Security Pacific is buying into Westamerica.

The big Los Angeles bank said it had obtained an option to purchase up to 9.9% of the common stock of Westamerica Bancorporation, a bank holding company headquartered in San Rafael. Security Pacific has an option on the 249,010 shares owned by Alpha Capital Co., an Oakland investment firm, at an exercise price of $47 a share. Westamerica, parent of Westamerica Bank, has assets of $1.06 billion and about 40 branches in Northern California.