Finite-source waveform inversion of Long Period (LP) volcanic events on Etna volcano (Italy): synthetic test
Abstract
Long period (0.1-2 Hz) events are frequently observed on active volcanoes and are often related to fluid and gas movements inside the edifice. Understanding the physical mechanism beyond their generation would improve our ability to detect volcanic unrest and define a more accurate state of the activity of the volcano. Nowadays full moment tensor (F-MT) inversions are usually carried out under the assumption of a point source. This assumption resides in the way classical F-MT inversions are performed: a grid search inside a specific volume of investigation. However source dimension of LP events may involve finiteness. We first carry out full moment tensor inversion to constrain a possible crack plane. Many sources along the identified plane are supposed to act simultaneously. Probability weights based on the pseudo-algorithm proposed by Zhang et al. (JGR, 2014) are given to each source to minimize the misfit between observed and retrieved signals. Subsequent iterations are continued until the best matching solution is obtained. Each point of the possible finite source gets different scaling factors, contributing to the synthetics. Sources with the low scaling factors are removed and we finally obtain a set of preferable sources which could act simultaneously (or time-shifted). We test this methodology on synthetic signals computed in a 3D realistic velocity model of Etna volcano (Italy) with topography and a source grid spacing of 40 meters. We generate synthetics for: a point source; a finite source (250m. wide with 5 sources acting at a delayed time of 0.2 s) both representing a horizontal crack. First results show how misfit values are very high for the point source solution (0.75) while the finite source shows a lower value (0.35). The point source solution doesn’t offer enough resolution, with many possible sources acting at the same time, and is rejected. The finite source solution gives an image of a finite source of the same dimensions of the real one but slightly shifted. This shift could come from some limitations of resolution due to the low frequencies of interest. These results suggest that this approach could be potentially applicable to volcanic LP events.