Legal Question in Landlord & Tenant Law in California

That would depend on how "unknown." If it was some random act of vandalism, no; the owner has to repair that. If they were throwing a house party and one of the guests did it but nobody can say who it was, then yes, the tenant is still responsible. What it really boils down to is whether the tenant was somehow involved in the breakage happening. If so, tenant pays. If the tenant was wholly uninvolved, then the owner pays.