Cal. Civ. Code § 3289

(a) Any legal rate of interest stipulated by a contract remains chargeable after a breach thereof, as before, until the contract is superseded by a verdict or other new obligation.
(b) If a contract entered into after January 1, 1986, does not stipulate a legal rate of interest, the obligation shall bear interest at a rate of 10 percent per annum after a breach.
For the purposes of this subdivision, the term contract shall not include a note secured by a deed of trust on real property.