Analysis for the curve chart of distribution of epicenter(Depth)

 To know a distribution of discrete values, histogram is usually used. There is Frequent tendency of the occurrence of Earthquakes in specific depth according with the local area. For such characteristics of discrete variable of the depth, the curve chart of depth class is used. 

The curve chart of the depth distribution in Iwate-oki Area including of Great East Japan Earthquake as below,

As the  deeper area's frequency is low, it seems to be redundant. So, double logarithmic graph is used as below,

This pattern seems to be ultimate one because of M9 huge earthquake. No need to say, not all curve have the peak of the curve in the shallow depth. This curve seems to be one of the reason why I believe earthquakes come from deeper epicenters. As 0km can't be used for logarithmic scale, 1km is used as the class value instead of it.

The double logarithm graph is Earthquake frequency per depth Chart in Niizima&Kozu island Area as below,

If the data of "Details unknown" with no magnitude is regarded as 1 km depth, the curve can become the similar line of blue as "in Sanriku-oki". So, I believed data of "Details unknown" as 1km depth.It seems to be too shallow to de fixed the depth, or to be slow slip epicenters. Anyway, in this site,  "Details unknown" is regarded as 1km depth data.