Rock pulverization at high strain rate near the San Andreas fault

In the damage zone around faults, strain is usually localized along fractures, whereas the blocks enclosed by the fractures remain relatively undamaged(1,2). Some rocks near the San Andreas fault, however, are pervasively pulverized at distances of up to 400m from the fault's core(3); intense fragmentation at such distances is rarely observed along other fault zones. Moreover, these rocks preserve their original grain shapes, indicating that they experienced low total strain(3). Here we use laboratory experiments to show that the intense fragmentation of intact rocks sampled near the San Andreas fault requires high rates of strain (> 150 s(-1)). Our calculations suggest that the combination of the low amount of strain experienced by the pulverized rocks and the high rates of strain indicated by our experiments could be explained by a supers-hear rupture-a rupture that propagated along the fault at a velocity equal to or greater than that of seismic shear waves.