Dark, aphanitic material occurs as a thin (<3 mm) fault vein along the fault plane on which the principal slip took place during the 2008 Mw 7.9 Wenchuan, China, earthquake. It also forms injection veins in a >5-cm-wide coseismic shear zone within the Longmen Shan thrust belt. Powder X-ray diffraction data and microstructural analysis indicate that the veins contain little or no glassy material, and are composed of fine-grained fragments set in an ultrafine-grained matrix, all sourced from the country rock of interbedded siltstone and mudstone. On the basis of X-ray diffraction and mesostructural and microstructural features, it is concluded that these veins are ultracataclastic veins generated by coseismic comminution with little melting, accompanied by rapid fluidization in the coseismic shear zone during the 2008 Wenchuan earthquake. The findings support earlier suggestions that (1) ultracataclastic veins that resemble pseudotachylyte veins in appearance can be generated by crushing and rapid injection accompanied by fluidization of fine-grained material during seismic faulting, with little or no melting; and (2) ultracataclastic veins formed in this way may record seismic slip events in seismogenic fault zones, acting as “earthquake fossils” in much the same way as pseudotachylyte veins.