How To Monitor ALTER TABLESPACE Ddl Change

I can use trigger to monitor ALTER TABLESPACE ddl statement against a particular tablespace in a schema.

CREATE OR REPLACE TRIGGER HOT_MAIL_DDL_CHANGE
AFTER ALTER TABLESPACE <tbs_name>
ON SHANNURA.SCHEMA
BEGIN
INSERT INTO HOT_MAIL_DDL_AUDIT_LOG VALUES
(SYSDATE,
SYS_CONTEXT('USERENV', 'SESSION_USER'),
ORA_SYSEVENT,
ORA_DICT_OBJ_TYPE,
ORA_DICT_OBJ_NAME
          );
END;
/

I think, line no.2 isn't a valid statement. Or can I use AUDIT instead - in fact, I want to monitor/audit only ALTER TABLESPACE ddl statement and that too for a particular tablespace only.