East Coast vs West Coast Earthquakes: The Good and the Bad

kmayeda's picture

In the past several years, damaging earthquakes appear to be on the rise and it seems that every few months a big event happens (Sumatra, Samoa, Haiti, Chile, Italy etc).  It turns out that globally, large earthquake rates have not changed, but unfortunately the past several earthquakes have occurred in populated areas, some of which have poor building practices.  This only brings home the need to better understand our risk in North America.  Though the west coast of the United States has significantly higher seismicity rates and risk, there are important distinctions between the other side of the U.S.  In the east coast, earthquakes are not nearly as common (that's a good thing!) and usually are smaller.  However, the crust in the eastern United States is markedly more efficient at propagating seismic waves than in the western United States (it's older and colder).  This means that for the same magnitude earthquake, the one in the eastern U.S. will be felt much more widely than one in California.  For example, a magnitude 5 in San Jose, CA would be moderately felt in San Francisco (about ..., 45 miles away) with almost no major damage to structures.  If you were in an office building sitting down, you'd probably feel a bit of swaying, however if you were walking about, you may not even feel it.  In contrast, a magnitude 5 in the east coast could be felt for several hundred miles.   In addition to efficient wave propagation, the earthquakes in the east coast tend to radiate much more energy at higher frequencies than those in the west coast (we call this "stress-drop" in earthquake parlance).  What this means for those in the east coast is that smaller structures (i.e., homes and small buildings) could experience about a factor of 5 more shaking.  The final distinction between the two coasts is of course building practice and codes.  In the west coast, all modern structures are designed to withstand strong lateral shaking and are flexible (to help dissipate the wave energy) and for older structures they are often retrofitted.   East coast homes, especially the older ones, are usually stiffer and could likely be made of masonry which will not fare well during an earthquake, even if the earthquake is not that large.

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