SEISMOLOGICAL SOCIETY OF AMERICA
VLP Displacements Observed with Broadband Seismometers at Mount Erebus Volcano Modeled as a Lava Lake Seiche
Persistent Strombolian explosions in the approximately 10-m diameter phonolitic lava lake of Mount Erebus,Antarctica, observed with broadband seismometers, are ubiquitously seen to excite strongly resonant very long period (VLP) signals with periods of approximately 20, 11, and 7 s. VLP signals persist for several minutes and dominate the displacement field at ranges up to several km. The lake is a unique window into a degassing shallow magma chamber, where the density and incompressibility decrease rapidly in the top few hundred meters due to vesiculation. We interpret the VLP signals as the elastic response of the upper volcano to a linear gravitational surface and/or internal seiche excited in a circular lake via the displacement of lava by large near-centrally exiting gas bubbles associated with the Strombolian eruptions. For a radially-symmetric seiche in a deep lake, the spatial eigenfunctions are given by Ui = J0(zir), where zi is the ith antinode of the order zero Bessel function J0(r), and the associated nonharmonic modal periods are proportional to zi-1. Even with these simplifying assumptions, theoretical modal period ratios (1:0.545:0.375) are within 10% of observed values (1:0.55:0.35). Observed near-field seismic displacements are the quasi-static elastic response of the upper volcano to conduit pressure variations from the seiche. Second order variations in observed VLP waveforms from three field seasons suggest that VLP signals are sensitive to the temporal evolution of the shallow conduit system. Event size, azimuthal, and radial VLP amplitude variations, modal Q, background VLP spectra, and video observations of the lake provide information that will allow further refinement of this model.