Query concerning Napier Earthquakes 1931 1932

[JennyLeez]

I dont understand the below and hopefully one of you may have the answer.
On the 3rd Feb 1931 an earthquake 7.8 tore a part Napier and became known as the 1931 Napier Earthquake.
(New Zealand’s deadliest earthquake which devastated the cities of Napier and Hastings. At least 256 people died in the magnitude 7.8 earthquake – 161 in Napier, 93 in Hastings, and 2 in Wairoa. Many thousands more required medical treatment.)

On the 13th of Feb another rocked Napier of 7.3 and was classed as an aftershock.

On the 5th of May 1932 some 15 months later another quake rocked Napier 5.9 and again this was considered an aftershock.
Why it was 15 months later?

Then to confuse me further. 19 months from the 1931 Napier earthquake a 6.9 hit Wairoa some 100 kms north.
This also is documented as being an aftershock of the 1931 Napier earthquake.
Why?
Why an aftershock. Why not an earthquake within its own right.

Thanks in advance

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