The iMUSH (imaging Magma Under St. Helens) project is a multidisciplinary study designed to image the details of the magmatic system that feeds Mount St. Helens from the subducting Juan de Fuca plate to the surface. The active-source seismic component of the project involved 23 borehole shots and around 6000 seismograph locations deployed over 17500 km2 (Figure 1). Initial 2D results from this data set have provided insight into the locations of shallow reservoirs in the region and the pathways along which magma migrates through the lower and middle crust (Figure 2).
Figure 1: Summary of the iMUSH active-source seismic deployment. (Left) Seismograph (dots) and borehole shot (stars) locations. Red and blue dots show the locations of instruments temporarily deployed to record the borehole shots. These instruments were deployed twice, the first deployment shown in blue and the second deployment shown in red. Pink dots are the locations of nodal instruments within ~15 km of Mount St. Helens that recorded continuously for 2 weeks. (Right) Volunteers for the iMUSH experiment. The iMUSH active-source deployment team included 70 volunteers from 36 institutions in the United States and Europe.
Figure 2: 2D results along two lines that intersect at Mount St. Helens. The background model is Vp. Red transparent bodies are regions with Vp/Vs greater than or equal to 1.9. White dots are earthquakes associated with the May 18, 1980 eruption. The high Vp/Vs body beneath Mount St. Helens (F1) is inferred to be the primary shallow magma reservoir that feeds this volcano. A low Vp column (L1) in the middle and lower crust is bordered to the west by DLP events (red squares), suggesting the presence of melt. Seismicity immediately following the May 18, 1980 eruption extends to the top of the high Vp middle/lower crustal anomaly beneath Mount St. Helens (H1). Lateral migration of magma between the deep and shallow reservoirs may occur along the boundary of this body. In addition to the feature beneath Mount St. Helens, a second high Vp/Vs body is observed beneath the Indian Heaven Volcanic Field (F4). Both Vp/Vs anomalies sit above lower crustal high Vp regions that may represent cumulates (H1 and H2).
Geometry of the magma reservoir beneath Mount St. Helens as estimated from a 3D Vp model (Kiser et al., 2018). The tan surface is exaggerated topography and the red volume is the 3% melt isosurface of the magma reservoir.