  We propose a model which explains how power-law crossover behaviour can arise
in a system which is capable of experiencing cascading failure. In our model
the susceptibility of the system to cascades is described by a single number,
the propagation power, which measures the ease with which cascades propagate.
Physically, such a number could represent the density of unstable material in a
system, its internal connectivity, or the mean susceptibility of its component
parts to failure. We assume that the propagation power follows an upward
drifting Brownian motion between cascades, and drops discontinuously each time
a cascade occurs. Cascades are described by a continuous state branching
process with distributional properties determined by the value of the
propagation power when they occur. In common with many cascading models, pure
power law behaviour is exhibited at a critical level of propagation power, and
the mean cascade size diverges. This divergence constrains large systems to the
subcritical region. We show that as a result, crossover behaviour appears in
the cascade distribution when an average is performed over the distribution of
propagation power. We are able to analytically determine the exponents before
and after the crossover.
