When the San Andreas fault system of California and the Alpine fault system of New Zealand are compared within the reference frame of their regional slip vectors, their major tectonic elements are seen to occupy similar positions. This argues that a genetic relationship exists between the development of these complexities.

Furthermore, three distinctive modes of slip, or ‘seismic style’, can be recognized for different segments of these major transform fault systems. These are: (1) slip during great (M ≥ 8) earthquakes separated by long periods of quiescence; (2) slip during more frequent large (6.5 ≤ M ≤ 7.5) earthquakes, and (3) aseismic slip (creep). These different seismic styles occur in distinctive tectonic settings and it is argued that they represent fundamental variations in the mechanical properties of the faults. I suggest that style 1 occurs along sections of the fault orientated such that a component of tectonic convergence occurs, leading to a high effective normal stress and hence high frictional strength. Style 2 occurs along sections of faults which strike close to the regional slip vector, and hence have a lower normal stress. Aseismic slip occurs only along sections of faults in central California where anomalously high pore pressures reduce the effective normal stress to a very low value and the fault is entirely within the stable sliding frictional field.

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