[1] Long-period (LP) seismicity on volcanoes is thought to be associated with moving fluids or resonating fluid-filled conduits, hence LP moment tensor (MT) source inversions might have a direct bearing on our understanding of the plumbing system. Using 3-D full wavefield simulations and 2-D sensitivity kernels in a digital elevation model of Mount Etna, we investigate the influence of near-surface volcanic structure on LP signals and on moment tensor inversions. Contrary to common wisdom in crustal seismology we find that, despite their relatively long wavelengths, LPs are severely distorted by near-surface structures including layering and topographic features. In particular near-surface low-velocity structures which are commonly observed on volcanoes play a critical role in controlling the nature of LP signals. If not accounted for, these path effects leak into the source solution, leading to the emergence of erroneous source geometries, spurious forces and incorrect source time functions. This is particularly problematic if one adopts an ''unconstrained'' solution space for the source, with many free parameters. Hence there is a fine balance in the trade-off between the velocity model and the source. In the absence of high-resolution near-surface velocity control we demonstrate the importance of employing a priori source information from other fields (e.g., structural geology), for shallow LPs, constraining the number of free parameters in the inversion. A probabilistic approach should then be taken, as the model with the ''best fit'' is not necessarily the ''true'' solution.Citation: Bean, C., I. Lokmer, and G. O'Brien (2008), Influence of near-surface volcanic structure on long-period seismic signals and on moment tensor inversions: Simulated examples from Mount Etna,

A passive seismic study was carried out underneath Glacier d’Argentière, Mont Blanc, France, where an array of seismometers was installed in a subglacial access tunnel. The data show a very high emissivity from the glacier. Fracturing can be discriminated from serac falls using the signal characteristics. We apply seismic array methods to locate the sources of these signals, using a two-step grid search in the parameter space. Four clusters of activity are found close to the network, showing that this fracturing does not take place uniformly over the glacier, but rather in isolated small zones. We compute a local magnitude using regional earthquakes for calibration. The magnitudes follow a classical Gutenberg–Richter law in the range ML = −3 to 0.15, showing that no characteristic size events dominate the process. We suggest that those spatial clusters of icequakes could reveal the heterogeneous nature of the friction at the base of the glacier, with patches of high frictional stresses locally generating intense fracturing within the ice mass.

[1] During the second half of June, 2008, 50 broadband seismic stations were deployed on Mt Etna volcano in close proximity to the summit, allowing us to observe seismic activity with exceptionally high resolution. 129 long period events (LP) with dominant frequencies ranging between 0.3 and 1.2 Hz, were extracted from this dataset. These events form two families of similar waveforms with different temporal distributions. Event locations are performed by cross-correlating signals for all pairs of stations in a twostep scheme. In the first step, the absolute location of the centre of the clusters was found. In the second step, all events are located using this position. The hypocentres are found at shallow depths (20 to 700 m deep) below the summit craters. The very high location resolution allows us to detect the temporal migration of the events along a dikelike structure and 2 pipe shaped bodies, yielding an unprecedented view of some elements of the shallow plumbing system at Mount Etna. These events do not seem to be a direct indicator of the ongoing lava flow or magma upwelling. Citation:

[1] One hundred twenty-nine 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°SE (family 1) and 50°NW (family 2). For family 1 events, the crack geometry is nearly orthogonal to the dikelike structure along which events are located, while for family 2 the location gave two pipelike bodies that 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 10 May 2008 as opposed to the flank lava flow.

[1] We performed numerical simulations of seismic waveforms with frequencies up to 10 Hz in heterogeneous media with topography to investigate the effects of topography and structural heterogeneity on seismic scattering. We used the simulated waveforms to test the source location method assuming isotropic radiation of S waves for long-period events and tremor at volcanoes. The assumption of isotropic radiation has been shown previously to be valid in a high-frequency band because of the path effect caused by the scattering of seismic waves. Our simulation results showed that distortion of the wavefields increased as the correlation distance of structural heterogeneity (a) decreased, as the frequency increased, and as the travel distance increased. Topography alone considerably distorted the wavefields. However, we found that strong scattering due to topography was suppressed if the correlation distance of structural heterogeneity was longer than the S wave wavelength. Isotropic radiation of S waves by scattering due to topography was not achieved in our simulations. Our results indicated that scattering due to structural heterogeneity becomes stronger than that due to topography at ka ffi 1 and a = 50 m, where k is the wave number of the S waves. This suggests that strong short-scale structural heterogeneity is required to achieve isotropic radiation of S waves. Although we could not reproduce isotropic radiation, our results support the validity of the assumption of isotropic radiation in the source location method.