Attenuation and scattering tomography of the deep plumbing system of Mount St. Helens

Abstract

We present a combined 3-D P wave attenuation, 2-D S coda attenuation, and 3-D S coda scattering tomography model of fluid pathways, feeding systems, and sediments below Mount St. Helens (MSH) volcano between depths of 0 and 18 km. High-scattering and high-attenuation shallow anomalies are indicative of magma and fluid-rich zones within and below the volcanic edifice down to 6 km depth, where a high-scattering body outlines the top of deeper aseismic velocity anomalies. Both the volcanic edifice and these structures induce a combination of strong scattering and attenuation on any seismic wavefield, particularly those recorded on the northern and eastern flanks of the volcanic cone. North of the cone between depths of 0 and 10 km, a low-velocity, high-scattering, and high-attenuation north-south trending trough is attributed to thick piles of Tertiary marine sediments within the St. Helens Seismic Zone. A laterally extended 3-D scattering contrast at depths of 10 to 14 km is related to the boundary between upper and lower crust and caused in our interpretation by the large-scale interaction of the Siletz terrane with the Cascade arc crust. This contrast presents a low-scattering, 4-6 km 2 "hole" under the northeastern flank of the volcano. We infer that this section represents the main path of magma ascent from depths greater than 6 km at MSH, with a small north-east shift in the lower plumbing system of the volcano. We conclude that combinations of different nonstandard tomographic methods, leading toward full-waveform tomography, represent the future of seismic volcano imaging.

Investigating the Apparent Seismic Diffusivity of Near-Receiver Geology at Mount St. Helens Volcano, USA

“…Since S-waves do not propagate through fluids this suggests that they are present close to the source of type B events, that is, fluid and molten materials are responsible for their generation [15]. Here, we focus on the near-receiver effects affecting coda waves, that is, those created by strong near-receiver scattering, observed by De Siena et al (2014) [12]. Using type B events, the authors observe high scattering and high attenuation anomalies up to 6 km depth, indicative of fluid-rich zones within the volcanic plumbing system during the last eruption of the volcano.…”

“…High-density accumulates, which may have a magmatic origin, are also observed and are thought to play an important role in the transport of such fluids within the crust. The low-velocity vertical anomaly gently dips towards SE; a low-scattering, high-intrinsic scattering anomaly retrieved at~10 km depth by scattering tomography [12]; an high-conductivity anomaly retrieved by means of magnetotelluric tomography and interpreted as an extended crustal magma sill [21]; and a deep low-velocity anomaly, obtained using full-waveform tomography and interpreted as the shallowest extension of a mantle wedge [22].…”

“…Firstly, type A events, located at some distance from the edifice (typically greater than 10 km) and in the frequency range of 5-15 Hz [12]. Secondly, type B events, which originate from within the volcanic edifice itself, and are of lower frequency, generally between 1 Hz and 5 Hz [15,17].…”

“…Secondly, type B events, which originate from within the volcanic edifice itself, and are of lower frequency, generally between 1 Hz and 5 Hz [15,17]. These differences are dependent on both the source dynamics and media properties close to the source [12,13,15]: whilst type A events show clear P-and S-wave arrivals, type B do not show a clear S arrival. Since S-waves do not propagate through fluids this suggests that they are present close to the source of type B events, that is, fluid and molten materials are responsible for their generation [15].…”

“…A lower diffusion value shows that energy is not leaving the local medium-just being constantly scattered around locally. Both are difficult to measure in the field but have wide implications for geophysical imaging in volcanoes [12,13].…”