San Andreas

[Leanne]

I.

It is -- personally I'd join these two lines together, simply because "it is" isn't super powerful, whereas "fault" definitely is, and it sets up the metaphor
a fault,
a tear in the crust,
a place where plates slip
past each other and grind
their edges in unchecked fury. -- you could consider dropping "their"

It is -- whereas here, "it is" alone works simply because it's a continuation
the constant threat of quakes,
the uncertainty of life,
the remembrance of tremors past,
the fear of what is to come— I'm being pedantic, but maybe a colon would be better here?
the big one.

II.

He folds the map of California in his lap,
shoves a Marlboro in his mouth
and lights his way to freedom.

She watches him drive away,
the candy-apple red Mustang -- to make this slightly more active, you could try something like "she watches as the candy-apple red Mustang shrinks into a blip of color (damn you anti-u spellers!)
a shrinking blip of color
in the bland Central Valley scrubland.

Her shadow wavers in the hot sun,
a dust-covered caricature
with outstretched arms
and trembling fingers.

Beneath her feet, the ground
shifts without warning
and gives way.