The spike corresponds to about 15min delay between the quake at 9:31 UTC. But some kind of spike visible in all stations at that time, even very far away ones, like Mendocino bay.

Honestly, this looks to me rather like some bad data due to the buoys switching mode because of radio commanding 15min after the quake rather than any wave arriving. (Like the other poster suggested)

Even if the shock would travel through an Earth made out of steel, it wouldn't have reached Mendocino bay that fast.

EDIT: There is actually a time shift and it seems to be enough, as wiredfool pointed out. Thanks!