Discussion of Likelihood of a Large Earthquake at a Certain Location
What is the likelihood of a large earthquake at location X?
Our analyses do not answer your question as stated, "probability of large earthquake at a location," but in a more suitable manner for safety. Danger comes, not only from large earthquakes AT a location, but also large earthquakes further away, and close, smaller earthquakes. Inasmuch as smaller earthquakes are more likely to occur, some attention has to be paid to the contribution to hazard from these events, too. By focusing on GROUND SHAKING caused by earthquakes of all magnitudes and distances from the site, rather than the size of the earthquake "at" a site, we get a truer picture of the hazard.
The maps at our web site: http://eqhazmaps.usgs.gov/,
give a good impression of overall relative earthquake ground motion shaking hazard for various locations in the vicinity of the locations you are interested. You can see which locations look more or less hazardous than others.
More particularly, our maps give the probability of various levels of ground motion being exceeded in 50 years.
For instance, using the data available at our web site for Chico, California, and for San Diego, California, one can get an idea of the approximate probability of strong, damaging ground motion at each location. Inspecting peak ground acceleration maps for ground motions having 10 percent probability of being exceeded in 50 years and ground motions having 2 percent probability of being exceeded in 50 years, we would find approximately that around San Diego, the ground motion that has 10 percent probability of being exceeded in 50 years is about equal to the ground motion around Chico that has 2 percent probability of being exceeded in 50 years. That ground motion is 30 percent g. (See the data from the zip-code look-up, below.)
From one of the answers at the Frequently-Asked Questions page at our site,
<<2.What is "% g"? What is the relation to building damage? >> we find that an approximate threshold for shaking that causes damage is 10 percent g. So, 30 percent g is very strong shaking indeed and has about 1 chance in 10 of occurring in San Diego in 50 years. In 20 years this would be about 1 chance in 25. In Chico, this level of shaking has about 1 chance in 50 of occurring in 50 years or about 1 chance in 125 in 20 years. This may be a reasonable number to use for the likelihood of the occurrence of a big, nearby earthquake.
However, you know, earthquake shaking doesn't kill people; falling buildings kill people. Earthquake shaking double this level was experienced in Santa Cruz at the time of the Loma Prieta earthquake (world series earthquake) and in well-built homes no one was killed. In California, because of the level of seismic engineering, the likelihood of widespread or long-lasting disorder after an earthquake is pretty low, so maybe earthquake probability should not be a prime consideration in deciding where to move.
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If the maps are hard to read, the next best way to get ground motion probability information is to use our Java Application available for free download on our website. (There is a postal service book that gives zip codes for various towns. The Rand McNally Road Atlas has latitudes and longitudes along the edges, so that approximate locations can be found for cities.)
For a given location, this application returns a table giving the ground motions having 10, 5 and 2 percent probability of being exceeded. Using these entries, an enquirer can estimate the probability of a damaging level of ground motion occurring at the location in question.
Here is some output (truncated) for a zip code in San Diego and a zip code in Chico
The input zip-code is 92101.
ZIP CODE 92101
LOCATION 32.7211 Lat. -117.1643 Long.
DISTANCE TO NEAREST GRID POINT 4.0831 kms
NEAREST GRID POINT 32.7 Lat. -117.2 Long.
Probabilistic ground motion values, in %g, at the Nearest Grid point are:
10%PE in 50 yr 5%PE in 50 yr 2%PE in 50 yr
PGA 30.679359 43.902100 62.643879
The input zip-code is 95926.
ZIP CODE 95926
LOCATION 39.7582 Lat. -121.8513 Long.
DISTANCE TO NEAREST GRID POINT 6.2394 kms
NEAREST GRID POINT 39.8 Lat. -121.9 Long.
Probabilistic ground motion values, in %g, at the Nearest Grid point are:
10%PE in 50 yr 5%PE in 50 yr 2%PE in 50 yr
PGA 14.041360 19.045530 27.313931
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In the example, the ground motion value used, 30 percent g, was convenient for comparison of both sites. For other sites, lower or higher ground motions may be more convenient. This also means that the answer to the initial question depends on the ground motion value used. For other ground motions not only will the probability be different, the contrast in probability will be different between the same two sites.
For example, for the two cities above, it is likely that if 10 percent g were used, the probability for threshold damage level ground motions would be three and a half times larger for San Diego, and eight times larger at Chico. The contrast in hazard at this ground motion level would be only a factor of two rather than the previous factor of 5.
