Comment on ‘‘Material contrast does not predict earthquake rupture propagation direction’’ by R. A. Harris and S. M. Day

[1] Harris and Day [2005] (hereinafter referred to as HD) concluded from some numerical simulations and low resolution analysis of 8 M4-M6 earthquakes near Parkfield, CA, that material contrast in fault zone structures is not important for earthquake propagation direction. The limited theoretical and observational results of HD do not justify such a general conclusion. Considerations of a larger body of results suggest that material contrast may produce a statistically preferred direction of earthquake ruptures. The inferred diversity of the 8 discussed events is consistent with their small number and relatively small size. In addition, the behavior has a natural explanation in a structure with two material interfaces – the San Andreas Fault (SAF) and the Southwest Fracture Zone (SFZ) – on the opposite sides of a deformation/damage region. Large earthquakes propagating predominantly as mode II ruptures, and affected primarily by properties of the bounding crustal blocks (rather than the local structure), are likely to exhibit a smaller amount of diversity.