Shear cracks with finite cohesive forces can propagate by skipping past barriers. The barriers left behind may remain unbroken or may eventually break because of subsequent increase in dynamic stress depending on the ratio of barrier strength to tectonic stress. This model can explain a variety of observations on rupture in the earth, including (1) segmentation of the fault or ruptured zone in earthquakes and rock bursts, (2) ripples in seismograms which cannot be explained by path effect, and (3) departure of the scaling law of the seismic spectrum from that based upon the similarity assumption. The model also explains why the simple uniform dislocation model sometimes works better than the crack model without barriers. It also predicts, contrary to common belief, that an earthquake with low average stress drop may generate relatively greater amounts of high-frequency waves than an earthquake with high average stress drop. One important consequence of of our barrier model is the possibility of predicting the occurence of aftershocks by analyzing the source spectrum of the main shock.