This section of the San Andreas Fault Zone is best known for the 1906 San Francisco earthquake. This magnitude 7.9 earthquake ruptured 430 km of the fault, from northwest of San Juan Bautista to Cape Mendocino. The Bolinas earthquake with a moment magnitude of 4.7 is ~60,000 times smaller than the 1906 event.
This section of the San Andreas fault has almost been devoid of earthquakes since the 1906 earthquake. Very large displacements were observed on the segment of the San Andreas extending north from the Golden Gate (5.1 m on average) during 1906. This pattern of "aseismic" activity following a major earthquake is commonly observed on many active faults. It likely reflects the high frictional properties in the fault zone, as opposed to weaker faults that exhibit combined creep and microearthquake activity. Ultimately, the accumulation of strain from the relative plate motions is relieved in major earthquakes. In contrast, other segments of the San Andreas fault involved in the 1906 earthquake do show more seismic activity (see the historical seismicity plot), such as the 1957 Daly City earthquake on the Peninsula segment and the 1989 Loma Prieta earthquake on the Santa Cruz Mtns. segment.
Most earthquakes in the San Andreas Fault Zone show strike-slip motion and this is the dominate style of motion along the plate boundary (see, for example, the Annual Moment Tensor Maps). However, this event shows primarily reverse motion and is likely to have occurred on a reverse fault that is subparallel to the main trace of the San Andreas. This type of faulting reflects the fact that the San Andreas Fault Zone is not perfectly parallel to the plate boundary and that some compression is the result. This compression expresses itself, in part, through the uplift of the Coast Range mountains. The Daly City earthquake had a very similar mechanism to the Bolinas event, and the Loma Prieta earthquake was a mixture of strike-slip and reverse-slip motion. For more information on the different types of earthquakes, we recommend these fault motion animations.
Seismologists and geologists are studying Bay Area faults in order to assess the seismic hazard. In 1990, the North Coast segment (extending north from the Golden Gate to Cape Mendocino) was given a 2% probability of experiencing a magnitude 8 earthquake over 30 years. A new study of Bay Area faults is under way (a preliminary report was issued in 1996) and should be completed this year.
The San Andreas Fault System is similar in some ways to the fault system that generated the 08/18/99 M7.4 Turkey earthquake. The North Anatolian Fault Zone is also a strike-slip boundary. However, this fault fails much more frequently (in this century) in M>6.7 earthquakes than has the San Andreas. The North Anatolian Fault Zone has become a classic example of how some faults fail in a progressive manner. It should be noted that the close temporal correspondence between the Bolinas and Turkey earthquakes is only a coincidence! There are over 700 magnitude 5 earthquakes every year worldwide. Calculations show that the effects of stress transfer from the Turkish event are limited to 100 km or so.
For additional information, please see our Suggested Reading List.. In particular, the USGS Professional Paper 1515 is highly recommended. John Martin and Associates have created an online version of this book. Another online resource on the San Andreas is The San Andreas Fault - a USGS General Interest Publication.
