This exercise involves determining rates and directions of plate motion at trans

Question

This exercise involves determining rates and directions of plate motion at transform plate boundaries, employing information obtained from use of the NUVEL-1 Plate Velocity calculator (http://www.ldeo.columbia.edu/~menke/plates2.html for two location along the San Andreas fault. The first aim is to determine the rate (in mm/yr) and direction (azimuth) of plate motion for the following locations:

N. California: Motion of the Pacific plate near San Francisco, CA (Plate 1 = N. American, Plate 2 = Pacific) at 37°N, 122°W.

S. California: Motion of the Pacific plate near Los Angeles, CA (Plate 1 = N. American, Plate 2 = Pacific) at 34°N, 118°W.

1. How uniform are the rates and direction of transform motion for the Pacific plate along the San Andreas fault? Are the rates and direction of transform motion of the Pacific plate in these two locations (N. California & S. California) similar to the rates and direction of seafloor spreading determined in Part A of this exercise for the Gulf of California?

2. The present distance between Los Angeles and San Francisco is about 480 km. When (approximately) in the future (millions of years) will Los Angeles become a suburb of San Francisco (and vice versa)?

Explanation / Answer

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1. San Andreas fault forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). The fault divides into three segments, each with different characteristics and a different degree of earthquake risk. The slip rate along the fault ranges from 20 to 35 mm (0.79 to 1.38 in)/yr. The Pacific Plate, to the west of the fault, is moving in a northwest direction while the North American Plate to the east is moving toward the southwest, but relatively southeast under the influence of plate tectonics. The rate of slippage averages about 33 to 37 millimeters (1.3 to 1.5 in) a year across California.

2. San Andreas fault, the deep gash beneath California that spawned the devastating San Francisco earthquake of 1906 — one among many — and today also threatens the Los Angeles area. The fault marks the seam where the Pacific plate slides relentlessly northward past the North American plate.

Ten million years from now, Los Angeles will be abreast of San Francisco. And in another 50 million years, Los Angeles will have moved up the west coast into Alaskan waters.

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