Two members of the Sequoia String Quartet--first violinist Yoko Matsuda and cellist Robert Martin--will leave the locally based ensemble this summer, violist James Dunham said Monday.

Dunham, with the quartet since its founding at CalArts in 1972, told The Times that he and violinist Miwako Watanabe, also a founding member, would remain to keep the Sequoia alive.

The decision to split up came several months ago, and “was well thought out and carefully done,” Dunham said. While refusing to list specific reasons for the unexpected breakup, Dunham cited “the escalation of our career” as playing a major role. “We got to a juncture,” he commented, without elaboration.

The violist stressed that the split is an amicable one, noting that Martin (the group’s fourth cellist, who joined in 1974) wanted to pursue some of his “multiple interests, to explore his many talents. The chances are his next move won’t be in music.” Of Matsuda, a key figure in organizing the original quartet, Dunham said that she needed “a change of pace.”

Though Matsuda will maintain her teaching post at CalArts, Martin will resign his position there, Dunham noted. Neither Matsuda nor Martin could be reached for comment.

The future of the Sequoia Quartet, Dunham insisted, is secure: “There is confidence in the continuity.” Dunham and Watanabe are currently auditioning players, though the violist pointed out that “we are as much auditioned as they are. They have to like us too.”

The formal announcement of the new players will be made June 23. The present ensemble’s final concert will be July 10 as part of the Long Beach Summer Institute of Chamber Music.