  Interbank markets are often characterised in terms of a core-periphery
network structure, with a highly interconnected core of banks holding the
market together, and a periphery of banks connected mostly to the core but not
internally. This paradigm has recently been challenged for short time scales,
where interbank markets seem better characterised by a bipartite structure with
more core-periphery connections than inside the core. Using a novel
core-periphery detection method on the eMID interbank market, we enrich this
picture by showing that the network is actually characterised by multiple
core-periphery pairs. Moreover, a transition from core-periphery to bipartite
structures occurs by shortening the temporal scale of data aggregation. We
further show how the global financial crisis transformed the market, in terms
of composition, multiplicity and internal organisation of core-periphery pairs.
By unveiling such a fine-grained organisation and transformation of the
interbank market, our method can find important applications in the
understanding of how distress can propagate over financial networks.
