Cal. Lab. Code § 4554

In case of the willful failure by an employer to secure the payment of compensation, the amount of compensation otherwise recoverable for injury or death as provided in this division shall be increased 10 percent. Failure of the employer to secure the payment of compensation as provided in Article 1 (commencing at Section 3700) of Chapter 4 of Part 1 of this division is prima facie evidence of willfulness on his part.