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We find that foreshocks provide clear evidence for an extended nucleation process before some earthquakes. In this study, we examine in detail the evolution of six California foreshock sequences, the 1986 Mount Lewis (M_L = 5.5), the 1986 Chalfant (M_L = 6.4), the 1986 Stone Canyon (M_L = 4.7), the 1990 Upland (M_L = 5.2), the 1992 Joshua Tree (M_W = 6.1), and the 1992 Landers (M_W = 7.3) sequence. Typically, uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution. However, the similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms that we cross correlate to obtain extremely accurate relative locations. We use these results to identify small‐scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mainshock …