All large earthquakes are followed by an increase of seismicity
("aftershocks"), likely triggered by the static stress change
induced by the mainshock. Aftershock rate decays approximately as
the inverse of the time since the mainshock. This property can be
explained by the seismicity model of Dieterich , based on
the rate-and-state friction law. We have introduced heterogeneity
of the stress change in this model. This new model explains why
most aftershocks are located on or close to the mainshock rupture,
where the stress has decreased on average after the mainshock.
Stress heterogeneity also modifies the temporal decay of
aftershocks, compared to the case of a uniform stress step
[Dieterich 1994]. We can thus estimate the stress distribution
from the aftershock rate.
However, some aftershock sequences have a faster temporal decay than expected by this model. This suggests that postseismic stress relaxation is not negligible, or that the friction law should be modified.
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