  We study how network structure affects the dynamics of collateral in presence
of rehypothecation. We build a simple model wherein banks interact via chains
of repo contracts and use their proprietary collateral or re-use the collateral
obtained by other banks via reverse repos. In this framework, we show that
total collateral volume and its velocity are affected by characteristics of the
network like the length of rehypothecation chains, the presence or not of
chains having a cyclic structure, the direction of collateral flows, the
density of the network. In addition, we show that structures where collateral
flows are concentrated among few nodes (like in core-periphery networks) allow
large increases in collateral volumes already with small network density.
Furthermore, we introduce in the model collateral hoarding rates determined
according to a Value-at-Risk (VaR) criterion, and we then study the emergence
of collateral hoarding cascades in different networks. Our results highlight
that network structures with highly concentrated collateral flows are also more
exposed to large collateral hoarding cascades following local shocks. These
networks are therefore characterized by a trade-off between liquidity and
systemic risk.
