Associate Professor Yokose has been analyzing seismic patterns in the area since quality data became available in the 2000s. He has identified two key phases of activity.
The first lasts about five days, with a rapid increase in quake frequency. That's often followed by a second phase lasting about two weeks, when the number of tremors drops but the risk of a larger, more powerful earthquake rises.
In fact, four years ago, a magnitude 6.1 earthquake struck on day six of such a sequence, reaching an intensity of 5+ on Japan's seismic scale.
Yokose says the current situation appears to be the first phase. And though it may not follow the same pattern as before, there is a chance of an earthquake up to magnitude 6. He says residents should be on high alert for the next week or so.