UC San Diego

In the summer of 2015, a geophysical survey was completed across Okmok Caldera in the Aleutian Islands. Okmok is an active volcano with eruptions every 10-20 years in recent history (Miller et al., 1998). In order to better understand the melt system beneath the volcano, data were collected at 54 marine magnetotelluric (MT) and 23 onshore MT stations in an amphibious array crossing from the forearc into the backarc. Thirteen temporary broadband seismometers were also installed onshore for one year to supplement the network of 13 permanent seismometers on the island. Initial processing the marine MT data reveals complex features across many of the stations, with full wrapping of the phase in both transverse electric (TE) and transverse magnetic (TM) modes. Attempts to invert these data using a two-dimensional finite element based inversion code resulted in poor RMS goodness-of-fit values, and evaluation of skew and polarization plots indicates that much of the data is impacted by 3D geologic structure. In order to understand what subsurface features could be consistent with the complex responses seen in the marine MT data, forward modeling was completed using a version of the freely-available 3D code ModEM that was modified to accommodate the marine environment. The 3D forward modeling shows that the complex offshore topography strongly distorts the MT responses. Simple models of seafloor resistivity represented as a half-space with bathymetry are able to account for a significant fraction of the observed MT response structure. More quantitative interpretation of the offshore MT data requires a 3D inversion code capable of including the complex 3D seafloor bathymetry.