Understanding seismic path biases and magmatic activity at Mount St Helens volcano before its 2004 eruption

Abstract

SUMMARY In volcanoes, topography, shallow heterogeneity and even shallow morphology can substantially modify seismic coda signals. Coda waves are an essential tool to monitor eruption dynamics and model volcanic structures jointly and independently from velocity anomalies: it is thus fundamental to test their spatial sensitivity to seismic path effects. Here, we apply the Multiple Lapse Time Window Analysis (MLTWA) to measure the relative importance of scattering attenuation vs absorption at Mount St Helens volcano before its 2004 eruption. The results show the characteristic dominance of scattering attenuation in volcanoes at lower frequencies (3–6 Hz), while absorption is the primary attenuation mechanism at 12 and 18 Hz. Scattering attenuation is similar but seismic absorption is one order of magnitude lower than at open-conduit volcanoes, like Etna and Kilauea, a typical behaviour of a (relatively) cool magmatic plumbing system. Still, the seismic albedo (measuring the ratio between seismic energy emitted and received from the area) is anomalously high (0.95) at 3 Hz. A radiative-transfer forward model of far- and near-field envelopes confirms this is due to strong near-receiver scattering enhancing anomalous phases in the intermediate and late coda across the 1980 debris avalanche and central crater. Only above this frequency and in the far-field diffusion onsets at late lapse times. The scattering and absorption parameters derived from MLTWA are used as inputs to construct 2-D frequency-dependent bulk sensitivity kernels for the S-wave coda in the multiple-scattering (using the Energy Transport Equations—ETE) and diffusive (AD, independent of MLTWA results) regimes. At 12 Hz, high coda-attenuation anomalies characterize the eastern side of the volcano using both kernels, in spatial correlation with low-velocity anomalies from literature. At 3 Hz, the anomalous albedo, the forward modelling, and the results of the tomographic imaging confirm that shallow heterogeneity beneath the extended 1980 debris-avalanche and crater enhance anomalous intermediate and late coda phases, mapping shallow geological contrasts. We remark the effect this may have on coda-dependent source inversion and tomography, currently used across the world to image and monitor volcanoes. At Mount St Helens, higher frequencies and deep borehole data are necessary to reconstruct deep volcanic structures with coda waves.

Sensitivity kernels for static and dynamic tomography of scattering and absorbing media with elastic waves: a probabilistic approach

“…The assumption of an initially uniform distribution of heterogeneity is common to most studies that investigated the spatial variability of heterogeneity and attenuation properties with scattered coda waves. Therefore, previous studies are restricted to a first order mapping of deviations from uniform heterogeneity (De Siena et al 2014a,b;Prudencio et al 2015a;Zieger et al 2016;Gabrielli et al 2020;Sketsiou et al 2020) An iterative tomography of this nonlinear problem is therefore impossible so far and requires developments such as those presented here.…”

Fast Changes in Seismic Attenuation of the Upper Crust due to Fracturing and Fluid Migration: The 2016–2017 Central Italy Seismic Sequence

“…At low frequencies, high Q c −1 anomalies map surface geology and sedimentary basins in the Pyrenees , the Alps (Mayor et al, 2016), and Vrancea (Borleanu et al, 2017). At higher frequencies, the change in coda composition from surface to body waves can increase depth sensitivity (De Siena et al, 2016;Mayor et al, 2016;Gabrielli et al, 2020). This effect is dominant when measuring Q c −1 across volcanic structures, as documented along the Mount St. Helens volcano (US) where low-frequency waves allow to map the shallowest, most-heterogeneous volcanic structures, while deep feeding systems appear as high-Q c −1 anomalies at higher frequencies (De Siena et al, 2016;Gabrielli et al, 2020).…”

“…At higher frequencies, the change in coda composition from surface to body waves can increase depth sensitivity (De Siena et al, 2016;Mayor et al, 2016;Gabrielli et al, 2020). This effect is dominant when measuring Q c −1 across volcanic structures, as documented along the Mount St. Helens volcano (US) where low-frequency waves allow to map the shallowest, most-heterogeneous volcanic structures, while deep feeding systems appear as high-Q c −1 anomalies at higher frequencies (De Siena et al, 2016;Gabrielli et al, 2020). Di Martino et al (2022) recently obtained absorption maps using an active seismic experiment (~100 m 2 array with meter-scale station distance) at the Solfatara volcanic crater, in Southern Italy.…”

Small-Scale Volcanic Structures of the Aeolian Volcanic Arc Revealed by Seismic Attenuation

“…However, models of subsurface features are based on indirect observations. Imaging volcanic structures remains a challenge for the seismological community, even when similar structures have been identified for multiple volcanoes using different methods (e.g., velocity and attenuation from active and passive seismic sources; Zandomeneghi et al, 2008;Zandomeneghi et al, 2009;Rawlinson et al, 2010;García-Yeguas et al, 2012;García-Yeguas et al, 2014;De Siena et al, 2014;Koulakov and Shapiro, 2015;Prudencio et al, 2015a;Prudencio et al, 2015b;De Siena et al, 2017;Prudencio and Manga, 2020;Gabrielli et al, 2020). In particular, high contrast heterogeneities and the identification of structural changes related to magma transport are challenging for these methods (e.g., Castro-Melgar et al, 2021;Giampiccolo et al, 2021).…”