Fault pseudotachylyte is formed during an earthquake. The coseismic slip on the fault frictionally heats the surrounding rock and forms a melt. Rapid cooling of the melt forms a glass-like, dark rock, pseudotachylyte. A combination of the dynamic stresses imparted on the wall rock during the earthquake and dynamic pressurization will allow the melt to enter fractures in the wall rock.
In the photo above pseudotachylyte is the thin, black rock. The injection is at a 90 degree angle with the main pseudotachylyte vein.