Some Properties of “aftershocks”

Some properties of AftershocksDave Jackson

UCLA

Oct 25, 2011

UC BERKELEY

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.

ETAS model of earthquake triggering, from Zhuang et al 2008

Stochastic Declustering

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

Empirical, So Cal, mt=4.2 plus = triggered, triangle = spontaneous

Synthetic data, same parameters as for empirical study

Synthetic data, triggering of mag 4.2+ by mag 3.7+

Magnitude distributions for spontaneous and triggered quakes, California m4.7+ after 1933

Magnitude distribution after randomizing the independence weightings

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