  We investigate the geometrical structure of breaking bursts generated during
the creep rupture of heterogeneous materials. Based on a fiber bundle model
with localized load sharing we show that bursts are compact geometrical
objects, however, their external frontier has a fractal structure which
reflects their growth dynamics. The perimeter fractal dimension of bursts
proved to have the universal value 1.25 independent of the external load and of
the amount of disorder in the system. We conjecture that according to their
geometrical features breaking bursts fall in the universality class of
loop-erased self-avoiding random walks with perimeter fractal dimension 5/4
similar to the avalanches of Abelian sand pile models. The fractal dimension of
the growing crack front along which bursts occur proved to increase from 1 to
1.25 as bursts gradually cover the entire front.
