  Earthquake aftershock identification is closely related to the question "Are
aftershocks different from the rest of earthquakes?" We give a positive answer
to this question and introduce a general statistical procedure for clustering
analysis of seismicity that can be used, in particular, for aftershock
detection. The proposed approach expands the analysis of Baiesi and Paczuski
[PRE, 69, 066106 (2004)] based on the space-time-magnitude nearest-neighbor
distance $\eta$ between earthquakes. We show that for a homogeneous Poisson
marked point field with exponential marks, the distance $\eta$ has Weibull
distribution, which bridges our results with classical correlation analysis for
unmarked point fields. We introduce a 2D distribution of spatial and temporal
components of $\eta$, which allows us to identify the clustered part of a point
field. The proposed technique is applied to several synthetic seismicity models
and to the observed seismicity of Southern California.
