Cal. Unemp. Ins. Code § 1126

If any employing unit fails to make a return or report as required under this division, the director shall make an estimate based upon any information in his or her possession or that may come into his or her possession of the amount of wages paid for employment in the period or periods for which no return or report was filed and upon the basis of the estimate shall compute and assess the amounts of employer and worker contributions payable by the employing unit, adding thereto a penalty of 10 percent of the amount of contributions.