Ross Stein and David Jacobson report that the recent M=6.2 earthquake lies 25 km east of where the 2000 M=6.2 Tottori shock (which struck on a north-oriented left-lateral strike-slip fault), and 30 km west of the 1943 M=7.2-7.4 Tottori quake (which struck on one or more east-oriented right-lateral faults).
It is likely that both the 1943 and 2000 earthquakes contributed stress that brought the intervening faults, which ruptured today, closer to failure. However, at this point it is speculation as we know too little about the fault that just slipped. A 2011 study by Shinji Toda and Bogdan Enescu attempted to account for these stresses in an earthquake forecast for all of Japan. Focusing on the Tottori Prefecture, it is intriguing to see that the site of the 21 October 2016 shock lies in a zone of elevated (red) earthquake potential shown in the maps presented here.
The Earthquake Forecast layer in Temblor, which is based on the Global Earthquake Activity Rate (GEAR) model of Bird et al. (2015), reveals that an earthquake of M~6.5 has an occurrence rate of about 1% per year, which translates to about 57% in a typical lifetime (the color band in which the quake fell is boxed in the legend in the map presented). So this quake cannot be considered unexpected, and has a rate of about once a generation. So we all need to be ready for quakes like this one, too. Fortunately, no country is more prepared than Japan, an accomplishment the rest of us should envy and admire. Thus far, the damage does not appear to be widespread, a stark contrast to the heavy toll of the 24 August 2016 M=6.2 Amatrice, Italy, quake, an earthquake of the same size and depth.
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