  Aftershock sequences are of particular interest in seismic research since
they may condition seismic activity in a given region over long time spans.
While they are typically identified with periods of enhanced seismic activity
after a large earthquake as characterized by the Omori law, our knowledge of
the spatiotemporal correlations between events in an aftershock sequence is
limited. Here, we study the spatiotemporal correlations of two aftershock
sequences form California (Parkfield and Hector Mine) using the recently
introduced concept of "recurrent" events. We find that both sequences have very
similar properties and that most of them are captured by the space-time
epidemic-type aftershock sequence (ETAS) model if one takes into account
catalog incompleteness. However, the stochastic model does not capture the
spatiotemporal correlations leading to the observed structure of seismicity on
small spatial scales.
