Science Nation, the National Science Foundation’s online magazine, presents an excellent, short video on drilling into an active fault zone. Researchers from Penn State University examined rock recovered from the Parkfield, California, segment of the San Andreas Fault. What they discovered was incompetent clay-rich material, which explains the aseismic nature of deformation there.
Other fault segments host more competent rock that seize up until increasing stress causes the rock to fail brittlely. Earthquake!