Non-double-couple mechanisms in the seismicity preceding the 1991-1993 Etna volcano eruption

Abstract

Summary The temporal evolution of the complete source moment tensor is investigated for 28 earthquakes that occurred at Mt Etna in the period August 1990–December 1991 preceding the biggest eruption of the last three centuries. We perform several tests to check the robustness of the results of inversion considering different frequency ranges and different groups of stations. As well as the selection of good‐quality data, the error analysis, statistically significant at the 95 per cent confidence level, is employed to validate the findings of the inversion and to distinguish between physical solutions and artefacts of modelling. For events between 0.3 and 10 km depth, strike‐slip mechanisms prevail on normal, inverse and dip‐slip mechanisms; this is possibly due to the dyke‐induced stress dominating the overall stress field at the surface, producing a continuous switch of the tensile and compressive axes. The regional E–W tension prevails at depth, as indicated by the prevalence of normal mechanisms. An increment of the non‐double‐couple components is observed immediately before the eruption and can be related to movements of fluids, even though, for some events, the complex interaction between tectonic stress and volcanic activity cannot be excluded. The source time functions retrieved are in general simple and short but some show complexities, as one would expect in volcanic seismicity. From the seismic scalar moment found, we extrapolate an empirical moment–magnitude relation that we compare with other relations proposed for the same area and computed for the duration magnitude and the equivalent Wood–Anderson magnitude.

Probabilistic waveform inversion for 22 earthquake moment tensors in Hungary: new constraints on the tectonic stress pattern inside the Pannonian basin

“…Several procedures have been proposed to estimate the focal mechanism of low-magnitude earthquakes by using high-frequency seismograms recorded at local stations (e.g. Šílený et al 1992;Mao et al 1994;Panza & Sarao 2000;Sarao et al 2001;Vavryčuk & Kühn 2012).…”

Correlation between the dynamics of the Hyblean Foreland and Etnean volcanism

“…The focal mechanisms of 138 events with a good quality of the statistical parameters (Table 1) were computed, hypothesising double-couple focal sources. However, it is known that in active tectonic areas (Campus et al, 1993;Radulian et al, 2000;Aoudia et al, 2003), as well as in volcanic and geothermal areas (Saraó et al, 2001;Panza and Saraó, 2000), non-double-couple mechanisms are possible. In our opinion the double-couple focal source hypothesis is supported by conclusions reached in other studies carried out with different methodologies, which fit well with the results of our study of the focal mechanisms.…”

The Variability and Interpretation of Earthquake Source Mechanisms in The Geysers Geothermal Field From a Bayesian Standpoint Based on the Choice of a Noise Model

“…The very existence of this vapor-dominated field is a result of specific geological conditions involving permeable rocks that are suspected to be underlain by a magma body (e.g., Brook, 1981). The non-DC components are thought to be the most unstable part of source inversions, as they are sensitive to station coverage, inadequacies in the Green's functions due to hypocenter mislocation and imperfect structural models, and noise in the seismic waveforms (e.g., Kuge & Lay, 1994;Křížová et al, 2013;Saraò et al, 2001). Microearthquakes in the area were attributed to water injection and steam extraction (e.g., Majer et al, 2007).…”