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I was reading a book about tectonic plate boundaries, and I came across this paragraph: enter image description here

It says that when a plate edge slides with another plate edge it causes a vertical fault, as San Andreas fault. I thought that SAF is a horizontal fault i.e. strike-slip fault. So what does the book mean by vertical faults?

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  • $\begingroup$ The fault plane is vertical. It is also a strike slip fault. $\endgroup$ – bon Jun 20 '17 at 11:43
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    $\begingroup$ @bon, you mean it speaks about the fault plane not the direction of motion of the rocks? Do you mean that if it wrote "horizontal fault" some people would interpret this to be a thrust fault? $\endgroup$ – Asmaa Jun 20 '17 at 16:11
  • $\begingroup$ The fault itself is more or less vertical; the motion is horizontal. $\endgroup$ – BillDOe Jun 21 '17 at 0:34
  • $\begingroup$ @Asmaa Yes it refers to the orientation of the fault plane. That is how most people would interpret this I think. Vertical fault, referring to the direction of motion would be ambiguous between a reverse or a normal fault so not a very useful description. $\endgroup$ – bon Jun 21 '17 at 11:29
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See the image below. In the picture, the San Andreas Fault dip is nearly 90 degrees or exactly vertical; The strike is pointing North.

Most normal faults and reverse faults (such as the Sierra Madre) are not vertical, but rather dip in either direction. (In the picture the Sierra Madre appears to have a dip of about 45 degrees).

In real life, the dip of the San Andreas is not perfectly vertical, but is variable, depending on where in California you are.

Strike and dip can be measured using a Brutton Compass.

enter image description here

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  • $\begingroup$ So.....the dip of the fault plane will be different if I'm in, say, San Diego vs. Eureka? $\endgroup$ – Knob Scratcher Jul 3 '17 at 16:29
  • $\begingroup$ Yeah, it's possible. $\endgroup$ – J.W. Powell Jul 5 '17 at 22:35

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