pipe-like bodies which belong to the same plane as the crack mechanism. The orientations of the cracks are consistent with local tectonics, which shows a SW-NE weakness direction. The LP events appear to be a response to the lava fountain occurring on the 10th of May, 2008 as opposed to the flank lava flow.