To come up with an accurate view of the past, researchers need to know how far back in the historical record you can go before significant gaps first appear. Otherwise, any conclusions they reach about seismicity rates might be misleading.
A scientific report published in 1995 concluded that there had been too few large earthquakes in southern California since 1850, given the rate of slip along the plate boundary, and the amount of energy that would need to be released (as earthquakes) to accomodate that motion. Thus, this report reasoned that southern California should expect an increased rate of damaging earthquakes in the next few decades. Rates for earthquake insurance quadrupled as a result!
In 1998, another scientific paper cast doubt upon those findings. Southern California in 1850, it argued, was too sparsely settled for all damaging earthquakes to have been recorded. If a list of earthquakes were constructed from after a time that southern California had more complete coverage by sources that would record notable shaking, the newer paper argued, the apparent deficit of large earthquakes would vanish.
Is this earthquake deficit real, or only the result of an incomplete historical record? The activity below allows you to investigate this question for yourself.
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Have there been too few large earthquakes in southern California
in historic times? Piece together information to arrive at a conclusion
about the long-term seismicity rate, and determine for yourself if
there is really an "earthquake deficit". |
What did the activity above teach you about reaching conclusions regarding the long-term prospects for seismicity in southern California? Does it seem wise to use past recorded events to anticipate the future?
An Earthquake Deficit?