123W 



122 



Seismogratn from 1989 Loma Prieta eartliquake 



The photograph above, taken from a 

 "captive airship" 5 weeks after the great 

 earthquake of April 18, 1906, shows the 

 devastation wrought on the city of San 

 Francisco by the quake and subsequent 

 fire. In the city and surrounding region, 

 the official death toll was about 700, but 

 it is now believed that the actual loss of 

 life was three to four times greater. At 

 the time, property losses were estimated 

 to be more than $400 million. If a similar 

 earthquake occurred in northern Califor- 

 nia today, after many decades of rapid 

 urban growth, many thousands of people 

 might be killed, and economic losses 

 could be in the hundreds of billions of 

 dollars.The photograph at left shows a 

 fence near Bolinas, about 20 miles south- 

 east of Point Reyes, that was offset by 

 ground movement along the San Andreas 

 Fault in the 1906 quake. 



38N 



EXPLANATION 



FAULTS 

 MAJOR FAULTS 



The San Francisco Bay region lies on the boundary 

 zone between two of the major tectonic plates that 

 make up the Earth's outer shell. The continuous 

 motion between the Pacific and North American 

 Plates, distributed across this zone, is monitored by 

 geophysicists using the satellite-based Global Posi- 

 tioning System (GPS). Arrows on this map depict 

 recent (mid to late 1990's| rates of movement, rela- 

 tive to the interior of the North American Plate, of 

 reference markers anchored in rock or deep in solid ground. This relentless motion of the 

 plates strains the crustal rocks of the bay region, storing energy that eventually will be 

 released in earthquakes. During the time represented in this diagram, most of the faults in 

 the bay region have been "locked," not producing earthquakes. 



5 centimeters/year 



Earthquakes, Faults, and Tectonics 13 



