3D Seismic Reflection Images of the Mount St Helens Magma Plumbing System

Abstract:

We present 2 different seismic reflection images made with data from the iMUSH[1] active source experiment that provide complementary images of the Mount St Helens (MSH) magmatic plumbing system.

Autocorrelation of noise records from the ~5900 seismic stations in the iMUSH active source experiment produces a zero-offset 3D seismic volume (Claerbout, 1968), which was depth migrated using an average MSH crustal velocity model. The reflection images of the upper magma storage zone (~3-15 km bsl) are remarkably consistent with the complicated tomography image. We inverted individual traces for porosity, i.e. melt content (Chu et al, 2010). From tomography the upper storage zone is estimated to have an average melt content of 5.8%, with a maximum of 13%. The reflectivity inversions show that internally this zone consists of stacks of laterally extensive sills from 200m to > 1km thick which range from near solid to 35% melt, with an average melt content for the entire volume of 8.7%. Individual sills extend away from MSH beyond the limits of the tomographically defined melt bodies.

CMP stacks of the iMUSH shot data show bright lower crustal, Moho, and upper mantle reflections. Moho amplitudes decrease near MSH and the nearby Indian Heaven Volcanic Field where high velocity/density lower crustal bodies are tomographically imaged. Between the volcanic centers, lower crustal thru upper mantle reflectors are clearly imaged in a region at the southern end of a NS trending lower crustal magma reservoir (Bedrosian et al, 2018). The amplitude, depth distribution, and lateral extent of these reflections increase to the NE of MSH where Quaternary volcanism is absent and the lower crustal reservoir broadens. We hypothesize that magmas from the reservoir intersecting the high-density lower crustal bodies experience enhanced vertical transport through the crust due to the increase in melt-host rock density contrast, resulting in greater buoyancy forces. This produces the observed cluster of volcanic vents above the high-density bodies. This hypothesis predicts that lateral growth of the reservoir is limited adjacent to the high-density bodies. Further north, the larger lower crustal reservoir results from the lack of an efficient mechanism for siphoning magma away from the reservoir.

[1] imaging Magma Under St Helens