How fast did the ground move?
Based on what we learned from the 1989 Loma Prieta earthquake, the best guess is that in places where the offset approached 20 ft, it probably took 4 to 5 seconds to move this distance.
This would imply a speed of 4 to 5 ft/sec or approximately 3 miles/hr.
Note, however, that the rupture itself probably propagated at a speed of about 2.7 km/sec or 5,800 mi/hr. (See the paper by Wald and others.)
"At Point Reyes Station at the head of Tomales bay the 5:15 train for San Francisco was just ready. The conductor had just swung himself on when the train gave a great lurch to the east, followed by another to the west, which threw the whole train on its side. The astonished conductor dropped off as it went over, and at sight of the falling chimneys and breaking windows of the station, he understood that it was the Temblor. The fireman turned to jump from the engine to the west when the return shock came. He then leaped to the east and borrowing a kodak he took the picture of the train here presented." (From "The 1906 California Earthquake", David Starr Jordan, Editor, 1907, A.M. Robertson, San Francisco.)