Analysis for the line chart of epicenter(Depth)

As showed ""Analysis for the line chart of epicenter(Lat/Lng)", the trend of 0km(1km) data of epicenters is very important. The trend graph in Sanriku-Area(1919-2015) is showed as below,

There is the longest silent term of 0km(1km) epicenter in the black circle.The graph area without 0km(1km) epicenter looks like blank area towards deeper epicenters.The next graph shows MD trend. MD is defined as the product between the magnitude and the depth of each epicenter. closed up the blank area is as below,

To show the blank area of  0km(1km) in details is as below, 

MD range is limited to close up the blank area.2 red lines are envelope lines to be connected of local minimum values in the blank area. It seems they may be useful for the prediction of Great East Japan Earthquake(M9.0).

* All data was downloaded from the site of Japan Meteorological Agency. I have arranged the data into graphs.Please see this page.