Abstract
Since a few decades volcanic Long Period (LP) events have been recorded on many active volcanoes and their study has been recognized as an important tool to characterize volcanic activity. LP event analyses through moment tensor (MT) inversions have led to kinematic descriptions of various source mechanisms. The main challenge in these inversions is to strip out the propagation effect in order to isolate the source; hence the velocity model used controls the accuracy of the retrieved source mechanism. We first carry out several synthetic tests of inversions on Mt. Etna volcano (Italy). Four geological models with topography are considered with increasing complexity: the most complex model is used to generate synthetic data, while the other three models are used to calculate the Greens' functions for inversions. The retrieved solutions from the three velocity models are similar. The MT solutions for a deeper source are well retrieved, while a shallower source test suffers from high uncertainties and strong misinterpretation of the source orientation. The homogeneous model gives the lowest misfit value, but source location and mechanism decomposition are inaccurate. When a complex model different from the true one is used, a high misfit value and a wrong solution is obtained. We then incorporate our findings into the MT inversion of an LP event recorded on Mt Etna in 2008. We obtain very different solutions among the three models in terms of source location and mechanism decomposition. The overall shape of the retrieved source time functions are similar, but some amplitude differences arise, especially for the homogeneous model. Our work highlights the importance of including the unconsolidated surface materials in the computation of Green’s functions especially when dealing with shallow sources.
Highlights
The understanding of the origin of seismic signals on volcanoes is of fundamental importance to enhance our knowledge of volcanic systems and to monitor their activity
We evaluate the misfit from the moment tensor plus single force (MT+F) inversions
Four models with increasing geological complexity have been used in our synthetic test. Since both the source location and inversion are jointly affected by the uncertainties in the velocity model, we suggest that, when possible, long period (LP) events should be first located by other location methods, and inverted for the best source position
Summary
The understanding of the origin of seismic signals on volcanoes is of fundamental importance to enhance our knowledge of volcanic systems and to monitor their activity. Bean et al, (2013) extended the observations of Harrington and Brodsky, (2007) and proposed an alternative model for explaining shallow LP seismicity They analyzed the pulse-like nature of some LP events recorded on volcanoes and explained their origin as a slow failure of the weak shallow volcanic edifice close to the brittle-ductile transition. Previously thought that kilometres long LP wavelengths are mostly insensitive to small variations of the volcanic structure, it has been shown that hundred meters thick superficial layers and features (volcanics above the sediments, dykes, magma pathways etc.) can significantly degrade source inversion efforts (Bean et al, 2008; Cesca et al, 2008; Kumagai et al, 2005; Neuberg and Pointer, 2000) This is because the complex stratigraphy of volcanoes has a strong impact on the seismic wavefield (Bean et al, 2008; Neuberg and Pointer, 2000). We extend the work of De Barros et al, (2011) analyzing a real LP event by means of the three geological models with variable complexity
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