  This paper discusses local features in the source function that generate in
the far zone simultaneously (a) a quadratic decay of the source spectrum and
(b) the loss of radiation directivity at high frequencies. A. Gusev drew
attention to this problem and suggested that a positive solution can be
obtained for earthquake rupture front with a rather complex ("lace") structure.
Below we give a theoretical solution of the problem and show that the front
structure can be simple enough, but not smooth.
