Hi cool16, would just like to add few points to AjiteshArjun's post.

Your observation about the need to repeat the verb with the correct tense, if the tense is changing, is correct.

However, GMAT shows flexibility in this regard if the implied verb after the comparison indicator is a simple linking verb (especially is/was).

This is what's going on here:

....the planet's crust harbors up to three times as much water as (was) previously thought.

Since a simple linking verb (was) is being implied in the second part, it's fine to assume it, despite the tense change.

There are multiple other examples in OG:

Industry analysts said that the recent rise in fuel prices may be an early signal that prices of gasoline and heating oil will stay higher than usual through the end of the year.

Again, the actual sentence is:

...prices of gasoline and heating oil will stay higher than (is) usual....

Since a simple linking verb (is) is being implied in the second part, it's fine to assume it, despite the tense change.

Another example:

Heating-oil prices are expected to be higher this year than last because refiners are paving about $5 a barrel more for crude oil than they were last year.

Again, the actual sentence is:

Heating-oil prices are expected to be higher this year than (were) last year

Since a simple linking verb (was) is being implied in the second part, it's fine to assume it, despite the tense change.

On the other hand, instead of a simple linking verb, if an action verb is involved and there is a tense change, then it becomes mandatory to explicitly mention the verb. So, following would be incorrect:

Peter works harder now than one year back.

Since an action verb (work) is involved, we need to explicitly mention the verb with the correct tense. So, the correct sentence would be:

Peter works harder now than he did one year back.