LOS ANGELES (AP) _ As California's regional interstate banking law goes into effect, the pace of consolidation between lenders in the West and Southwest is expected to pick up steam, industry officials say.

That consolidation is expected to continue for 3 1/2 years when California will begin permitting nationwide interstate banking, giving the financial behemoths of New York an opportunity to tap California's rich deposit base.

''We are doing everything we can now to make sure we are one of the survivors,'' said an official of one California bank, who asked not to be identified.

California's Legislature, in a compromise, last year agreed to permit regional interstate banking beginning Wednesday. But it deferred until Jan. 1, 1991, the opening of California's borders nationwide.

Already, many of California's major banks and thrifts have agreed to buy institutions in the West and Southwest, and Arizona has received particular attention by lenders as a region where growth is expected in the future.

But the biggest deals were farther afield. Security Pacific Corp. of Los Angeles agreed to acquire Seattle-based Rainier Bancorp for $1.1 billion, and Los Angeles-based First Interstate Bancorp agreed to gobble up Allied Bancshares of Houston for between $415 and $450 million.

The states covered by the California's regional banking law are Alaska, Arizona, Hawaii, Idaho, Nevada, New Mexico, Oregon, Texas, Utah and Washington.

Besides going into Washington, Security Pacific, the nation's sixth-largest bank holding concern, has spread into Oregon and Arizona. First Interstate, the nation's ninth-largest banking company, is in New Mexico, and Union Bank bought an institution in Arizona.

Among savings and loan companies, H.F. Ahmanson, Great Western, CalFed and First Nationwide have acquired foothoolds in Arizona, while Gibraltar and Great American First Savings are expanding in Washington.

''I don't see any immediate flurry of new mergers because of the July 1 date,'' says Nancy Sheppard of the California Bankers Association in San Francisco. ''But I expect there will be some by 1991.''

That's when the nationwide interstate banking law takes effect.

By moving to expand now, lenders are seeking to make themselves less vulnerable to being acquired themselves.

Two big New York-based banking concerns - Citicorp and Chase Manhattan Corp. - have acquired financial institions in Arizona.

But they are prohibited by California law from using those institutions to in turn acquire California banks or thrifts - a process known in the industry as ''leapfrogging.''

''But it's not that long until 1991, and it's pretty well agreed that Citicorp is probably going to take over one of the Big Four,'' said one high- level bank official. ''It's just a question of which one.''

The four to which he was referring are BankAmerica Corp. and Wells Fargo & Co., both based in San Francisco, and Security Pacific and First Interstate.

Most of the mergers are expected to involve the majors and their smaller and weaker competitors. But some observers suggest there may even be a merger between the majors to create an institution more able to challenge Citicorp.

Last year, Wells Fargo took over Crocker National Corp. for $1.07 billion, to create a stronger regional institution. There have been two or three smaller mergers.

Earlier this year, First Interstate abandoned a $3.2 billion bid to take over beleaguered BankAmerica, the nation's second-largest holding company after Citicorp.

''It's too bad,'' one First Interstate official said after the takeover attempt collapsed. ''That probably was the last chance anyone was going to have a create a true world-class bank based on the West Coast.''