using article "the" before three scientific methods

Solution 1:

I would suggest saying :

We analyzed aftershocks according to Gardner (1974) according to Knopo (1986) ... etc.

'According to' is academic speak for 'according to the methods of'. Or, at least, it was when I was involved in laboratory methodology.

The 'according to' needs to be repeated, otherwise 'according to Gardner (1974), Knopo (1986) etc' becomes ambiguous, in that Gardner/Knopo could refer to a single method reported by multiple authors.

Multiple authors of a single method, that is to say a single paper written by multiple authors, would usually be reported as 'according to Smith et al (2021)', et al being short for et alia meaning 'and others'.

There are also other ways of stating this information.