Source Mechanism of Seismic Explosion Signals at Santiaguito Volcano, Guatemala:New Insights From Seismic Analysis and Numerical Modeling

Volcanic activity at the Santiaguito dome complex (Guatemala) is characterized by lava extrusion interspersed with small, regular, gas-and-ash explosions that are believed to result from shallow magma fragmentation; yet, their triggering mechanisms remain debated. Given that the understanding of source processes at volcanoes is essential to risk assessments of future eruptions, this study seeks to shed light on those processes. We use data from a permanent seismic and infrasound network at Santiaguito volcano, Guatemala, established in 2018 and additional temporary stations, including a seismic array deployed during a 13-day field investigation in January 2019 to analyze and resolve the source characteristics of fragmentation leading to gas-and-ash explosions. Seismic data gathered within a distance of 4.5 km from the vent show a weak seismic signal 2–6 s prior to the explosions and associated main seismic signal. To resolve the source location and origin of the seismic signals, we first used ambient noise analysis to assess seismic velocities in the subsurface and then used two-dimensional spectral element modeling (SPECFEM2D) to simulate seismic waveforms. The analyzed data revealed a two-layer structure beneath the array, with a shallow, low-velocity layer (v$_{s}$ = 650 m/s) above deeper, high-velocity rocks (v$_{s}$ = 2,650 m/s). Using this velocity structure, possible source mechanisms and depths were constrained using array and particle motion analyses. The comparison of simulated and observed seismic data indicated that the precursory signal is associated with particle motion in the RZ-plane, pointing toward the opening of tensile cracks at a depth of ∼600 m below the summit; in contrast, the main signal is accompanied by a vertical single force, originating at a shallow depth of about ∼200 m. This suggests that the volcanic explosions at Santiaguito are following a bottom-up process in which tensile fractures develop at depth and enable rapid gas rise which leads to the subsequent explosion. The result indicates that explosions at Santiaguito do not occur from a single source location, but from a series of processes possibly associated with magma rupture, gas channeling and accumulation, and fragmentation. Our study provides a good foundation for further investigations at Santiaguito and shows the value of comparing seismic observations with synthetic data calculated for complex media to investigate in detail the processes leading up to gas-ash-rich explosions found at various other volcanoes worldwide

Statistical evidence of transitioning open-vent activity towards a paroxysmal period at Volcán Santiaguito (Guatemala) during 2014–2018

Long-term eruptive activity at the Santiaguito lava dome complex, Guatemala, is characterised by the regular occurrence of small-to-moderate size explosions from the active Caliente dome. Between November 2014 and December 2018, we deployed a seismo-acoustic network at the volcano, which recorded several changes in the style of eruption, including a period of elevated explosive activity in 2016. Here, we use a new catalogue of explosions to characterise changes in the eruptive regime during the study period. We identify four different phases of activity based on changes in the frequency and magnitude of explosions. At the two ends of the spectrum of repose times we find pairs of explosions with near-identical seismic and acoustic waveforms, recorded within 1–10 min of one another, and larger explosions with recurrence times on the order of days to weeks. The magnitude-frequency relationship for explosions at Santiaguito is well described by a power-law; we show that changes in b-value between eruptive regimes reflect temporal and spatial changes in rupture mechanisms, likely controlled by variable magma properties. We also demonstrate that the distribution of inter-explosion repose times between and within phases is well represented by a Poissonian process. The Poissonian distribution describing repose times changes between and within phases as the source dynamics evolve. We find that changes in source properties restrict the extrapolation of explosive behaviour to within a given eruptive phase, limiting the potential for long-term assessments of anticipated eruptive behaviour at Santiaguito

Volcanic tremor associated with eruptive activity at Bromo volcano

Three broadband stations were deployed on Bromo volcano, Indonesia, from September to December 1995. The analysis of the seismograms shows that the signals produced by the volcanic sources cover the frequency range from at least 25 Hz down to periods of several minutes and underlines, therefore, the importance of broadband recordings. Frequency analysis reveals that the signal can be divided into four domains. In the traditional frequency range of volcanic tremor (1-10 Hz) sharp transitions between two distinct values of the tremor amplitude can be observed. Additional tremor signal including frequencies from 10 to 20 Hz could be found during late November and early December. Throughout the whole experiment signals with periods of some hundred seconds were observed which are interpreted as ground tilts. For these long-period signals a particle motion analysis was performed in order to estimate the source location. Depth and radius can be estimated when the source is modeled as a sudden pressure change in a sphere. The fourth frequency range lies between 0.1 and 1 Hz and is dominated by two spectral peaks which are due to marine microseism. The phase velocity and the direction of wave propagation of these signals could be determined using the tripartite-method

Volcanic tremor associated with eruptive activity at Bromo volcano

Three broadband stations were deployed on Bromo volcano, Indonesia, from September to December 1995. The analysis of the seismograms shows that the signals produced by the volcanic sources cover the frequency range from at least 25 Hz down to periods of several minutes and underlines, therefore, the importance of broadband recordings. Frequency analysis reveals that the signal can be divided into four domains. In the traditional frequency range of volcanic tremor (1-10 Hz) sharp transitions between two distinct values of the tremor amplitude can be observed. Additional tremor signal including frequencies from 10 to 20 Hz could be found during late November and early December. Throughout the whole experiment signals with periods of some hundred seconds were observed which are interpreted as ground tilts. For these long-period signals a particle motion analysis was performed in order to estimate the source location. Depth and radius can be estimated when the source is modeled as a sudden pressure change in a sphere. The fourth frequency range lies between 0.1 and 1 Hz and is dominated by two spectral peaks which are due to marine microseism. The phase velocity and the direction of wave propagation of these signals could be determined using the tripartite-method