This study emphasizes the relationship between the rupture mode of earthquakes and the nature and distribution of heterogeneities along a fault plane. A primary objective is to determine whether heterogeneities are structural features or are features induced by stress and slip variations that change with time. We will investigate the rupture mode of earthquakes relative to the fault heterogeneities through analysis of waveforms of the largest events and their aftershocks. Heterogeneities are mapped along the length and depth of a fault plane by analyzing refracted waves generated by local seismicity. This information reflects the velocity contrast between differing materials that are juxtaposed in the fault zone. Our analyses reveal that the velocity contrasts are not randomly distributed, but rather occur in patches where one range of values predominates. We propose to extend our dataset of relocated earthquakes and to refine the refraction angle analyses for mapping patches of velocity contrast. Finite-difference modeling of the waveforms will better define the nature of the heterogeneities that are mapped by refraction analysis. Preliminary results indicate that the largest earthquakes in the study region (San Andreas fault, central California) since 1936 nucleate in the vicinity of small patches with high variance of velocity contrast and rupture into larger patches of more homogeneous velocity contrast. Combined with waveform analysis of the mainshocks, we may be able to characterize fault zone heterogeneities and the mainshock rupture process to a greater degree than has previously been done.