[logback-user] All error in a separate file

Hi, I am new to logback configuration. I have read the online manual but clearly not in depth. I would like to log in a separate file all ERROR logs but keep these logs at the same time where they are supposed to be. I understand I could use root with an ERROR level and a file appender but then I woul lose all the details (logs lower than ERROR). Is there any obvious way that I might have missed? Regards, Bertrand Dechoux