A Question to plate tectonics

 Here is a question for plate tectonic.

While explaining the major Earthquakes ,it is satiated that , when the stresses are built up over two plate boundaries, the pales move with respect to one another and Earthquake occurs .

The time required to built up this much stresses over plate boundaries could be hundred of years .

 (A) Now here are the number of major quakes at PNG from 1st January 2000 till today.( Area selected is -9.846 to +1.706 Latitude and 120.023 to 153.105 Longitudes)

1) 6+ quakes ....297

2) 7+ quakes.... 32

3) 8+ quakes....1

So ,now question is ...

In a span of 24 years ,how come the stresses built up so frequently ,to produce 32 quakes of magnitude 7+

( B) Similarly at Japan region ( 26.385 to 54.542 latitudes , 126.914 to 155.742 Longitudes) . The number of quakes during 1st January 2000 till today are 

1) 6+ quakes ..  354

2) 7+ quakes  ..38

3)8+ quakes......4

4) 9+ quakes ....1

So how the number of major quakes and stresses built up slowly can be expalined