  We present a detailed description of seismic activity in Romania, Italy, and
Japan, as well as the California seismic zone in the United States of America,
based on the statistical analysis of the underlying earthquake networks used to
model the aforementioned zones. Our results on network connectivity and simple
network motifs allow for a complex description of seismic zones, while at the
same time reinforcing the current understanding of seismicity as a critical
phenomenon. The reported distributions on node connectivity, three-, and
four-event motifs are consistent with power-law, i.e., scale-free,
distributions over large intervals and are robust across earthquake networks
obtained from different discretizations of the seismic zones of interest. In
our analysis of the distributions of node connectivity and simple motifs, we
distinguish between the global distribution and the powerlaw part of it with
the help of maximum likelihood estimation (MLE) method and complementary
cumulative distribution functions (CCDF). The main message is that the
distributions reported for the aforementioned seismic zones have large
power-law components, extending over some orders of magnitude, independent of
discretization. All the results were obtained using publicly-available
databases and open-source software, as well as a new toolbox available on
GitHub, specifically designed to automatically analyze earthquake databases.
