Chris;

> Roger, I think I know what your columns are but am not sure: the first columb is degree of latitude: but is the box to the north or south? (is 37 from 37 to 38, or 36.5 to 37.5, or 36 to 37, or what?

It's the integer value of latitude and since these are North lats it's the box to the north.

> the second is total number of M3 quakes in that degree band.

3.0 to 9.9.

> The 3rd is total number of M3+ quakes within 20 days of a M5.5 located south of Lat 33?

Yes.

> If this is correct, the distribution is very odd. Part of this, or perhaps all of it, would be incomplete catalogues for M3 quakes since 1900, and very incomplete catalogues for Mexico.

Yes, necessarily so.

> What do you think you have and what do you think it means? Less concise,
please.

What you have here is what there is. I expected the 2 columns to have similar distributions, reflecting that the number of quakes was a function of where quakes actually happen. But that doesn't seem to be true. It seems that some locations are having more than expected while others have less.

Why this happens is pure speculation without more information.

Roger