re: #198 jaunte

West coast earthquakes don’t propagate as far for a couple of reasons. First is that they tend to be relatively shallow, less than 15 miles so basement rock doesn’t get as disturbed as for deep quakes. The exception is for quakes on the Cascadia subduction zone off of Oregon and Washington. The second reason is the rocks on the west coast have been sutured on to the North America continent as island arcs have moved across the Pacific and finally met up with the continent. As a result, these rocks are more broken up due to the number of faults that have appeared where these island arcs have subducted and had material scraped off to form land. Seismic waves don’t travel as far through that kind of rock.

For a fascinating description of this process, have a look at John McPhee’s book, Assembling California. It’s the last in a series of 4 books on the geology of the United States.