Plan for THREE M6.0 - M6.9 Fukushima Aftershocks between June 18 and July 1

The Japanese Meteorological Agency issues aftershock outlook predictions for major earthquakes. Unfortunately it does not appear that they have publicly issued such outlooks for the Tohoku Earthquake (Fukushima), and that raises the question, how does one schedule and plan rebuilding, recovery, or preparedness operations without a milestone date to meet?

Fortunately the aftershock planning question can be answered because aftershocks tend to follow a pattern that is predictable enough for the purposes for civil defense operations. The analysis below builds off of an earlier analysis and will give a good indication why it would be wise to plan for three magnitude 6 range aftershocks by July 1, 2011; and why if those aftershocks don't occur there is an expedited potential of a magnitude 7.0 aftershock.

The following chart plots the daily cumulative number of actual aftershocks minus the number of expected aftershocks. Given a long enough historical view of the data, the values would distribute around the zero line. For scheduling purposes the assumption is that any short period non-randomness is indicative of the earthquake fault line structure. From that assumption, it follows that positive numbers indicate that earthquake stresses are decreasing, and negative numbers indicate a growth in earthquake stresses. As a result, the greater the trend towards negative values the more likely the aftershock is to happen in the near future.

Examining the chart below, as of 6/17/11, there has been a deficit of almost three M6.0-M6.9 aftershocks. The history also shows no previous deficit of greater than four M6 aftershocks. Based on those boundary condition, several M6+ aftershocks would be expected before July 1, 2011. Additionally, the trending history shows that a M6 aftershock occurs whenever the downward slope (stress build up) of the M5 line crosses the M6 line. That cross over point is trending to happen on June 19, 2011.

While not predicting that a series of three M6.0-M6.9 aftershocks will happen by 7/1/11, that is the scheduling window one would follow for the purposes of recovery operations. If those M6 quakes did not occur by that time period, the schedule would have to be reassessed for an increased risk of a nearer term M7.0-M7.9 aftershock occurring before December 7, 2011.