Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Scale-invariant statistics of period in directed earthquake network   A new law regarding structure of the earthquake networks is found. The
seismic data taken in California is mapped to a growing directed network. Then,
statistics of period in the network, which implies that after how many
earthquakes an earthquake returns to the initial location, is studied. It is
found that the period distribution obeys a power law, showing the fundamental
difficulty of statistical estimate of period.

9
The categories of this paper are: nlin
28
The categories of this paper are: alg-geom
35
The categories of this paper are: mtrl-th
27
The categories of this paper are: comp-gas
21
The categories of this paper are: adap-org
3
The categories of this paper are: physics
16
The categories of this paper are: stat
36
The categories of this paper are: cmp-lg
18
The categories of this paper are: econ
19
The categories of this paper are: eess
22
The categories of this paper are: chao-dyn
26
The categories of this paper are: bayes-an
10
The categories of this paper are: q-bio
20
The categories of this paper are: acc-phys
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex