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| Titel |
Empirical Green's Functions Analysis of Volcanic Hybrid Earthquakes Simulated in the Laboratory |
| VerfasserIn |
Rebecca Harrington, Philip Benson |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250050405
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| Zusammenfassung |
| Volcanic hybrid earthquakes often precede explosive volcanic eruptions by hours to
days, and are therefore frequently used for short term eruption forecasting. In spite
of their predictive capabilities, their high-frequency onsets which transition to a
protracted, low-frequency ringing make inferring a source mechanism a perplexing task.
Complex models involving some combination of elastic shear, fluid, fluid shear, and
their interactions are commonly invoked to explain their mechanism. However,
some field observations suggest that the highly attenuating, complex travel path in a
volcanic edifice may be responsible for some portion of the low-frequency part of the
waveform. Resolving the ambiguity in the role of fluids in hybrid generation is an
important factor in understanding eruption dynamics, as it would better facilitate
our ability to provide accurate forecasts of how explosive a given eruption may
be.
Here we present a new analysis of experimental simulations of volcanic hybrid
signals, in efforts to better understand their origin. We examine the waveforms of
laboratory microseismic events generated during two rock deformation experiments
performed on samples of Mt. Etna basalt to determine their source characteristics and
establish evidence for a mode of failure. Events were recorded during deformation
under (a), unsaturated (dry) conditions, and (b), samples saturated with water. We
employ an empirical Green’s function approach to isolate the acoustic emission
source spectra from attenuation and travel path effects, and estimate the spectral
corner frequency using a least-squares fit to a Brune spectral model. Spectral fits
indicate that the acoustic emission events occurring under dry conditions follow the
expected scaling of moment and corner frequency for standard brittle failure in an
elastic medium with constant stress drop, namely M0 - fc-3 . Events occurring
during the decompression phase of the saturated experiment have estimated corner
frequencies not easily described by any simple scaling relationship. The observed
moment-corner frequency scaling also suggests that event durations change in a
predictable way with increasing moment for the events occurring under dry conditions.
Conversely, events occurring under wet conditions do not show any distinctive
relationship between duration and event size. The specific size dependence on duration
exhibited by the events in the dry experiment must consequently rule out fluid flow as a
source, as there is no plausible reason for the driving pressure for fluid flow to be
dependent on duration in such a specific way. Similar scaling observations between AE
events occurring during the dry experiment and volcanic hybrid earthquakes at
Mount St. Helens volcano suggest that a brittle failure mechanism explains both
laboratory and field results where M0 - fc-3 scaling occurs. The consistency of
laboratory and field observations suggest that the presence of brittle-failure scaling
must exclude fluid flow as a source of the seismic signal for a particular group of
events. Conversely, the absence of such scaling may suggest a fluid induced seismic
signal. |
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