Backprojection and waveform inversion of volcanic tremor during the 2008 eruption of Okmok Volcano

Abstract:

At volcanoes, seismic signals are often characterized by emergent onsets, a lack of phase arrivals, and long durations. Volcanic tremor, in particular, is a ubiquitous type of eruption seismicity that remains poorly understood. Large-N deployments of seismometers at actively erupting volcanoes offer a chance to make headway into understanding the processes of tremor generation. To illustrate these challenges, here I discuss imaging, waveform inversion, and tracking of volcanic tremor.

During the 2008 Okmok eruption, low frequency tremor radiated continuously at frequencies as low as 0.2 Hz from beneath the active intracaldera cone. At these low frequencies and over the aperture of the Okmok seismic network, surface waves dominated the wave field and propagated in an effective laterally homogeneous medium with little scattering besides that due to topography. Since unknown path effects were negligible, I invert the waveforms of the low frequency tremor between 0.2-0.3 Hz for a point moment source. The inversion resolves a shallow subhorizontal sill beneath the active cone, with a mostly diagonal moment tensor dominated by the Mzz component.

Backprojection has become a powerful tool for imaging the rupture process of global earthquakes. At the volcano scale, it can monitor changes in tremor location as well. I apply the method to the Okmok seismic network in the 0.2-0.3 Hz band at the time of an escalation in tremor during the 2008 eruption. Although backprojection focuses the wavefield close to the location of the active cone, the network array response lacks sufficient resolution to reveal kilometer-scale changes in tremor location. By deconvolving the response in successive backprojection images, I enhance resolution and find that the tremor source moved 1-2 km toward an intracaldera lake prior to its escalation. The increased tremor therefore may have resulted from magma-water interaction, in agreement with the overall phreatomagmatic character of the eruption. Imaging and tracking of eruption tremor shows that time reversal methods, such as backprojection, can provide new insights into the temporal evolution of volcanic sources.