Rupture directivity of small earthquakes at Parkfield

[1] Theoretical modeling of strike-slip ruptures along a bimaterial interface suggests that earthquakes initiating on the interface will have a preferred rupture direction. We test this model with 450 small earthquakes (2   3 exhibiting southeast rupture characteristics. Some spatial and temporal variability in rupture directivity is also apparent. We observe a higher proportion of northwest directivity ruptures following the 2004 M 6 Parkfield earthquake, which ruptured toward the northwest. Our results are generally consistent with the preferred southeast rupture directivity model but suggest that directivity is likely due to several contributing factors.