California Code, Code of Civil Procedure - CCP § 1297.73

(a) An arbitration agreement is in writing if its content is recorded in any form, including, but not limited to, in a document signed by the parties or in an exchange of letters, telex, telegrams, electronic mail, or other means of telecommunication accessible for subsequent reference that provides a record of the agreement.

(b) The requirement that an arbitration agreement be in writing is met by an electronic communication if the information contained therein is accessible so as to be usable for subsequent reference.

(c) An arbitration agreement is in writing if it is contained in an exchange of statements of claim and defense in which the existence of an agreement is alleged by one party and not denied by the other.

(d) The reference in a contract to any document containing an arbitration clause constitutes an arbitration agreement in writing, provided that the reference is such as to make that clause part of the contract.