A C152 ASSIGNED 3500 FT PASSING NEAR SNA ARPT OBSERVED A B737 DEPART SNA AND TURN TOWARD HIS POS. THE TWR INSTRUCTED THE B737 TO MAINTAIN 3000 FT FOR SEPARATION; BUT THE C152 PLT; APPARENTLY UNAWARE ATC WAS PROVIDING SEPARATION; CONTINUED DSCNT BELOW 3500 FT IN AN ATTEMPT TO AVOID THE B737. THE PLTS ON THE B737 WERE REQUIRED TO TAKE EVASIVE ACTION ON A TCASII RA AND A VISUAL ON THE C152.

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FO WAS PF. DURING INITIAL CLBOUT FROM SNA ON THE MUSEL 5 SID; WITH THE APPROVED NOISE ABATEMENT PROC; SNA TWR INSTRUCTED US TO LEVEL AT 3000 FT; INSTEAD OF THE CLRED 5000 FT; DUE TO TFC AT 3500 FT. AFTER I ACKNOWLEDGED THE NEW ALT TWR HANDED US OFF TO SOCAL. ALL OF THIS OCCURRED DURING THE PWR CUTBACK AND TURNS REQUIRED BY THE NOISE ABATEMENT PROC. SOCAL POINTED OUT TFC AT 1 O'CLOCK POS; LEVEL AT 3500 FT. WE LEVELED OUT AT 3000 FT WITHOUT VISUAL CONTACT AND THE TCASII CALLING 'TFC.' (THE TIME FRAME FROM THE TCASII ALERT TO THE EVASIVE ACTION WAS ABOUT 10 SECONDS.) SOCAL GAVE US A VECTOR TURN TO THE R FOR SXC. THE FO BEGAN A R TURN AS I REACHED UP TO THE MCP TO MOVE THE HEADING SELECTOR TO THE NEW HEADING. AT THIS POINT; BOTH OF US SAW A C152 AT 12 O'CLOCK POS AT APPROX 1/4 MI AT OUR ALT. THE TCASII CALLED 'DSND.' THE FO BEGAN A DSCNT AND AN EVASIVE TURN TO THE R IN ORDER TO PASS BEHIND THE CESSNA. WE PASSED THE CESSNA AT WHAT APPEARED TO BE SLIGHTLY LESS THAN 1/4 MI AND ABOUT 300 FT BELOW. THE PLT OF THE CESSNA STATED TO SOCAL CTLR THAT HE HAD BECOME DISTRACTED AND HAD INADVERTENTLY DSNDED. SUPPLEMENTAL INFO FROM ACN 403754: I WAS ON RADAR WITH SOCAL APCH AT THE TIME OF THE INCIDENT. I HAD RECEIVED PERMISSION TO DSND TO 3500 FT FROM 5500 FT MSL AT THE LONG BEACH SHORELINE; HEADING TO PALOMAR ARPT. AS I WAS FLYING BY JOHN WAYNE ARPT AT 3500 FT I NOTICED THAT A COMMERCIAL AIRLINER HAD JUST TAKEN OFF AND WE APPEARED TO BE ON A COLLISION COURSE. SOCAL APCH DID NOT RADIO ME THAT A JET AIRLINER WAS APCHING MY FLT PATH. I THOUGHT THAT SOCAL APCH WAS NO LONGER AWARE WHERE I WAS NOR WAS THE JET AIRLINE AWARE. MY FIRST REACTION WAS TO AVOID A COLLISION. I KNEW I COULD NOT OUT CLB HIM; SO I BEGAN TO GRADUALLY DSND IN CASE I NEEDED TO GO UNDERNEATH. AT APPROX 1 1/2 MI FROM ME; THE JET AIRLINER VEERED N. I HEARD THE PLT OF THE OTHER ACFT SAY THAT I WAS AT 3000 FT NOT 3500 FT. THIS WAS THE FIRST TIME THAT I WAS AWARE THAT I WAS ON THE SAME FREQ AS THE AIRLINE PLT. AT THE TIME THAT I FIRST NOTICED THE JET ACFT COMING TOWARDS ME I WAS ABOUT 5 NM FROM JOHN WAYNE AND LESS THAN THAT FROM THE AIRLINER; WHO WAS COMING AT ME VERY QUICKLY.