erc-query-display

erc-query-display is a variable defined in `erc.el'.
Its value is

window

Documentation:
Indicates how to display query buffers when using the /QUERY
command to talk to someone.

The default behavior is to display the message in a new window
and bring it to the front. See the documentation for
`erc-join-buffer' for a description of the available choices.

See also `erc-auto-query' to decide how private messages from
other people should be displayed.

You can customize this variable.