Lecture #20: Earthquakes on Strike-Slip Faults
(Abbott, pp. 71-73, 84-86, 132-133, 138-160)
Note: Know the names and years
of the three major California earthquakes mentioned
in this lecture.
Also, review Lectures 6-10 and make sure that you understand the
various concepts associated with faults and earthquakes, such as focus, epicenter, the
different types of faults, P waves, S waves, etc.
San Andreas Fault, California
As mentioned earlier, the
San Andreas Fault
is a 1,200 kilometer long
located on a boundary between the
(p. 139; Figure 3.3, p. 52; Figure 6.10, p. 139; Figures 6.11 and 6.12, p. 140;
Figure 6.13, p. 141).
Although the San Andreas and other strike-slip faults appear very
sharp and linear (Figure 6.13, p. 141), they actually consist of very complex zones of
fractured and interlocking rocks that extend to depths of several kilometers (p. 84-85).
During earthquakes, the rocks fracture, slide and grind past each other (
The San Andreas also has numerous branching and subparallel faults
(Figure 6.10, p. 139; Figure 6.36, p. 156).
The San Andreas Fault passes through the Gulf of California and northward into western
California. The fault eventually moves off the northern California coast near San
Francisco and merges with the right-lateral Mendocino transform fault, which forms the
southern border of the oceanic Gorda Plate (Figure 5.12, p. 120).
The Gorda Plate is
located to the south of the Juan de Fuca Plate (Figure 9.5, p. 220).
section of the Pacific Plate is located on the western side of the San Andreas Fault and
includes Baja California and much of western California.
The "continental" sections of
western California also include oceanic crust that has been uplfited above sea level and
combined with the continental materials. Western California and Baja California are
If this northward movement continues, San Francisco will reach
Alaska several tens of millions of years from now (p. 133).
As shown in Figure 6.1 (p. 132), the San Andreas Fault did not exist 30 million years
A subduction zone extended along the entire western coast of North America.
Juan de Fuca
, Explorer, Gorda, and
plates were all part of the oceanic
(Figure 6.1, p. 132).
The San Andreas formed about 28 million years ago once the
spreading zone between the Pacific and Farallon plates subducted (p. 132; Figure 6.1, p.
San Andreas Fault of Southern California: