IMUSH: MAGMA RESERVOIRS FROM THE UPPER CRUST TO THE MOHO INFERRED FROM HIGH-RESOLUTION VP AND VS TOMOGRAPHY BENEATH MOUNT ST. HELENS

Seismic investigations following the 1980 eruption of Mount St. Helens have led to a detailed model of the magmatic and tectonic structure directly beneath the volcano. These studies suffer from limited resolution below ~10 km, making it difficult to estimate the volume of the shallow magma reservoir beneath the volcano, the regions of magma entry into the lower crust, and the connectivity of this magma system throughout the crust. The latter is particularly interesting as one interpretation of the Southern Washington Cascades Conductor (SWCC) suggests that the Mount St Helens and Mount Adams volcanic systems are connected in the crust (Hill et al., 2009).

The multi-disciplinary iMUSH (imaging Magma Under St. Helens) project is designed to investigate these and other fundamental questions associated with Mount St. Helens. Here we present the first high-resolution 2D Vp and Vs models derived from travel-time data from the iMUSH 3D active-source seismic experiment. Significant lateral heterogeneity exists in both the Vp and Vs models. Directly beneath Mount St. Helens we observe a high Vp/Vs body, inferred to be the upper/middle crustal magma reservoir, between 4 and 13 km depth. Southeast of this body is a low Vp column extending from the Moho to approximately 15 km depth. A cluster of low frequency events, typically associated with injection of magma, occurs at the northwestern boundary of this low Vp column. Much of the recorded seismicity between the shallow high Vp/Vs body and deep low Vp column took place in the months preceding and hours following the May 18, 1980 eruption. This may indicate a transient migration of magma between these two reservoirs associated with this eruption.

Outside of the inferred magma bodies that feed Mount St. Helens, we observe several other interesting velocity anomalies. In the lower crust, high Vp features bound the low Vp column. One explanation for these features is the presence of lower crustal cumulates associated with Tertiary ancestral Cascade volcanism. West of Mount St. Helens, high Vp/Vs regions in the upper and middle crust have eastern boundaries that are close to the eastern boundaries of the accreted Siletzia terrain inferred from magnetic data. Finally, a low Vp channel northeast of Mount St. Helens between 14 and 18 km depth correlates well with the location of the SWCC.