  The topological properties of interbank networks have been discussed widely
in the literature mainly because of their relevance for systemic risk. Here we
propose to use the Stochastic Block Model to investigate and perform a model
selection among several possible two block organizations of the network: these
include bipartite, core-periphery, and modular structures. We apply our method
to the e-MID interbank market in the period 2010-2014 and we show that in
normal conditions the most likely network organization is a bipartite
structure. In exceptional conditions, such as after LTRO, one of the most
important unconventional measures by ECB at the beginning of 2012, the most
likely structure becomes a random one and only in 2014 the e-MID market went
back to a normal bipartite organization. By investigating the strategy of
individual banks, we explore possible explanations and we show that the
disappearance of many lending banks and the strategy switch of a very small set
of banks from borrower to lender is likely at the origin of this structural
change.
