A remarkable seismic phenomenon of Erebus is the ubiquitous association of Strombolian eruptions with repeatable very long period (VLP ; > 1 s) oscillations that persist for several minutes. VLP signals presage eruptions by approximately 4 seconds, indicating that they are excited by the upward rise of gas slugs in the shallowest portion of the conduit system (probably less than 30 meters below the surface of the lava lake). The Erebus VLP spectrum is observed to contain numerous harmonics, dominated by three strong harmonics at 21 s, 11 s, and 8 s (see Figure at Right).
Possible explanations for the VLP phenomenon include the slow nonlinear resonance of a crack-like feeding system during magma recharge or slow internal gravity waves in the density-stratified near surface of the lava lake excited by the passage of the eruptive gas slug. During the three field seasons for which we have broadband seismic data (1996-1997, 1999-2000, and 2000-2001) VLP modal periods have become a few percent graver, indicating a relaxation and/or broadening of the summit conduit system over the past few years. The accompanying figure shows a sonogram constructed from a stack of 35 1999-2000 VLP signals recorded at station E1S. VLP signals have also recently been observed on many other active volcanoes, such as Stromboli, Kilauea, and Popocatepetl, and offer a unique geophysical view into the dynamics and geometry of volcanic plumbing and moving magma.