It is still shapeable, even if the area of effect isn't defined in cubes. Cubes are only sometimes used, to make it more convenient, but spells without areas defined by cubes are still shapeable if they're marked as shapeable:
(S) Shapeable: If an area or effect entry ends with "(S)," you can shape the spell. A shaped effect or area can have no dimension smaller than 10 feet. Many effects or areas are given as cubes to make it easy to model irregular shapes.
“Many,” but not all! Earthquake is one of those exceptions.
An area defined as a (spherical) spread gives the volume available to shape. You just have to use geometry skills and equations to figure out the volume you have available to work with, and the size of the legal reshapings you could choose. (Or leverage WolframAlpha, I guess!)
In practical terms, this means that, among other shapes, you could make a long, narrow earthquake (a cylinder of diameter 10 feet, up to 27,307 feet long or [spell range] distance from you, whichever is closer) rip open a crack in the ground and do other earthquake-y things in that narrow tube.
Wait! Why so big? Because shapeable does not limit you to the original dimensions of the area of effect — shapeable lets you rearrange its cubic feet however you like. The printed area of effect is what you start with, and provides a volume limit — it doesn't limit you to only play “inside” that original shape.
However, despite that lack of dimension limit from the area of effect line, there still is an upper limit on how far away you can make any part of your modified area of effect though:
[…] A spell's range is the maximum distance from you that the spell's effect can occur, as well as the maximum distance at which you can designate the spell's point of origin. If any portion of the spell's area would extend beyond this range, that area is wasted.
So although you can make a 10-foot diameter tube of earthquake goodness that is theoretically 27,307 feet long, in practice you're limited by the 400 feet + 40 feet/level range of earthquake, so even a 20th-level druid could only make that 10-foot diameter tube a maximum of 1200 feet long if it was centered on herself.
It means that you can reshape the volume of the spell to any contiguous volume, with a minimum dimension of 10', so long as the volume remains inside the range of the spell. You can even put holes in it to exempt yourself and your allies.
... can have no dimension smaller than 10 feet. Many effects or areas are given as cubes to make it easy to model irregular shapes. Three-dimensional volumes are most often needed to define aerial or underwater effects and areas.
So for an 80' spread the volume could be broken up into 10' cubes for easy shaping, making \$((4/3)\pi\times 80^3)/1000\$ of them, or about 2144 10' cubes. (That's a lot!) The range is \$ 400'+40'\times CL\$, presuming 15th level, that's 1000', and 1200' at 20th level. Just keep them contiguous.
Remember that the range is centered on the caster, so that 1000 to 1200' range can be a total length of up to 2400', if you extend it in opposing directions from the caster's position. It could be 80' wide at one end, and 90' for almost the entire length.
This spell can cover a square area 460' on a side, with a few left over, if shaped in such a square.
If you have more cubes than you need, you can either be 'evil' and use them anyway, or be 'good' and push the excess into the air.
Regarding the oddity of the area effect line, this is an instance that proves that the 'many effects or areas are given as cubes' really means 'many' and not 'all', since this effect is obviously not given as cubes.