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| Titel |
High volcanic seismic b-values: Real or artefacts? |
| VerfasserIn |
Nick Roberts, Andrew Bell, Ian G. Main |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105568
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| Publikation (Nr.) |
 EGU/EGU2015-5101.pdf |
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| Zusammenfassung |
| The b-value of the Gutenberg-Richter distribution quantifies the relative proportion of large to
small magnitude earthquakes in a catalogue, in turn related to the population of fault rupture
areas and the average slip or stress drop. Accordingly the b-value is an important
parameter to consider when evaluating seismic catalogues as it has the potential to
provide insight into the temporal or spatial evolution of the system, such as fracture
development or changes in the local stress regime. The b-value for tectonic seismicity is
commonly found to be close to 1, whereas much higher b-values are frequently
reported for volcanic and induced seismicity. Understanding these differences is
important for understanding the processes controlling earthquake occurrence in different
settings. However, it is possible that anomalously high b-values could arise from small
sample sizes, under-estimated completeness magnitudes, or other poorly applied
methodologies. Therefore, it is important to establish a rigorous workflow for analyzing these
datasets.
Here we examine the frequency-magnitude distributions of volcanic earthquake
catalogues in order to determine the significance of apparently high b-values. We first derive a
workflow for computing the completeness magnitude of a seismic catalogue, using synthetic
catalogues of varying shape, size, and known b-value. We find the best approach involves
a combination of three methods: ‘Maximum Curvature’, ‘b-value stability’, and
the ‘Goodness-of-Fit test’. To calculate a reliable b-value with an error ≈¤0.25, the
maximum curvature method is preferred for a “sharp-peaked” discrete distribution. For a
catalogue with a broader peak the b-value stability method is the most reliable with the
Goodness-of-Fit test being an acceptable backup if the b-value stability method
fails.
We apply this workflow to earthquake catalogues from El Hierro (2011-2013) and Mt
Etna (1999-2013) volcanoes. In general, we find the b-value to be equal to or slightly greater
than 1. However, reliable high b-values of 1.5-2.4 at El Hierro and 1.5-1.8 at Mt Etna are
observed for restricted time periods. We argue that many of the almost axiomatically ‘high’
b-values reported in the literature for volcanic and induced seismicity may be attributable to
biases introduced by the methods of inference used and/or the relatively small sample sizes
often available. |
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