Normal noise vs. unreasonable behavior
This is a common and frustrating situation, and the answer depends on whether the noise is unreasonable behavior or just the normal sound of shared living in a building with thin floors.
Everyday sounds — walking, moving chairs, children playing at reasonable hours — are usually considered a normal part of apartment life, even when floors are thin. Landlords generally are not required to soundproof a building or eliminate ordinary living noise, and thin construction by itself is usually not a violation.
The picture changes if the upstairs neighbor is doing something unreasonable — like stomping, loud noise at all hours, or clearly disruptive behavior — especially if they are another of the landlord's tenants breaking lease rules. Then reporting it in writing and documenting the pattern can give the landlord grounds to act.
So it helps to honestly separate "the building is just noisy" from "this neighbor is being unreasonable." If it is the latter and it is serious and ongoing, a free, confidential review can help you understand whether your landlord has a duty to step in.
This is general information about California tenant rights, not legal advice. Every situation is different.