In the late afternoon of October 17, 1989 the San Andreas fault ruptured
in its first major earthquake since the great San Francisco earthquake of
1906. Centered along a remote segment of the fault in the southern Santa
Cruz Mountains, the Loma Prieta earthquake reruptured the southernmost
40km of the 1906 fault break. Click on the blue map to the right to see
the relation of the Loma Prieta fault to the Bay Area, and the San
Andreas Fault. The San Andreas fault is the boundry between the North
American and Pacific
plates. As the Pacific plate slides north along the San Andreas relative to
the North American plate, earthquakes are generated along the San
Andreas. Displacement along the San Andreas is termed right lateral
because the
plates appear to be moving to the right relative to each other as can be
seen on the map.
The earthquake broke the San Andreas fault where it makes a conspicuous leftward bend, connecting more or less straight segments to the north and south. Movement during the earthquake involved comparable amounts of right-lateral strike slip and reverse slip. The occurrence of reverse slip means that the two sides of the fault were not only moving to the right relative to each other, but they were also being compressed up and over each other. Whereas the average strike slip displacement during the quake was approximately 1.6 meters (about 6.2 feet), the reverse slip displacement (verticle displacement) averaged 1.2 meters (about 4.3 feet). The reverse slip is due to the leftward bend of the fault. The bend is called a restraining bend, and its presence impeeds the right lateral northerly movement of the Pacific plate. During the earthquake, the Pacific plate tried to overcome the restraining bend by thrusting itself over the North American Plate. As a result of the thrusting, the Santa Cruz Mountains grew about 14 inches in the area of the quake. This kind of thrusting has occured for millions of years. In fact, repetition of this kind of thrusting over several million years has formed the Santa Cruz Mountains.
The earthquake hypocenter occurred
at a depth of 18km below the surface. One of the surprises of the
earthquake was the discovery that the San Andreas has a dip of 70 degrees to
the southwest. The locations of the main
shock and aftershocks can be seen by double clicking on the image to the
left. The view is looking to the north-northwest along the fault. The blue
cross indicates the main shock, and the green lines are topographic
features. You should be able to make out the San Francisco Bay in the
upper right, and the Monterey Bay to the left.
Notice how you can make out
the strike and dip of the fault. Notice also the small cluster of
aftershocks to the left of the San Adreas. This is thought to represent
conjugate faulting along the Zayante fault.
The image to the right shows the same image as above, but this time the
view is looking towards the fault plane. The Monterey Bay is in the
foreground.
