ON DELETE CASCADE Question

I would like for corresponding records in t_a to be deleted when I
delete a record from t_b. This deletes from t_b when I delete from t_a,
but not the other way around. I am unable to create a foreign key
constraint on t_a because this table holds records from several other
tables. I added a simple script below that demonstrates my problem.

Any suggestions?

/*******************************************************************/
drop table IF EXISTS t_b;
drop table IF EXISTS t_a;

CREATE TABLE t_a
(
id bigint NOT NULL,
CONSTRAINT pk_a PRIMARY KEY (id)
);

CREATE TABLE t_b
(
id bigint NOT NULL,
CONSTRAINT pk_b PRIMARY KEY (id),
CONSTRAINT fk_b_a FOREIGN KEY (id) REFERENCES t_a (id) ON DELETE
CASCADE
);

INSERT INTO t_a VALUES (1),(2),(3);
INSERT INTO t_b VALUES (1),(2),(3);

delete from t_b where id = 2;

select * from t_a;