Pulverized rocks were found near the San Andreas Fault, as far as 400m away from the fault core. It is exceptional to observe such a dense fragmentation so far from the core. Here we show that extreme strain rate is necessary to intensely fragment intact rocks sampled near the fault. We performed laboratory experiments on natural rocks to understand this pulverization, using a Split Hopkinson Pressure Bars apparatus. When strain rate exceeds 150/s, samples break into numerous fragments, to a scale smaller than the initial grain size. To satisfy such strain rate, we propose that the pulverization damage is associated with supershear rupture.