Hi Petra. The two epicenter locations of the 1906 earthquake will probably be with us for all times. However there is another possibility that some seismologist say that everyone is missing.

When reading newspaper accounts of the 1906 quake some people reported feeling two jolts. One jolt was felt just a couple of seconds before the “big one” giving an indication that there may have been a foreshock to the big quake and the epicenter of that foreshock was Daly City. That is pretty flimsy evidence and can’t be ruled out or in. It’s just another story about the 1906 quake.

The rupture of the fault was from Cape Mendocino in the north to San Juan Bautista in the south. The rupture south of the epicenter was about 6 feet and less. The rupture north of the epicenter was from 9 feet to 21 feet. As you well know the 21 feet was seen in the area of Sir Francis Drake Boulevard.

The thinking by some for the differences in the rupture length is that Black Mountain may have slowed the rupture down in that the fault makes a slight fog to the east to go around the mountain. This is also the reason for the current thinking that the stress in the Peninsula wasn’t completely relived and another major quake on the Peninsula is possible. This is also the area of the “Bay Area seismic gap” (Portola Valley and San Francisco).

The San Andreas fault ranks among the longest such structures in the world. Overland, it is a visible break that can be traced for 625 miles from Point Arena, north of San Francisco, to the Salton Sea, near the border with Mexico.

The inferred extensions of the fault are even more dramatic. Northward, the break is thought to extend under the sea another 400 miles, possibly merging into the Murray Fracture Zone on the deep sea floor. At the southern end, the fault is believed to follow the axis of the Gulf of California for almost 1,000 miles.

Because of the fault's great length, as much as 2,000 miles, movement is intermittent as to time, and random as to place. At any place along the fault, movement may be a one-time event, or movement may be recurrent over thousands of years. In any one episode, displacement may be a fraction of an inch or it may be many feet.

Actually, the term San Andreas fault is kind of misleading in that it suggests that the fault is a single break. It is more accurately called the San Andreas fault system, or zone, with several major and numerous minor branches, particularly in southern California.

The age of the fault varies depending on what segment of the fault you’re looking at. In Northern California it is about 30 to 40 million years old. In Southern California it is about 12 million years old and starting in the Gulf of California southward it is about 6 million years old. The progressively younger age of segments of the fault from north to south is consistent with ideas concerning plate movement along the west coast of North America. Of course with any theory there are different opinions.

Of course I can’t end here. There is some evidence that the Pilarcitos Fault may be an east-dipping thrust fault that has emplaced Franciscan rocks over Salinian granites. The Pilarcitos Fault may represent an old segment of the San Andreas Fault system that accommodated pre-Quaternary right-lateral slip, and so it is a high-angle structure.

A temporary seismograph network was deployed on the San Francisco peninsula during the first half of 1995 to use local earthquake and controlled source travel times to create a detailed upper-crustal structural model of the San Andreas and associated faults. The goal of this study was to determine which of the two possible models for the arrangement of the Salinia-Franciscan terranes on the peninsula is correct.

The answer to this question is important because an active Pilarcitos thrust fault could imply far more hazard to nearby San Francisco and peninsula cities than an extinct proto-San Andreas segment. At present time I don’t know if the study is still on going, or has ended and what the results are or were.

One last note. Did you know the San Andreas fault is named after Saint Andrew? I wonder what his thoughts are about that? Take Care…Don in creepy town

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