Electrical Survey

The only MT study to date focusing on the crustal magmatic system at Mount St Helens is the work of Hill et al. (2009). Their 3D resistivity model, based on inversion of MT responses at 20 frequencies spanning 10-3 Hz – 384 Hz, from 67 sites in a 35 km x 35 km area centered of Mt St Helens, images a ~5 km wide zone of enhanced conductivity beneath the summit at ~2.5 km depth (Fig. 4 a). The location and width of this feature correspond well with the inferred shallow magma storage zone of Waite and Moran 2009, however the resistivity model has high conductivity extending much deeper than the corresponding low-velocity zone. At greater depth, this conductive zone elongates in the north-south direction (Fig. 4 b) and is roughly parallel to the St Helens seismic zone (Weaver & Smith, 1983). Due to the finite aperture of Hill et al.’s survey, it is unclear if this feature extends further to the north. Below mid-crustal depths this zone expands outward to the north and east (Fig. 4 c) into what is likely a portion of the Southern Washington Cascades Conductor (SWCC). We have independently corroborated Hill et al’s (Fig. S7, Supplemental material from that paper) resolution/sensitivity study that shows the wideband MT dataset was capable of resolving the key model features, as indicated in Fig. 4b below.

Earthscope MT TA studies (Fig. 4 c and d; Bedrosian, unpubl.; Patro and Egbert 2008) provide the larger framework in which to view Mount St Helens and the SWCC. Though strongly aliased by the 70 km site spacing and with little resolving power above ~10 km depth because of the low frequency nature of the EarthScope MT dataset (~10-4 Hz – 10-1 Hz), the 3D nature of the SWCC, its relation to Mount St Helens, Mount Adams, and Mount Rainier, and a clear eastward dip are evident. Studies at an intermediate scale include numerous investigations of the SWCC and its relationship to regional seismicity and volcanism (Stanley et al. 1987; Stanley et al. 1990; Stanley et al. 1992; Egbert and Booker 1993; Stanley et al. 1996). This conductive feature has been interpreted as metasediments associated with the Eocene suturing of Siletzia (Stanley et al. 1987), a subducted seamount complex (Egbert and Booker 1993), and more recently as a zone of partial melt (Hill et al. 2009).

It is one of the primary goals of the MT component to discriminate between the competing origins for the SWCC. Stanley et al.’s interpretation of the SWCC as forearc basin and accretionary prism sediments is based upon the imaged high conductivity coming to within 2 km of the surface, a strong eastward dip in the top of the conductor, and its spatial correspondence with the St Helens and Rainier seismic zones. It suffers, however, from data of limited bandwidth (10-2 to 10 Hz to 100 sec), and more seriously from a primarily 1D interpretation of a clearly 3D structure. Hill et al.’s (2010) magmatic interpretation of the SWCC is based primarily on modeling of their MT data, which requires that a conductive Mount St Helens anomaly beneath Mount St Helens be attached to the broader SWCC anomaly that extends over to Mount Adams. Though Hill’s model for Mount St Helens is based upon wider bandwidth data and a 3D inversion, it suffers from the connection between Mount St Helens and Mount Adams being based solely upon a 2D inversion of a single profile crossing the SWCC.