| We are pleased to hear that Iditarod racer Melanie Gould has been found after 12 days and is physically unhurt. However, what we actually had in mind were missing earthquakes. Normally, we don't publish earthquakes below 5.0 Richter. Sometimes high 4s are useful for predictions of future seismic events, and a swarm (hundreds, perhaps thousands) of Richter 1s and 2s almost always signals moving magma and a looming volcanic eruption. In the last forty years, and certainly in the last four years, earthquake reporting has improved dramatically. As a rule of thumb, when we look at older data we are expecting an order of magnitude reduction as we increase a the lower limit by a full Richter. If there were 2000 Richter 5.0 or greater events in an area we expect to see 200 Richter 6.0 or greater; 20 Richter 7.0 or greater and 2 Richter 8.0 or greater events. We generally get interested when there are more 7s and 8s than we would expect. By symmetry we also take a longer look if there are too few powerful earthquakes. Sometimes, this is due to topology. Other times, the record-keeping (ours or various reporting agencies) is deficient. A third possibility is that there simply aren't as many large earthquakes because stress is building up. This can be a very dangerous situation. There was a Richter 7.2 on June 24, 2011 at 3:09 UTC near the Fox Islands (a portion of the Aleutians) in Alaska. Alaska has had some gigantic earthquakes [March 27,1964 was a 9.2] and Richter 7.2s are not normally tsunamiagenic, so one might glance at the event and pay no further attention. Well, it certainly is good that no one was hurt and there was apparently no damage. It is even better than some seismic stress was released. However, we are obliged to point out that on March 9, 2011 at 2:25 UTC there was a Richter 7.3 near eastern Honshu in Japan. It was, in retrospect, a foreshock for the deadly 9.0 51 hours later. This was unusual. |
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