Aftershock patterns ... demonstrated that actual fault rupture teminated approximately 5 km below the ground surface.
Apparently, this earthquake was "unusually deep" - 11 miles deep compared to "typical California earthquake focal depths" of 4 to 6 miles, as this passage explains:

Surface Effects

Surface displacements with offsets of up to 3 or 4 feet along a zone about 20 miles long would normally be expected to accompany an earthquake of this magnitude. Instead many cracks have been found over several discontinuous and indistinct zones. There are several possible explanations for this lack of clear surface expression. The earthquake was unusually deep, making it difficult for the bedrock rupture to propagate to the ground surface. The combination of rugged topography, thick soil, and forest cover could also make surface breaks less distinct. The State Commission report of the 1906 earthquake described very similar surface rupture characteristics along the Santa Cruz Mountains portion of the San Andreas Fault.