Some Properties of “aftershocks” Some properties of Aftershocks Dave Jackson UCLA Oct 25, 2011
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Some Properties of “aftershocks”
Some properties of AftershocksDave Jackson
UCLA
Oct 25, 2011
UC BERKELEY
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Conclusions
• Aftershocks are not clearly defined• Whether aftershocks have different
magnitude dependence or triggering potential depends upon their definition
• For most reasonable definition, there is suggestive but weak evidence that triggered earthquakes have different magnitude distributions and triggering potential.
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ETAS model of earthquake triggering, from Zhuang et al 2008
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Stochastic Declustering
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Definitions
• Mainshock: largest earthquake in a cluster• Foreshock : An earthquake in a cluster,
occuring before the mainshock• Aftershock: An earthquake in a cluster,
occuring after the mainshock• Triggered event: An earthquake with a low
value of the independence probablity f, independent of its own magnitude
• Spontaneous event: An earthquake with a high value of the independence probability
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Empirical, So Cal, mt=4.2 plus = triggered, triangle = spontaneous
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Synthetic data, same parameters as for empirical study
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Synthetic data, triggering of mag 4.2+ by mag 3.7+
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Magnitude distributions for spontaneous and triggered quakes, California m4.7+ after 1933
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Magnitude distribution after randomizing the independence weightings
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Conclusions
• Aftershocks are not clearly defined• Whether aftershocks have different magnitude
dependence or triggering potential depends upon their definition
• For most reasonable definition, there is suggestive but weak evidence that triggered earthquakes have different magnitude distributions and triggering potential. Conclusions depend on clustering model