  We study numerically the growth of a crack in an elastic medium under the
influence of a travelling shockwave. We describe the implementation of a fast
algorithm which is perfectly suited for a data parallel computer. Using large
scale simulations on the Connection Machine we generate cracks with more than
10000 sites on a $\scriptstyle 1024 \times 1024$ lattice. We show that the
resulting patterns are fractal with a fractal dimension that depends on the
chosen breaking criterion and varies between $\scriptstyle 1.$ and
$\scriptstyle 2.$
