Earthquakes: Whose Fault Is It?

Remember in science class when you learned all about earthquakes and their causes and the differences between a strike-slip fault and a dip-slip fault and that other kind of fault? Not really? Don’t worry, we’re a bit rusty on that stuff, too. That’s why we’ve compiled a couple of really interesting resources to help jog our adult memories.

This article from National Geographic does a pretty good explaining the basics.

“The Earth’s crust is made of a jigsaw puzzle of continental and oceanic plates that are constantly ramming each other, sliding past each other, or pulling apart.” The subsequent energy created by all this seismic activity is released along fault lines–the places where the puzzle pieces don’t quite snap together–and is felt as earthquakes and tremors. Obviously, there’s a lot more to it than that. Just take a look at this map.

The type of plate activity whether crashing, slipping or pulling is dependent on the faults along which it occurs. For example, on a strike-slip fault, earthquakes occur when plates slide past each other. A well-known example of a strike-slip fault is the  notoriously unstable San Andreas fault in California that promises one day to swallow whole the city of San Francisco.

A dip-slip fault by comparison is when “the ground above the fault zone either drops (a normal fault) or is pushed up (a reverse fault).” If you’re having a hard time visualizing how either of these kinds of activities may look in IRL, here are a couple of good images:



As you can see in the roadwork in the image on the left, the ground is displaced not up or down but parallel along the fault. One side moved left, one side moved right. That’s due to strike-slip activity.

The image on the right shows normal dip-slip activity. The footwall slid upwards while bearing against the hanging wall which slid downwards. The result is about a 10 foot downward displacement of the hanging wall. In reverse, the hanging wall would have slid over the the footwall by 10 feet.

Finally, we have the oblique fault. “Faults that combine sideways with up-and-down motions are called oblique by seismologists.” In the following graph, you can see what plate activity along an oblique fault line might look like.

As one side of the rock formation moves parallel to the other, the other side moves upward. True to the definition of oblique this kind of fault moves “neither at a right angle or parallel.” It does both at the same time.











Thanks for reading our post! Hopefully, you found it informative. Obviously, there is a lot more to be said about earthquakes especially since they’ve been in the news so much lately, but if you found yourself fuzzy about the science behind them, you’re welcome!

About the author

Seattle CPA+John Huddleston has written extensively on tax related subjects of interest to small business owners. Since 2002, he has owned his own small business, Huddleston Tax CPAs. He is a graduate of Washington State University and the University of Washington School of Law.


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