It isn't redwood but Sequoiadendron giganteum and it likes more dryer and colder places than the redwood (Sequoia semperviens).
It grows hight in the mountains, where the winds had already lost the main part of their rain (over the redwood land, lower in the valleys).
Its root swell looks really odd. It's probably a nursery tree grown under bad practices, like many nurseries do for town people. Easy work and quick earn money, but prone to give a root system in a very bad shape.
You can dig a little around the stump to look at the buttress roots. Not too deep, but enough to see well all the collar.
For me this tree is already lost, sick or not.
I think that the better is to replace it with a well grown and healthy tree.