Rafael Abella

Applying Fractal Dimensions and Energy-Budget Analysis to Characterize Fracturing Processes During Magma Migration and Eruption: 2011–2012 El Hierro (Canary Islands) Submarine Eruption

The impossibility of observing magma migration inside the crust obliges us to rely on geophysical data and mathematical modelling to interpret precursors and to forecast volcanic eruptions. Of the geophysical signals that may be recorded before and during an eruption, deformation and seismicity are two of the most relevant as they are directly related to its dynamic. The final phase of the unrest episode that preceded the 2011–2012 eruption on El Hierro (Canary Islands) was characterized by local and accelerated deformation and seismic energy release indicating an increasing fracturing and a migration of the magma. Application of time varying fractal analysis to the seismic data and the characterization of the seismicity pattern and the strain and the stress rates allow us to identify different stages in the source mechanism and to infer the geometry of the path used by the magma and associated fluids to reach the Earth’s surface. The results obtained illustrate the relevance of such studies to understanding volcanic unrest and the causes that govern the initiation of volcanic eruptions.

Array analysis of the seismic wavefield of long‐period events and volcanic tremor at Arenal volcano, Costa Rica

We use wavefield decomposition methods (time-domain crosscorrelation and frequency-domain MUSIC) to analyze seismic data recorded by a dense, small-aperture array located 2 km West of Arenal volcano, Costa Rica, and operated during 2.5 days. The recorded wavefield is dominated by harmonic tremor and includes also spasmodic tremor and long-period (LP) events. We find that the initial stages of LP events are characterized by three different wave arrivals. These arrivals propagate with similar back-azimuths pointing to the volcano summit (~80◦N) and increasing apparent slownesses of 0.4, 1.1, and 1.7 s/km. Spasmodic tremors can not be regarded as coherent signals. On the contrary, harmonic tremors are highly coherent, characterized by the stability of the apparent slowness vector estimates. Apparent slownesses lay in the range 1-2 s/km. Back-azimuths point in the general direction of the volcano, but with a large variability (40-120◦N). Nevertheless, there are long-term variations and evidences of multiple simultaneous components in the harmonic tremor wavefield. These observations suggest that LP events and tremor are generated in a shallow source area near the volcano summit, although they do not share exactly the same source region or source processes. The tremor source is located in the shallowest part of the plumbing system, beneath the lava crust. This dynamic region is subject to complex fluctuations of the physical conditions. Degassing events at different locations of this region might generate variable seismic radiation patterns. The effects of topography and heterogeneous shallow structure of the volcano may amplify these variations and produce the wide directional span observed for volcanic tremor. On the other hand, the LP source seems to be more repeatable. LP events are likely triggered by fragmentation of the fluid flow in a slightly deeper portion of the volcanic conduits.

Time‐Dependent Spatial Amplitude Patterns of Harmonic Tremor at Arenal Volcano, Costa Rica: Seismic‐Wave Interferences?

Seismograms recorded at the receivers of a small‐aperture seismic array usually display very similar waveforms and amplitudes, as a consequence of their close proximity. During the analysis of the volcanic tremor wave field at Arenal volcano, Costa Rica, we detected significant differences in the amplitudes of harmonic tremor recorded at the stations of a small‐aperture (∼210  m) seismic array. The amplitude distributions are geometrically complex and characterized by strong gradients. They occur just for harmonic tremors; any other type of seismic event produces nearly uniform amplitudes across the array. This suggests some relation with harmonic frequency content. Moreover, the spatial amplitude patterns change with time. Some of these observations could be explained by an extreme combination of source, path, and site effects. But they also could be produced by interference of different components of the seismic wave field. We use numerical calculations to investigate the amplitude pattern generated by two interfering plane waves, and are able to reproduce the main features of the observed amplitude patterns. We propose mechanisms that might generate seismic wave fields with multiple components and conclude that interference can explain the complexity and variability of the harmonic tremor wave field at Arenal volcano. Online Material: Wave‐field animation.