AUTOCORRELATION REFLECTIVITY IMAGING INSIDE THE UPPER MAGMA STORAGE ZONE OF THEMOUNT ST HELENS TRANSCRUSTAL MAGMA SYSTEM

Mount St Helens (MSH) is the most active volcano in the contiguous United States, having erupted 14 times in 4 ka, including the catastrophic 18 May 1980 Plinian eruption. Geophysical and geochemical evidence suggest a complex magmatic plumbing system, consistent with a transcrustal magma system (Cashman et al, 2017). This consists of interconnected mush layers (<35-40% melt content) at different crustal depths through which basaltic melt from the mantle is refined into the more felsic compositions of the continents.

A shallow magma chamber (1-3 km) east of the edifice was imaged by local earthquake P-wave tomography (Waite and Moran, 2009). This is fed by an upper magma storage zone (UMSZ) at 3-14 km depth, the geochemical source region of the 1980 eruption (Pallister et al, 2008). The UMSZ was imaged in 3D tomographically by the active source iMUSH experiment from which a melt model (TM) was derived (Kiser et al, 2018). The TM model contains ~48 km³ of melt with an average melt fraction of 5.8%, and a maximum melt fraction of 11.7%.

Here we used the noise data from ~5000 iMUSH active experiment seismic stations to image the internal structure of the UMSZ. The 1-5Hz noise data were autocorrelated to form a 3D reflectivity image in time (Claerbout, 1968), which was then Kirchhoff depth migrated using a locally averaged 1D velocity model. The depth migrated autocorrelation reflectivity shows a remarkable degree of correspondence to the TM model. We inverted the depth migrated AR data for melt content using Gassman’s relations for a crystalline mush with <35% melt content (Chu et al, 2010).

In map view the UMSZ consists of two large lobes lying SW of the Saint Helens Seismic Zone: An upper lobe from 3-9 km depth is elongate NW-SE (N59W) and a lower lobe from 10-14 km depth is elongate NE-SW (N43E), roughly aligned with the regional minimum and maximum principal stress axes, ~N20E and ~N60W (Baker and Malone, 1991). A smaller third body lies at 9-11 km depth and extends NE of the SHSZ. The AR inversion shows that each element of the UMSZ consists of sills several hundred meters thick, ranging in melt content from near 0 (i.e. frozen) to 35%. High melt content sills are concentrated at the tops and bottoms of the upper and lower lobes. The AR melt models have mean melt fractions >10% with total melt estimated to be 72 to 109 km³.