When predicting an earthquake, there are two goals. The first is to have great accuracy
, meaning the predictions are usually correct. The second is to have great lead time
, meaning the predictions can be made far in advance of the actual event. In general, we can achieve one of these at a time, but not both together.
During an earthquake, two types of waves are produced. P-waves (P for primary) are fast-moving waves that cause little damage. S-waves (for secondary) move much slower than the p-waves, but are the ones that cause trouble for us. The difference in the velocities of these waves is a few kilometers per second, which isn't huge, but since the waves travel hundreds of kilometers, by the time they reach a major city the two waves can be separated by a minute or more. In the recent Tohaku earthquake in Japan, sensors detected the p-waves heading towards Tokyo before any damage had been done. Japan's Early Warning System
was then able to go into effect, sending text messages to residents to allow them to prepare for the earthquake. These messages were received 80 seconds before the s-waves hit Tokyo
, allowing a small amount of time to move to a safer location.
Measuring p-waves can only provide a lead time of a minute or two. Other methods are currently being studied as means to detect earthquakes earlier, but they are still in exploratory stages. In 1999, a very large earthquake decimated parts of Turkey. Data from this earthquake is still being analyzed. Recently, physicists found evidence of vibrations in seismic waves
beginning 44 minutes before the earthquake. These vibrations are believed to be caused by the two plates slipping against each other in the fault region. Of course, it's not beneficial if the earthquake can only be predicted ten years after it occurs, but if these signals can be registered and analyzed quickly, then this could provide evidence of an upcoming earthquake an hour before the event.
Some scientists believe more accurate earthquake prediction exists in radon. Radon concentrations in groundwater are believed to increase tremendously in the days before an earthquake. Since radon is radioactive with a half-life of 3.8 days, this could be detectable. This was studied in the 1980s, with inconclusive results. However, with modern technology, including high-precision space-based observatories, this method is again being considered. Currently, physicists are analyzing data from the days before the 2010 Chilean earthquake to see if radon concentrations could have been used here in order to detect the earthquake days before the event.
Hopefully, in the future one of these methods will prove viable, and earthquake predictions will be accurately made days or even weeks beforehand!
Thanks for the question,
(published on 03/13/2011)