We don't allow nested Boolean operators, but instead read them in order, left to right. You can get the results you need, expanding with Boolean algebra laws:
A AND (B OR C) = (A AND B) OR (A AND C)
This way you can just use 1 query: Mexico AND earthquake OR Mexico AND quake OR Mexico AND aftershock
However, it is not true that you get 60 tweets if the individual queries were 10, 20, 30. Some of the results could have 'Mexico', 'earthquake' and 'quake' all together, in which case, they only get counted once in the composite query (and would get double counted with the separate calls).
Since our search goes not just through tweet text, but also link title/description, it is likely that a result will have 'Mexico' and 2 or all of the other 3 words appear together.
hth, -vh