Oceanography and Geology of the Gulf of the Farallones 



Earthquakes, Faults, and Tectonics 



Holly F. Ryan, Stephanie L. Ross, and Russell W. Graymer 



On April 18, 1906, the San Francisco 

 Bay region was rocked by one of the 

 most significant earthquakes in history. 

 This magnitude 7.8 earthquake began 

 on a segment of the San Andreas Fault 

 that lies underwater in the Gulf of the 

 Farallones, just a few miles offshore 

 of San Francisco. The quake ruptured 

 nearly 270 miles of the fault from 

 San Juan Bautista to Cape Mendocino. 

 Damage from the intense shaking 

 during the earthquake, along with 

 the devastation from the ensuing fire, 

 wreaked havoc in San Francisco, a 

 city of 400,000 inhabitants at the time. 

 Although the official death toll from 

 the earthquake was reported to be about 

 700, it is now widely believed that the 

 actual loss of life was three to four 

 times greater. In addition, more than 50 

 percent of the population of the city 

 was homeless following the earthquake. 



Today, a large metropolitan area 

 of more than 6 million people covers 

 more than 7,000 square miles around 

 San Francisco Bay. Many small earth- 

 quakes occur in the bay region every 

 year, although only those greater than 

 about magnitude 3 are usually felt. The 

 1989 magnitude 6.9 Loma Prieta earth- 

 quake was a strong reminder of the 



potential for large, destructive earth- 

 quakes in the region. The Loma Prieta 

 earthquake killed 63 people, injured 

 more than 3,700 others, and caused 

 property damage in excess of $6 bil- 

 lion. It should be kept in mind, how- 

 ever, that devastation occurred only in 

 limited areas because the epicenter of 

 this earthquake was on a somewhat 

 remote segment of the San Andreas 

 Fault, 70 miles south of San Francisco 

 in the Santa Cruz Mountains. An earth- 

 quake of similar magnitude located 

 closer to the center of a densely popu- 

 lated urban area is capable of causing 

 much greater damage and loss of 

 life. This was shown by the 1995 

 Kobe, Japan, earthquake (magnitude 

 6.9), which caused more than 6,000 

 deaths, injured 35,000 people, resulted 

 in $100 billion in property damage, 

 and destroyed the homes of more than 

 300,000 people. 



The rigid outer shell of the Earth 

 is made up of large "tectonic plates" 

 that move horizontally relative to one 

 another. The Gulf of the Farallones 

 includes part of the boundary between 

 two of the Earth's largest tectonic 

 plates. Tectonic motion along this 

 boundary is what makes the San Fran- 



cisco Bay region so susceptible to 

 earthquakes and is a significant factor 

 in creating the geology and geomor- 

 phology of the region. The Pacific 

 and North American Plates are sliding 

 relentlessly past each other at an aver- 

 age rate of about 2 inches per year. 

 Most of this motion occurs in cat- 

 astrophic bursts of movement earth- 

 quakes along the San Andreas Fault 

 system. Near San Francisco, the San 

 Andreas Fault system is a complex 

 zone of faults about 50 miles wide. 

 It stretches from offshore to as far 

 east as the cities of Vallejo and 

 Livermore. In addition to the San 

 Andreas Fault, the numerous faults 

 that are part of the San Andreas 

 Fault system include the San Gre- 

 gorio, Hayward, Rogers Creek, and 

 Calaveras Faults. 



Much of the geomorphology (sur- 

 face features) of the San Francisco 

 Bay region is a consequence of the 

 location and motion of past and pres- 

 ently active faults within the San 

 Andreas Fault system, and of the jux- 

 taposition of blocks of different rock 

 types by movements along these faults. 

 For example, the Farallon Islands off- 

 shore and Montara Mountain located 



onshore north of Half Moon Bay 

 are parts of large fault blocks that 

 contain granitic rocks believed to be 

 originally derived from the southern 

 Sierra Nevada. At least 17 such fault- 

 bounded structural blocks (terranes) of 

 different sizes and rock types have 

 been identified in the bay region. 



To help reduce injuries and prop- 

 erty damage from future earthquakes 

 in the San Francisco Bay region, it is 

 necessary to have a good understand- 

 ing of the geology of this region. This 

 must encompass both the onshore and 

 offshore geology, including that of the 

 Gulf of the Farallones. 



12 Oceanography and Geology of the Gulf of the Farallones 



