Ambient noise tomography across Mount St. Helens using a dense seismic array

Abstract

We investigated upper crustal structure with data from a dense seismic array deployed around Mount St. Helens for 2 weeks in the summer of 2014. Interstation cross correlations of ambient seismic noise data from the array were obtained, and clear fundamental mode Rayleigh waves were observed between 2.5 and 5 s periods. In addition, higher‐mode signals were observed around 2 s period. Frequency‐time analysis was applied to measure fundamental mode Rayleigh wave phase velocities, which were used to invert for 2‐D phase velocity maps. An azimuth‐dependent traveltime correction was implemented to mitigate potential biases introduced due to an inhomogeneous noise source distribution. Reliable phase velocity maps were only obtained between 3 and 4 s periods due to limitations imposed by the array aperture and higher‐mode contamination. The phase velocity tomography results, which are sensitive to structure shallower than 6 km depth, reveal an ~10–15% low‐velocity anomaly centered beneath the volcanic edifice and peripheral high‐velocity anomalies that likely correspond to cooled igneous intrusions. We suggest that the low‐velocity anomaly reflects the high‐porosity mixture of lava and ash deposits near the surface of the edifice, a highly fractured magmatic conduit and hydrothermal system beneath the volcano, and possibly a small contribution from silicate melt.

Retrieval of Interstation Local Body Waves From Teleseismic Coda Correlations

“…In general higher mode Rayleigh waves arrive earlier than the fundamental modes. This has been documented in other studies such as Wang et al (). For a similar aperture, large‐N array, Wang et al () found the average Rayleigh wave phase velocities at around 3.98 km/s for 1.5 s and 4.42 km/s for 2.5 s across Mount St. Helens.…”

“…This has been documented in other studies such as Wang et al (). For a similar aperture, large‐N array, Wang et al () found the average Rayleigh wave phase velocities at around 3.98 km/s for 1.5 s and 4.42 km/s for 2.5 s across Mount St. Helens. We conduct a dispersion calculation for Rayleigh wave phase and group velocities of fundamental and first higher mode using the same velocity model in Figure for WRA (in supporting information).…”

Local Source Vp and Vs Tomography in the Mount St. Helens Region With the iMUSH Broadband Array

“…The Spirit Lake and Spud Mountain plutons to the north of MSH have been extensively imaged by previous geophysical studies, including local earthquake tomography (Lees & Crosson, ; Moran et al, ; Waite & Moran, ), ambient noise tomography (Flinders & Shen, ; Wang et al, ), controlled source tomography (Figure S17; Kiser et al, ), magnetotellurics (Bedrosian et al, ), gravity (Williams et al, ), and magnetic surveys (Finn & Williams, ). However, the increased station density of the iMUSH broadband array provides improved constraints on the size and depth of these anomalies, especially those farther from MSH that have not been analyzed by previous seismic studies, namely the Silver Star pluton and the McCoy Creek intrusive complex.…”

“…These anomalies do not appear in the 3‐D Vs model, perhaps due to resolution limitations. However, there is a high‐ Vs anomaly to the west of Marble Mountain imaged by ambient noise tomography using the dense iMUSH Nodal array (Wang et al, ). A local earthquake tomographic study by Lees and Crosson () also imaged high‐ Vp features in the region to the south of MSH.…”

Shear Velocity Structure From Ambient Noise and Teleseismic Surface Wave Tomography in the Cascades Around Mount St. Helens

“…Near MSH, several local tomography studies have imaged an upper‐crustal magma chamber at 5‐ to 8‐km depth with limited resolution in the lower crust (Lees & Crosson, ; Waite & Moran, ; De Siena et al, ; Kiser et al, ). Wang et al () used ambient noise methodologies to image low velocities in the upper 6 km near the MSH edifice, using a small‐aperture high‐frequency Nodal array concentrated within 15 km of the MSH summit. Regional studies give hints of a region of partial melt in southwestern Washington between MSH, Mount Adams, and Mount Rainier from active‐source seismology (Kiser et al, ), magnetotelluric imaging (Bedrosian et al, ; Hill et al, ), and regional ambient noise tomography (Flinders & Shen, ).…”