Analysis of thermal, seismic, and video data, collected between 17 and 19 February, 2002, reveals that Erta Ale lava lake cycles between low (~0.03 m s -1 ) and high (~0.1 m s -1 ) convective regimes, with 60 to 220 minute periods. We attempt to characterize the seismic signature of each convective regime using spectral content, polarization analysis, and amplitude-based location of the continuous tremor. We identify the distinguishing spectral characteristics of each convective regime from continuous spectrograms. This information, combined with the covariance analysis method of Jurkevics (1988), is used to analyze the wavefield composition. For both convective regimes, we find tha t the wavefield from 0.85-3 Hz is dominated by rectilinear polarization, with azimuths and angles of incidence most consistent with P waves. At higher frequencies, for both convective regimes, the wavefield becomes more complex, and planar polarization dominates, suggesting that the higher frequency energy is mostly comprised of scattered S and Rayleigh waves. Because the majority of energy is concentrated at the lower frequencies, where body waves dominate, we assume an isotropic source, and locate windows of tremor from each convective regime with a method based on Gottsch�mer and Surono (2000). Our modified method uses least-squares inversion, based on tremor amplitudes recorded at three separate 3-component stations, to determine tremor epicenters and source power. By dividing each time window into shorter segments, and locating each segment of data, we find that the tremor source regions obtained for the low and high convective phases differ significantly. We also find a weak relationship between average frequency and tremor location. The similar wavefield composition of both convective phases suggests that they may share a common source process. However, their locations argue strongly that the source process is not only non-stationary, but is distributed over a small, unique volume for each of the two convective phases.

Seismic characteristics of lava lake convection at Erta Ale, Ethiopia

CARNIEL, Roberto
2003

Abstract

Analysis of thermal, seismic, and video data, collected between 17 and 19 February, 2002, reveals that Erta Ale lava lake cycles between low (~0.03 m s -1 ) and high (~0.1 m s -1 ) convective regimes, with 60 to 220 minute periods. We attempt to characterize the seismic signature of each convective regime using spectral content, polarization analysis, and amplitude-based location of the continuous tremor. We identify the distinguishing spectral characteristics of each convective regime from continuous spectrograms. This information, combined with the covariance analysis method of Jurkevics (1988), is used to analyze the wavefield composition. For both convective regimes, we find tha t the wavefield from 0.85-3 Hz is dominated by rectilinear polarization, with azimuths and angles of incidence most consistent with P waves. At higher frequencies, for both convective regimes, the wavefield becomes more complex, and planar polarization dominates, suggesting that the higher frequency energy is mostly comprised of scattered S and Rayleigh waves. Because the majority of energy is concentrated at the lower frequencies, where body waves dominate, we assume an isotropic source, and locate windows of tremor from each convective regime with a method based on Gottsch�mer and Surono (2000). Our modified method uses least-squares inversion, based on tremor amplitudes recorded at three separate 3-component stations, to determine tremor epicenters and source power. By dividing each time window into shorter segments, and locating each segment of data, we find that the tremor source regions obtained for the low and high convective phases differ significantly. We also find a weak relationship between average frequency and tremor location. The similar wavefield composition of both convective phases suggests that they may share a common source process. However, their locations argue strongly that the source process is not only non-stationary, but is distributed over a small, unique volume for each of the two convective phases.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11390/679311
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