Regional bank stocks tumbled on Thursday after Zions Bancorp said it would write off fraudulent loans made to two borrowers, adding to investors’ fears about lending standards and stress in credit markets. Zions Bancorp (ZION) on Thursday said it had recently identified “what it believes to be apparent misrepresentations and contractual defaults” by two borrowers. As a result, it plans to write off $50 million of the $60 million outstanding on the affected loans. Shares of Zions dropped 13% on Thursday, leading regional banks lower. The KBW Regional Banking Index fell 6%. The recent bankruptcies of two companies in the...